Your FREE MCAT Practice Test 2026 – 460+ Q&A

Correct answer: Phosphorylation of serine or threonine found near a basic amino acid

PKA reads a short consensus sequence in its substrates (Arg-Arg-X-Ser/Thr), so its specificity comes from "Phosphorylation of serine or threonine found near a basic amino acid". "Phosphorylation of serine or threonine followed by an acidic residue" is the acidophilic pattern used by casein kinase 2, and "Phosphorylation of serine or threonine followed by a proline residue" is the pattern of proline-directed kinases such as CDKs and MAP kinases. "Phosphorylation of serine or threonine inside an exposed alpha helix" is wrong because PKA recognizes the linear amino acid sequence around the target residue, not a secondary-structure element; its sites usually sit in flexible, unstructured stretches.

Correct answer: Vmax is unchanged and Km is unchanged

Vmax and Km are constants of the enzyme under fixed conditions, so adding substrate leaves "Vmax is unchanged and Km is unchanged"; only the measured rate climbs toward Vmax. "Vmax is increased and Km is unchanged" confuses the initial velocity, which does rise with substrate concentration, with the maximum velocity, which is set by the amount of enzyme and its turnover number. "Vmax is unchanged and Km is increased" is the pattern a competitive inhibitor produces, not extra substrate. "Vmax is increased and Km is increased" wrongly treats both constants as values that scale with the substrate supply.

Correct answer: They help proteins fold correctly or refold once misfolded

Molecular chaperones such as Hsp70 and the chaperonins bind exposed hydrophobic stretches of polypeptide chains, prevent aggregation and give a chain repeated chances to reach its native shape, so "They help proteins fold correctly or refold once misfolded". "They link amino acids into proteins and extend nascent peptide chains" describes the ribosome, which forms the peptide bonds. "They determine the final shapes or sequences of proteins" is wrong because the gene specifies the sequence and the sequence determines the native fold; chaperones only help a chain reach it. "They stay attached to proteins and become permanent structural parts" is wrong because chaperones bind only transiently and release the folded protein.

Correct answer: To extend the telomeres at chromosome ends

Telomerase is a reverse transcriptase that carries its own RNA template and adds repeats to chromosome ends, so its job is "To extend the telomeres at chromosome ends", offsetting the shortening left by lagging-strand replication. "To seal the nicks between Okazaki fragments" is the job of DNA ligase, "To create the primers for Okazaki fragments" is the job of primase, which lays down short RNA primers, and "To unwind the parental helix at the origins" is the job of helicase.

Correct answer: They trigger the G-protein to swap its GDP for GTP

An activated GPCR acts as a guanine nucleotide exchange factor: "They trigger the G-protein to swap its GDP for GTP", and the GTP-bound alpha subunit then separates from the beta-gamma pair and signals. "They trigger the G-protein to swap its GTP for GDP" runs the exchange backward; ending up with GDP is the inactive state, which the alpha subunit reaches by hydrolyzing its own GTP. "They trigger the G-protein to swap its ADP for ATP" and "They trigger the G-protein to swap its ATP for ADP" name the wrong nucleotides: G-proteins are guanine nucleotide-binding proteins, while ATP and ADP are the currency of kinases and pumps.

Correct answer: They show peak activity at acidic pH

Lysosomal acid hydrolases have a low pH optimum, so "They show peak activity at acidic pH": a proton pump holds the lysosome lumen near pH 5, and enzymes that leak into the neutral cytosol lose most of their activity, which protects the cell. "They work optimally at alkaline pH" reverses this. "They are secreted into the gut lumen" describes pancreatic digestive enzymes, not enzymes that work inside the lysosome. "They are driven by ATP hydrolysis" is wrong because hydrolases use water to split their substrates; the ATP is spent by the proton pump that acidifies the lumen.

Correct answer: Fibrous proteins form long linear chains, while globular proteins fold into compact shapes

"Fibrous proteins form long linear chains, while globular proteins fold into compact shapes": collagen and keratin are long, linear polypeptide chains that build fibers and sheets, whereas enzymes and hemoglobin fold into compact, roughly spherical forms. "Fibrous proteins curl into round globules, while globular proteins extend as straight rods" swaps the two shapes. "Fibrous proteins are largely water soluble, while globular proteins tend to precipitate" reverses their solubility: fibrous proteins are generally insoluble, and globular proteins are generally soluble. "Fibrous proteins largely serve as enzymes, while globular proteins give structural support" reverses their roles, since most enzymes are globular and most fibrous proteins are structural.

Correct answer: To act as an electron carrier in redox reactions

NAD+ is a coenzyme that accepts a hydride ion to become NADH and later passes those electrons on, so its role is "To act as an electron carrier in redox reactions" in glycolysis, the citric acid cycle and the electron transport chain. "To act as a phosphoryl donor in kinase reactions" describes ATP. "To act as an acyl group carrier in fat metabolism" describes coenzyme A. "To act as a methyl group donor in DNA methylation" describes S-adenosylmethionine.

Correct answer: It binds the repressor and keeps the operator free

Allolactose is the natural inducer of the lac operon: "It binds the repressor and keeps the operator free", because binding changes the repressor's shape so that it releases the operator, and RNA polymerase can then transcribe the lac genes. "It helps the repressor lock firmly onto the operator" describes a corepressor, as tryptophan acts in the trp operon, which is the opposite effect. "It joins the CAP protein and draws in RNA polymerase" describes cAMP, the signal of low glucose. "It blocks RNA polymerase from copying the lacI gene" is wrong because lacI is expressed constitutively; allolactose acts on the repressor protein itself, not on its gene.

Correct answer: Helicase

"Helicase" uses ATP to break the hydrogen bonds between base pairs and separate the two strands at the replication fork. "DNA gyrase" is a topoisomerase that relieves the supercoiling that builds up ahead of the fork, but it does not separate the strands. "DNA ligase" seals the nicks between Okazaki fragments, and "Primase" lays down the RNA primers that DNA polymerase extends.

Correct answer: It labels proteins for destruction by the proteasome

A chain of ubiquitin molecules attached to a lysine side chain is the signal the 26S proteasome recognizes: "It labels proteins for destruction by the proteasome". "It keeps proteins from destruction by the proteasome" reverses its role. "It cuts proteins into short peptides for disposal" describes the proteasome's own protease activity; ubiquitin is only the tag and is recycled. "It refolds misfolded proteins into a stable shape" describes chaperones such as Hsp70, not ubiquitin.

Correct answer: It binds troponin, causing a conformational change that moves tropomyosin

In skeletal muscle, calcium released from the sarcoplasmic reticulum binds troponin C: "It binds troponin, causing a conformational change that moves tropomyosin", and the shifted tropomyosin uncovers the myosin-binding sites on actin. "It binds tropomyosin, causing it to rotate and uncover the myosin-binding sites" names the wrong partner: tropomyosin moves, but calcium binds troponin. "It binds myosin heads, which then attach directly onto actin" is wrong because the myosin heads are already energized by ATP hydrolysis and are held back only by the blocked sites. "It binds actin filaments, which then glide along myosin heads" is wrong because calcium does not bind actin; the sliding is driven by myosin cross-bridge cycling.

Correct answer: Oxygen

At complex IV (cytochrome c oxidase), electrons are handed to "Oxygen", which picks up protons and is reduced to water; without it the chain backs up. "NAD+" accepts electrons at the start of the process, in glycolysis and the citric acid cycle, and its reduced form NADH then feeds the chain. "Water" is the product formed when oxygen is reduced, not the acceptor. "Pyruvate" accepts electrons from NADH only in lactic acid fermentation, a pathway with no electron transport chain.

Correct answer: Km is increased and Vmax is unchanged

A competitive inhibitor competes with substrate for the active site, so more substrate is needed to reach half-maximal velocity, yet a high enough substrate concentration still outcompetes it: "Km is increased and Vmax is unchanged". "Km is unchanged and Vmax is decreased" describes a noncompetitive inhibitor, which binds away from the active site. "Km is decreased and Vmax is decreased" describes an uncompetitive inhibitor, which binds only the enzyme-substrate complex. "Km is increased and Vmax is lowered" describes a mixed inhibitor.

Correct answer: It carries out the rate-limiting step of fatty acid synthesis

Acetyl-CoA carboxylase uses biotin and ATP to carboxylate acetyl-CoA to malonyl-CoA, so "It carries out the rate-limiting step of fatty acid synthesis"; insulin activates it, and its product malonyl-CoA also blocks fatty acid entry into mitochondria. "It catalyzes the two-carbon elongation of fatty acid chains" describes fatty acid synthase, which uses the malonyl-CoA that acetyl-CoA carboxylase supplies. "It cuts fatty acid chains into acetyl groups during oxidation" describes the thiolase step of beta-oxidation. "It inserts cis double bonds into saturated fatty acid tails" describes the fatty acyl-CoA desaturases of the endoplasmic reticulum.

Correct answer: To link the Okazaki fragments together

DNA ligase forms a phosphodiester bond across a nick in the sugar-phosphate backbone, so its job is "To link the Okazaki fragments together" on the lagging strand, and it seals the final nick in the same way after excision repair. "To fill the gaps left by primer removal" is the job of DNA polymerase (DNA polymerase I in bacteria), which leaves a nick for ligase to seal. "To synthesize the short RNA primers" is the job of primase. "To relieve the strain ahead of the fork" is the job of topoisomerases.

Correct answer: It adds each amino acid to the nascent chain

Peptidyl transferase is the catalytic activity of the large ribosomal subunit's rRNA; it forms the peptide bond between the incoming amino acid and the growing polypeptide, so "It adds each amino acid to the nascent chain". "It matches each mRNA codon to its anticodon" describes base pairing between codon and tRNA in the decoding center, not peptide bond formation. "It moves each peptidyl-tRNA to the P site" describes translocation, driven by the elongation factor EF-G (eEF2 in eukaryotes). "It links each amino acid to its cognate tRNA" describes aminoacyl-tRNA synthetases, which charge tRNAs before they reach the ribosome.

Correct answer: RNA is built on ribose, while DNA is built on deoxyribose

The sugar is the defining structural difference: "RNA is built on ribose, while DNA is built on deoxyribose", the deoxyribose lacking the 2' hydroxyl group that ribose carries. "RNA is built with thymine, while DNA is built with uracil" swaps the bases; RNA uses uracil and DNA uses thymine. "RNA lacks a sugar-phosphate backbone, while DNA has one" is wrong because both are joined by phosphodiester bonds in a sugar-phosphate backbone. "RNA lacks a 2' hydroxyl on its sugar, while DNA has one" reverses the sugars: ribose has the 2' hydroxyl, which is also why RNA is less stable.

Correct answer: Hemoglobin binds oxygen cooperatively, while myoglobin does not

Hemoglobin is a tetramer whose subunits communicate, so oxygen binding at one heme raises the affinity of the others and gives a sigmoidal curve: "Hemoglobin binds oxygen cooperatively, while myoglobin does not", since myoglobin is a single chain with a hyperbolic curve. "Hemoglobin has copper ions, while myoglobin has heme iron" is wrong because both proteins bind oxygen at the iron of heme; copper carries oxygen in hemocyanin. "Hemoglobin has one polypeptide subunit, while myoglobin has four" reverses their subunit counts. "Hemoglobin shows the higher oxygen affinity, while myoglobin does not" reverses their affinities: myoglobin binds oxygen more tightly, reaching half saturation at about 1-3 mmHg compared with about 26 mmHg for hemoglobin, which lets it take oxygen from the blood and store it in muscle.

Correct answer: Metaphase

Condensation begins in prophase and peaks as the chromosomes line up at the metaphase plate, which is why karyotypes are prepared from cells arrested in "Metaphase". In "Prophase" the chromatin is only starting to condense. In "Anaphase" the sister chromatids are being pulled apart toward the poles, and in "Telophase" the chromosomes reach the poles and begin to decondense.

Correct answer: Plasma membrane

Bacteria have no mitochondria, so their respiratory complexes sit in the "Plasma membrane" (the cytoplasmic membrane), which pumps protons outward to drive ATP synthase. The "Outer membrane" of gram-negative bacteria is a porous barrier with no electron transport chain. A "Nuclear membrane" and a "Vacuolar membrane" belong to eukaryotic cells; a bacterium has neither a nucleus nor a vacuole of that kind.

Correct answer: Ribosomal RNA (rRNA) synthesis

The nucleolus forms around the rRNA genes, where RNA polymerase I transcribes them and the new rRNA is processed and packed with proteins into ribosomal subunits, so its function is "Ribosomal RNA (rRNA) synthesis". "Messenger RNA (mRNA) splicing" is carried out by spliceosomes in the nucleoplasm. "Transfer RNA (tRNA) charging" is done by aminoacyl-tRNA synthetases in the cytoplasm. "Messenger RNA (mRNA) transport" out of the nucleus runs through the nuclear pore complexes, not the nucleolus.

Correct answer: Marking proteins for breakdown

Ubiquitin's best-known role is "Marking proteins for breakdown": a polyubiquitin chain on a damaged or short-lived protein sends it to the proteasome. "Marking proteins for export" is the job of signal sequences read at the endoplasmic reticulum, not ubiquitin. "Cutting proteins into peptides" is done by the proteasome's proteases; ubiquitin is only the tag. "Folding proteins into shape" is the work of chaperones.

Correct answer: High specific heat

Hydrogen bonding gives water a "High specific heat", so it absorbs a great deal of heat with little temperature change and buffers organisms and environments against temperature swings. "Low enthalpy of fusion" is wrong because water's enthalpy of fusion, about 334 J/g, is high for so small a molecule. "Weak cohesion" is wrong because hydrogen bonds make water strongly cohesive, which gives it high surface tension. "Low heat of vaporization" is wrong because water's heat of vaporization is high, about 2260 J/g at 100 degrees C, which is the basis of evaporative cooling such as sweating.

Correct answer: Electron transport

Light absorbed by photosystems II and I excites electrons and pushes them along the thylakoid chain, so the process light drives directly is "Electron transport". "ATP synthesis" is driven by the proton gradient that electron transport builds, so it depends on light only indirectly. "Glucose production" and "RuBP regeneration" belong to the Calvin cycle, which runs on the ATP and NADPH made in the light reactions rather than on light itself.

Correct answer: The sequence of amino acids in the peptide chain

Primary structure is simply "The sequence of amino acids in the peptide chain", fixed by the peptide bonds that join the residues in the order the gene encodes. "The hydrogen bonds between nearby backbone groups" produce secondary structure such as alpha helices and beta sheets. "The packing of hydrophobic side chains in the core" helps set tertiary structure. "The contacts between separate polypeptide subunits" define quaternary structure.

Correct answer: Protein modification and sorting

Proteins arriving from the endoplasmic reticulum pass through the Golgi apparatus, which trims and adds sugar groups and sends each protein to the plasma membrane, a lysosome or a secretory vesicle, so its role is "Protein modification and sorting". "Polypeptide synthesis and folding" happen on ribosomes and in the rough endoplasmic reticulum. "Genome replication and DNA repair" take place in the nucleus. "Organelle digestion and recycling" is the job of lysosomes.

Correct answer: Hydrogen bonds

Alpha helices and beta sheets are held together by "Hydrogen bonds" between backbone carbonyl oxygens and amide hydrogens. "Ionic bonds" between charged side chains and "Disulfide bonds" between cysteines help stabilize tertiary structure. "Peptide bonds" link the amino acids into the chain and so define primary structure.

Correct answer: Ribulose-1,5-bisphosphate

RuBisCO attaches CO2 to the five-carbon sugar "Ribulose-1,5-bisphosphate", and the unstable product splits into two molecules of 3-phosphoglycerate. "1,3-bisphosphoglycerate" is formed later in the cycle, when 3-phosphoglycerate is phosphorylated before reduction, so it does not capture CO2. "Fructose-2,6-bisphosphate" is a regulator of glycolysis and gluconeogenesis, and "2,3-bisphosphoglycerate" lowers hemoglobin's oxygen affinity in red blood cells; neither takes part in carbon fixation.

Correct answer: Smooth endoplasmic reticulum

The enzymes that make fatty acid-derived oils, phospholipids and steroids sit in the membranes of the "Smooth endoplasmic reticulum", which is why steroid-secreting cells are packed with it. The "Rough endoplasmic reticulum" is studded with ribosomes and specializes in making secreted and membrane proteins. "Free cytosolic polyribosomes" translate mRNA into proteins, not lipids. The "Golgi apparatus" modifies and sorts the proteins and lipids that arrive from the endoplasmic reticulum; it does not make the bulk of the cell's oils, phospholipids or steroids.

Correct answer: Breakdown and recycling of cellular debris

Lysosomes hold acid hydrolases that digest worn-out organelles, engulfed material and macromolecules and return the building blocks to the cytosol, so their primary function is "Breakdown and recycling of cellular debris". "Glycosylation and sorting of new proteins" is the job of the Golgi apparatus. "Synthesis and folding of membrane proteins" happens on ribosomes bound to the rough endoplasmic reticulum. "Oxidation and shortening of fatty acids" is beta-oxidation, which takes place in mitochondria and peroxisomes.

Correct answer: Oxygen

In aerobic respiration, complex IV passes electrons to "Oxygen", which is reduced to water; that is why the chain stops without it. "Glucose" is the original electron source, not the acceptor, and "NADH" is the electron donor that feeds complex I. "Lactate" is the product of lactic acid fermentation, in which pyruvate rather than oxygen accepts the electrons from NADH.

Correct answer: Anaphase

When the cohesin holding sister chromatids together is cleaved, the chromatids separate and kinetochore microtubules pull them to opposite poles: that is "Anaphase". In "Prophase" the chromosomes condense and the spindle forms. In "Metaphase" the chromosomes line up at the cell's equator, still joined as sister chromatids. In "Telophase" the separated chromosomes have already reached the poles and new nuclear envelopes form.

Correct answer: To produce ATP from nutrients

Cellular respiration oxidizes fuels such as glucose and fatty acids and captures the released energy, so its purpose is "To produce ATP from nutrients". "To fix CO2 into sugars" describes the Calvin cycle of photosynthesis, the reverse of what respiration does to carbon. "To build DNA from nucleotides" describes an anabolic, ATP-consuming process. "To split water into O2" describes the light reactions of photosynthesis; respiration consumes oxygen and produces water.

Correct answer: The heterozygote phenotype blends the parental traits

In incomplete dominance neither allele fully masks the other, so "The heterozygote phenotype blends the parental traits", as when red and white snapdragons give pink offspring. "The heterozygote phenotype displays both alleles fully" describes codominance, as in AB blood type. "The heterozygote phenotype matches the dominant parent" describes complete dominance. "The heterozygote phenotype is fitter than both parents" describes heterozygote advantage (overdominance), a fitness effect rather than a pattern of trait expression.

Correct answer: Phosphorylation

Adding a phosphate group to serine, threonine or tyrosine residues of a finished polypeptide is a post-translational modification, so the answer is "Phosphorylation". "Polyadenylation" adds the poly-A tail to pre-mRNA, before translation. "Aminoacylation" attaches an amino acid to its tRNA, a step that prepares for translation rather than modifying a finished protein. "Decarboxylation" removes CO2 from small molecules, such as amino acids being converted to neurotransmitters, and is not a modification of finished proteins.

Correct answer: Large population size

Hardy-Weinberg equilibrium assumes a very "Large population size", along with random mating, no mutation, no migration and no natural selection, so that allele frequencies stay constant. "Population bottlenecks" shrink the population so that genetic drift changes allele frequencies by chance. An "Inbred population" breaks the random-mating assumption and raises homozygosity. "Gene flow into the population", the movement of alleles between populations through migration, also shifts allele frequencies.

Correct answer: To be the template that ribosomes read

Messenger RNA copies the coding sequence of a gene and carries it to the ribosome, where it is read codon by codon during translation, so its role is "To be the template that ribosomes read". "To carry each amino acid to the ribosome" is the job of tRNA, which pairs its anticodon with each codon. "To be the enzyme that forms peptide bonds" describes rRNA, the peptidyl transferase ribozyme of the large ribosomal subunit. "To splice each intron out of the pre-mRNA" is done by small nuclear RNAs in the spliceosome; mRNA is the molecule being processed, not the one doing the splicing.

Correct answer: Tight junctions

"Tight junctions" are strands of claudin and occludin proteins that seal neighboring epithelial cells together, closing the space between them so materials cannot leak through the paracellular route. "Gap junctions" are connexin channels that do the opposite, letting ions and small molecules pass directly from the cytoplasm of one cell to the next. "Adherens junctions" link the actin cytoskeletons of neighboring cells through cadherins and hold the cells together without sealing the gap. "Desmosomes" rivet neighboring cells together at spots where cadherins are linked to intermediate filaments; they give mechanical strength rather than a barrier.

Correct answer: DNA helicase

"DNA helicase" uses ATP to break the hydrogen bonds between base pairs at the replication fork, unwinding the double helix into two template strands. "DNA polymerase" builds the new strands by adding nucleotides to a primer, but only after the strands have been separated. "DNA ligase" seals the nicks between Okazaki fragments on the lagging strand. "DNA topoisomerase" works ahead of the fork to relieve the supercoiling that unwinding creates; it cuts and rejoins the backbone but does not separate the two strands.

Correct answer: Moving the chromosomes to opposite poles

Microtubules form the mitotic spindle: kinetochore microtubules attach to the chromosomes and shorten while polar microtubules push the poles apart, so their main job is "Moving the chromosomes to opposite poles". "Pinching the cell in two to complete cytokinesis" is done in animal cells by a contractile ring of actin microfilaments and myosin, not by microtubules. "Condensing the chromatin during prophase" is driven by condensin protein complexes. "Breaking down the nuclear envelope" follows phosphorylation of the nuclear lamins by cyclin-dependent kinase.

Correct answer: Mitochondrial matrix

The enzymes of the Krebs (citric acid) cycle are dissolved in the "Mitochondrial matrix", where acetyl-CoA is oxidized to CO2 and NADH and FADH2 are made for the electron transport chain. The "Inner mitochondrial membrane" holds the electron transport chain and ATP synthase; only one cycle enzyme, succinate dehydrogenase, is anchored there, and the cycle as a whole runs in the matrix. The "Mitochondrial intermembrane space" is where protons accumulate during electron transport. The "Outer mitochondrial membrane" is a porous boundary studded with porins and does not carry out the cycle.

Correct answer: To deliver amino acids to the ribosome

Each tRNA is charged with a specific amino acid by an aminoacyl-tRNA synthetase and pairs its anticodon with the matching mRNA codon, so its role is "To deliver amino acids to the ribosome". "To be the template that ribosomes read" describes mRNA, whose codon sequence the ribosome translates. "To catalyze the formation of peptide bonds" is the job of the rRNA in the large ribosomal subunit. "To guide the splicing of the introns" belongs to the small nuclear RNAs of the spliceosome, which act on pre-mRNA in the nucleus.

Correct answer: Programmed cell death

Apoptosis is "Programmed cell death": a regulated, caspase-driven process in which the cell shrinks, its chromatin condenses and its contents are packaged into apoptotic bodies that phagocytes clear without inflammation. "Necrosis" is the accidental form of cell death, in which injured cells swell and burst. "Inflammation" is a mark of necrosis rather than apoptosis: the contents spilled by burst cells trigger it, whereas apoptotic bodies stay wrapped in membrane until they are engulfed. "Uncontrolled proliferation" is a hallmark of cancer, which often arises when apoptosis fails.

Correct answer: Four genetically distinct haploid cells

Meiosis has one round of DNA replication followed by two divisions, and crossing over and independent assortment shuffle the alleles, so one diploid cell yields "Four genetically distinct haploid cells". "Two genetically identical diploid cells" is the outcome of mitosis. "Two genetically distinct haploid cells" is where meiosis stands after the first division only; meiosis II then splits each of those cells again. "Four genetically identical diploid cells" is what two rounds of mitosis would give; it misses both the halving of the chromosome number and the mixing of alleles.

Correct answer: Rough endoplasmic reticulum

Proteins bound for secretion are made on ribosomes attached to the "Rough endoplasmic reticulum": a signal sequence directs the growing chain into the ER lumen, where folding and early glycosylation begin. The "Smooth endoplasmic reticulum" lacks ribosomes and makes lipids and steroids, stores calcium and detoxifies drugs. The "Cis-Golgi network" receives newly made proteins from the ER in transport vesicles, and the "Trans-Golgi network" sorts and packages them into secretory vesicles; both modify and ship proteins that the rough ER has already synthesized.

Correct answer: tRNA

"tRNA" is the molecule with an anticodon, a three-base sequence in one of its loops that pairs with the complementary codon on mRNA so the right amino acid is added. "mRNA" carries the codons themselves. "rRNA" forms the structure and catalytic core of the ribosome. "snRNA" works in the spliceosome, pairing with splice sites on pre-mRNA so introns can be removed.

Correct answer: The repressor detaches from the operator, and lacZ transcription increases

When lactose is present, some of it is converted to allolactose, which binds the lac repressor and changes its shape so it can no longer hold the operator. RNA polymerase can then transcribe lacZ, lacY and lacA, so "The repressor detaches from the operator, and lacZ transcription increases". "The repressor binds to the operator, and lacZ transcription decreases" describes the operon when lactose is absent. "The repressor binds to the operator, and lacZ transcription increases" gets the repressor wrong: a repressor sitting on the operator blocks transcription. "The repressor detaches from the operator, and lacZ transcription decreases" gets the outcome wrong: freeing the operator lets transcription rise, most strongly when glucose is low and CAP-cAMP is bound.

Correct answer: It lets one tRNA recognize more than one codon

The first base of the anticodon pairs loosely with the third base of the codon (G can pair with U, and inosine with U, C or A), so "It lets one tRNA recognize more than one codon" and a cell needs far fewer than 61 different tRNAs. "It lets one tRNA deliver more than one amino acid to a codon" is wrong because each tRNA is charged with a single, specific amino acid, and each codon specifies only one amino acid. "It ensures precise tRNA-codon pairing" is the opposite of wobble, which relaxes the base-pairing rules at that one position. "It stabilizes tRNA-codon pairing" is also wrong, because wobble pairs such as G-U are weaker than standard Watson-Crick pairs.

Correct answer: S phase

DNA is copied during the "S phase" (synthesis phase) of interphase, so each chromosome enters mitosis as two sister chromatids. In "G1 phase" the cell grows and checks conditions before committing to replication. In "G2 phase" the DNA has already been replicated and the cell prepares for division, with a checkpoint for damaged or unreplicated DNA. "M phase" is mitosis and cytokinesis, when the replicated chromosomes are separated.

Correct answer: Peptide bond

A condensation reaction between the alpha-carboxyl group of one amino acid and the alpha-amino group of the next releases water and forms a covalent amide link, the "Peptide bond" of the protein backbone. A "Hydrogen bond" is a weaker attraction, such as the one between backbone N-H and C=O groups in helices and sheets, and does not join residues into a chain. A "Disulfide bond" is a covalent link between the side chains of two cysteines. An "Ester bond" forms when a carboxyl group reacts with a hydroxyl group rather than an amino group, as in triglycerides.

Correct answer: Active transport needs energy; facilitated diffusion does not.

Both processes use membrane proteins, but facilitated diffusion only lets solutes move down their concentration gradient, while active transport moves them against it by spending ATP directly or a stored ion gradient; so "Active transport needs energy; facilitated diffusion does not." The statement "Facilitated diffusion uses energy; active transport does not." swaps the two processes. "Facilitated diffusion moves solutes up their gradient; active transport does not." reverses their directions, since moving a solute up its gradient is what active transport does. "Active transport moves solutes down their gradient; facilitated diffusion does not." is wrong for the same reason: downhill movement is exactly what facilitated diffusion provides.

Correct answer: Glycolysis

"Glycolysis" splits one glucose into two pyruvate molecules with a net gain of 2 ATP and 2 NADH, and none of its ten steps removes a carbon as CO2. The "Krebs cycle" releases two CO2 per turn, at the isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase steps. "Alcoholic fermentation" releases CO2 when pyruvate decarboxylase converts pyruvate to acetaldehyde, which is then reduced to ethanol. The "Pentose phosphate pathway" releases CO2 in its oxidative phase, when 6-phosphogluconate is decarboxylated to ribulose 5-phosphate.

Correct answer: Meiosis I

Crossing over happens in prophase I, when homologous chromosomes pair up as tetrads and swap segments at chiasmata, so the process is "Meiosis I". "Meiosis II" separates sister chromatids, much as mitosis does, and the homologous chromosomes are no longer paired by then. "Mitosis" produces genetically identical daughter cells and does not pair homologous chromosomes. "Binary fission" is the asexual division of a prokaryotic cell with a single circular chromosome; prokaryotes have no homologous pairs to exchange segments.

Correct answer: By causing protein misfolding that disrupts cell function

Prions are misfolded forms of the prion protein (PrP) that convert normal PrP into the same misfolded shape; the aggregates damage neurons and cause spongiform encephalopathies, so prions act "By causing protein misfolding that disrupts cell function". "By integrating their DNA into the genome of host cells" describes retroviruses, but prions contain no nucleic acid at all. "By triggering an immune response that damages the host tissue" does not fit, because prions are built from a host protein and provoke little immune response or inflammation. "By lysing infected cells as they reproduce inside them" describes lytic viruses; prions spread by templating misfolding, not by replicating inside cells and bursting them.

Correct answer: Endosome

Material taken in by endocytosis goes first to early endosomes, which sort receptors back to the plasma membrane and pass the rest on as the compartment matures into a late endosome, so the structure is the "Endosome". The "Lysosome" is the final destination, where acid hydrolases degrade the cargo after late endosomes fuse with it; it does not do the sorting. The "Proteasome" is a cytosolic protease complex that degrades ubiquitin-tagged proteins, not membrane-enclosed endocytosed cargo. The "Golgi apparatus" modifies and sorts newly made proteins arriving from the endoplasmic reticulum on the secretory pathway; incoming endocytosed material is sorted in endosomes.

Correct answer: To establish ion gradients across the cell membrane

Each cycle of the Na+/K+-ATPase uses one ATP to move 3 Na+ out of the cell and 2 K+ in, which is how it serves "To establish ion gradients across the cell membrane"; those concentration gradients underlie the resting potential and drive secondary active transport. "To pump sodium in and potassium out across the cell membrane" reverses the direction of both ions. "To let ions leak across the membrane passively" describes a channel; the pump is a P-type ATPase that moves ions against their gradients. "To synthesize ATP as ions flow across the membrane" describes ATP synthase; the sodium-potassium pump hydrolyzes ATP rather than making it.

Correct answer: Amplifies a receptor signal to produce cellular responses

A first messenger such as a hormone binds a cell-surface receptor, and the activated receptor triggers production of second messengers such as cAMP, IP3 or Ca2+, each of which activates many downstream enzymes; the second messenger therefore "Amplifies a receptor signal to produce cellular responses". "Transmits signals from the extracellular environment directly to the nucleus" is wrong because second messengers are made inside the cell only after the first messenger has bound its receptor, and their effects reach the nucleus, if at all, through further steps such as kinase cascades. "Acts as a ligand that binds extracellular receptors" describes the first messenger, such as a hormone or neurotransmitter, not the second messenger. "Alters gene expression directly, with no signal cascade" fits a steroid hormone acting through an intracellular receptor; second messengers work by setting off further signaling steps.

Correct answer: Maintains the fluidity and stability of the bilayer

Cholesterol wedges between phospholipids: at high temperature its rigid rings restrain the fatty acid tails, and at low temperature it keeps the tails from packing into a gel, so it "Maintains the fluidity and stability of the bilayer". "Forms the channels and pores across the lipid bilayer" describes channel proteins; cholesterol actually makes the bilayer less permeable to small ions. "Serves as the energy store of the cell" describes triglycerides and glycogen; cholesterol is not broken down for energy. "Anchors the cytoskeleton to the cell surface" is the work of membrane proteins such as integrins and the spectrin-ankyrin network.

Correct answer: Reducing the surface area of solid reactants

Reaction rate depends on how often reactant particles collide with enough energy. "Reducing the surface area of solid reactants" exposes fewer particles to collision, so it slows the reaction rather than speeding it up. "Raising the concentration of dissolved reactants" increases the collision frequency. "Heating the reaction mixture to a higher temperature" gives more molecules enough energy to cross the activation barrier and makes collisions more frequent. "Adding a catalyst to the reaction mixture" provides a pathway with a lower activation energy.

Correct answer: Oxidation

A rise in oxidation state means the element has lost electrons, and loss of electrons is the definition of "Oxidation". "Reduction" is the gain of electrons, which lowers the oxidation state. "Sublimation" (solid to gas) and "Evaporation" (liquid to gas) are physical phase changes; they leave the oxidation state of every element unchanged.

Correct answer: 3.0

HCl is a strong acid and dissociates completely, so [H+] = 0.001 M = 1 x 10^-3 M and pH = -log(1 x 10^-3), which gives "3.0". "6.9" comes from pressing the natural logarithm instead of the base-10 logarithm: -ln(0.001) is about 6.9. "2.7" treats HCl as if it released two protons, so [H+] = 0.002 M and -log(0.002) is about 2.7; HCl is monoprotic. "2.0" comes from miscounting the decimal places and taking 0.001 M as 1 x 10^-2 M, so that -log(1 x 10^-2) = 2.0.

Correct answer: [Ar] 4s^1 3d^5

Chromium (Z = 24) has six electrons beyond the argon core. Moving one 4s electron into 3d gives a half-filled 3d subshell, which is lower in energy, so the ground state is "[Ar] 4s^1 3d^5". "[Ar] 4s^2 3d^4" is the configuration the simple aufbau order predicts, but chromium is one of the known exceptions. "[Ar] 4s^2 3d^5" has seven electrons beyond argon, which is manganese. "[Ar] 4s^1 3d^4" has only five electrons beyond argon, one too few for a neutral chromium atom.

Correct answer: Trigonal pyramidal

NH3 has four electron domains around nitrogen, three bonding pairs and one lone pair; describing only the positions of the atoms gives a "Trigonal pyramidal" molecule with H-N-H angles of about 107 degrees. "Tetrahedral" is the electron-domain geometry of NH3, and it would be the molecular shape only if all four domains were bonds, as in CH4. "Trigonal planar" ignores the lone pair: it is the shape of a molecule with three bonding domains and no lone pair on the central atom, as in BF3. "Trigonal bipyramidal" needs five bonding domains around the central atom, as in PCl5.

Correct answer: 0.80 V

By convention a standard electrode potential is quoted as a reduction potential, so the half-cell in which Ag+ is reduced to Ag(s) has the tabulated value, "0.80 V". "-0.80 V" is the potential for the reverse half-reaction, the oxidation of Ag to Ag+, which is not what this half-cell is doing. "1.60 V" comes from doubling the potential as if the half-reaction were written for two silver ions and two electrons, but electrode potentials are intensive and do not scale with the coefficients. "-1.60 V" makes both errors.

Correct answer: H2O

Water boils at 100 °C, far above the others, because each H2O molecule can form up to four hydrogen bonds (two through its hydrogens and two through its lone pairs) and so builds an extended network; "H2O" has the highest boiling point. "HF" forms stronger individual hydrogen bonds but only about two per molecule, mostly in chains, and boils near 20 °C. "NH3" has three hydrogens but only one lone pair, so its hydrogen bonding is weaker; it boils near -33 °C. "CH4" is nonpolar, has only London dispersion forces and boils near -162 °C.

Correct answer: The entropy change is positive

Melting turns an ordered crystal into a liquid whose particles can move past one another, so the number of accessible microstates rises and "The entropy change is positive"; at the melting point it equals the heat of fusion divided by the melting temperature. "The entropy change is less than zero" describes freezing, the reverse process. "The entropy change is zero" confuses the substance with the universe: at the melting point solid and liquid are in equilibrium, so delta G and the total entropy change of system plus surroundings are zero, but the entropy of the melting substance still rises. "The entropy change is equal to the heat of fusion" forgets to divide by the temperature; delta S = delta H / T, and the two quantities do not even share units.

Correct answer: Products are favored at equilibrium.

K is the ratio of product to reactant concentrations (or pressures) at equilibrium, so a value above 1 means "Products are favored at equilibrium." The statement "Reactants are favored at equilibrium." would require K less than 1. "The forward reaction is exothermic." and "The forward reaction is endothermic." both describe the enthalpy change, which K alone does not reveal: K reflects the standard Gibbs energy change, and reactions with large K can be either exothermic or endothermic.

Correct answer: It serves as a weak acid.

Acetic acid (pKa about 4.76) is only partly ionized, so in this buffer "It serves as a weak acid." It donates protons to neutralize added OH-, while the acetate ion from sodium acetate is the conjugate base that takes up added H+. "It serves as a weak base." describes the acetate ion, not the acid. "It serves as a strong acid." is wrong because a strong acid ionizes completely and cannot form a buffer with its conjugate base. "It serves as a strong base." mislabels it entirely, since acetic acid donates protons rather than accepting them.

Correct answer: PBr3

"PBr3" (phosphorus tribromide) reacts with the -OH group to form a phosphite ester leaving group that bromide then displaces in an SN2 step, turning a poor leaving group (hydroxide) into an alkyl bromide. "NaH" only deprotonates the alcohol to an alkoxide, which is an even poorer leaving group. "KMnO4" is an oxidizing agent that converts primary alcohols to carboxylic acids and secondary alcohols to ketones rather than activating the C-O bond. "H2O" is neither acidic enough to protonate the hydroxyl group nor able to replace it, so the alcohol is left unchanged.

Correct answer: Supplies protons to create a good leaving group

In Fischer esterification the acid catalyst protonates the carbonyl oxygen, making the carbonyl carbon more electrophilic, and later protonates an -OH of the tetrahedral intermediate so that it can leave as water; in short it "Supplies protons to create a good leaving group" and is regenerated at the end. "Serves as a nucleophile attacking the carbonyl carbon" describes the alcohol, not the acid. "Donates electrons to form a new ester bond" is wrong because the catalyst donates protons, not electron pairs; the new C-O bond is formed by the oxygen of the alcohol. "Acts as a reducing agent for carboxylic acids" is wrong because no oxidation state changes during esterification.

Correct answer: It reduces the rate

In an SN2 reaction the nucleophile must reach the back side of the carbon bearing the leaving group; bulky groups crowd that approach and the five-coordinate transition state, so "It reduces the rate" (methyl > primary > secondary, with tertiary substrates essentially unreactive by SN2). "It raises the rate" reverses the trend; alkyl groups speed up SN1 reactions by stabilizing carbocations, not SN2 reactions. "It decreases the activation energy" is wrong because steric crowding raises the energy of the transition state, and "It leaves the activation energy unchanged" is wrong for the same reason; that higher barrier is why the rate falls.

Correct answer: Benzene

"Benzene" has six pi electrons delocalized around a closed ring, giving an aromatic stabilization of roughly 150 kJ/mol (about 36 kcal/mol) compared with three isolated double bonds. "1,3,5-hexatriene" has the same number of conjugated pi electrons in an open chain, but without the cyclic, aromatic arrangement it gains only a small conjugation energy. "1,3-butadiene" is conjugated over just two double bonds, and "Ethene" has a single isolated double bond with no resonance stabilization at all.

Correct answer: 2-chloro-2-methylpropane

HCl adds across the C=C bond by electrophilic addition: the proton goes to the CH2 end, which carries more hydrogens, giving the more stable tertiary carbocation, and chloride then bonds to that carbon, so the Markovnikov product is "2-chloro-2-methylpropane" (tert-butyl chloride). "1-chloro-2-methylpropane" is the anti-Markovnikov product, which would need the proton on the more substituted carbon and a far less stable primary carbocation. "1-chloro-2-methylpropene" and "3-chloro-2-methylpropene" keep the double bond and swap a hydrogen for chlorine, as a substitution such as free-radical allylic chlorination would; HCl adds across the double bond instead.

Correct answer: B2

Bond order is (bonding electrons - antibonding electrons) / 2. "B2" has 6 valence electrons, filling sigma 2s, sigma* 2s and two half-filled pi 2p orbitals, for a bond order of 1. "C2" has 8 valence electrons and a bond order of 2; "O2" has 12 and a bond order of 2, with two unpaired electrons in its pi* orbitals; and "CO" has 10, like N2, giving a triple bond with a bond order of 3. B2 therefore has the lowest bond order.

Correct answer: Geometric isomerism

A carbon-carbon double bond cannot rotate, so the same groups can sit on the same side (cis) or on opposite sides (trans) while the molecular formula and the connectivity stay identical; that is "Geometric isomerism", also called cis-trans isomerism. "Optical isomerism" comes from a chiral center whose mirror images cannot be superimposed, not from a double bond. "Positional isomerism" means a double bond or functional group sits at a different position on the chain, and "Structural isomerism" more broadly means the atoms are connected differently; both change connectivity, whereas geometric isomers differ only in spatial orientation.

Correct answer: Oxymercuration-demercuration

In "Oxymercuration-demercuration" a mercury(II) ion activates the triple bond, water adds to the more substituted carbon (Markovnikov orientation), the mercury is then removed, and the enol that results tautomerizes to the ketone. "Hydroboration-protonolysis" adds a borane and then replaces the boron with hydrogen, giving a cis alkene rather than a ketone. "Acid-catalyzed dehydration" removes water from an alcohol to form an alkene, the reverse of adding it. "Free-radical hydrobromination" adds HBr with anti-Markovnikov orientation and installs a bromine atom, not an oxygen.

Correct answer: Diene and dienophile

The Diels-Alder reaction is a [4+2] cycloaddition between a conjugated diene and a dienophile (usually an alkene carrying an electron-withdrawing group) that builds a six-membered ring, so the answer is "Diene and dienophile". "Alkene and carbene" describes cyclopropanation, a [2+1] addition that gives a three-membered ring. "Enolate and aldehyde" describes the aldol reaction, a nucleophilic addition to a carbonyl group rather than a cycloaddition. "Alkene and alkyne" lacks the conjugated diene that supplies four of the six ring carbons, so no Diels-Alder reaction can take place.

Correct answer: A nucleophile displaces a substituent, and the product is aromatic

In nucleophilic aromatic substitution the nucleophile adds to the ring carbon that carries a leaving group (often a halide activated by electron-withdrawing groups), and that group is then expelled, so "A nucleophile displaces a substituent, and the product is aromatic". The anionic addition complex (Meisenheimer complex) is only a short-lived intermediate, so "A nucleophile forms a stable adduct, and the product is not aromatic" stops the reaction too early, and "A nucleophile adds across a pi bond, and the product is not aromatic" describes a simple addition, ignoring that loss of the leaving group restores the aromatic ring. "A nucleophile substitutes a hydrogen, and the product is aromatic" borrows from electrophilic aromatic substitution, where a ring hydrogen is the group that leaves.

Correct answer: Tertiary structure

Tertiary structure is the overall three-dimensional fold of a single polypeptide, and it is held together mainly by interactions among R groups: disulfide bridges between cysteines, salt bridges between charged side chains, side-chain hydrogen bonds, and hydrophobic packing. Primary structure is only the amino acid sequence linked by peptide bonds. Secondary structure (alpha helices and beta sheets) is stabilized by hydrogen bonds between backbone carbonyl and amide groups, not by side chains. Peptide-bond backbone geometry, the rigid, planar arrangement of atoms around each peptide bond, comes from resonance that gives the C-N bond partial double-bond character; it is fixed by the backbone itself, not by interactions among side chains.

Correct answer: Vmax is lowered and Km is unchanged

Vmax is lowered and Km is unchanged: a pure noncompetitive inhibitor binds free enzyme and the enzyme-substrate complex equally well at a site away from the active site, so it removes working enzyme (lower Vmax) without changing the apparent affinity for substrate (same Km). Vmax is unchanged and Km is increased is the pattern of a competitive inhibitor, which excess substrate can outcompete. Vmax is reduced and Km is lowered is the pattern of an uncompetitive inhibitor, which binds only the enzyme-substrate complex. Vmax is decreased and Km is increased is the pattern of mixed inhibition by an inhibitor that prefers the free enzyme.

Correct answer: 2 ATP and 2 NADH

2 ATP and 2 NADH: the payoff phase makes 4 ATP by substrate-level phosphorylation, but the investment phase spends 2 ATP (at hexokinase and phosphofructokinase-1), so the net gain is 2 ATP, and glyceraldehyde-3-phosphate dehydrogenase reduces 2 NAD+ to 2 NADH per glucose. 4 ATP and 2 NADH counts the gross ATP and forgets the 2 ATP invested. 2 ATP and 4 NADH gets the ATP right but doubles the NADH: glycolysis makes one NADH per glyceraldehyde-3-phosphate, so two per glucose, and a total of 4 adds the 2 NADH that pyruvate dehydrogenase makes in the mitochondrion when pyruvate is converted to acetyl-CoA, a step that comes after glycolysis. 4 ATP and 4 NADH makes both mistakes: it counts the gross ATP and adds the pyruvate dehydrogenase NADH.

Correct answer: Proline

Proline: its side chain bends back and bonds covalently to its own backbone nitrogen, closing a five-membered pyrrolidine ring. The ring locks the backbone angle and leaves no amide hydrogen for helix hydrogen bonding, so proline breaks alpha helices and is common at turns. Glycine also destabilizes helices, but because its side chain is a single hydrogen that makes the backbone too flexible, not because of a ring. Histidine has an imidazole ring containing nitrogen, but that ring belongs to the side chain alone and never bonds back to the backbone. Threonine is a beta-branched residue that forms helices poorly, yet its side chain carries a hydroxyl group and forms no ring.

Correct answer: Succinyl-CoA synthetase

Succinyl-CoA synthetase (also called succinate thiokinase) breaks the high-energy thioester bond of succinyl-CoA and uses that energy to phosphorylate GDP (or ADP), the only substrate-level phosphorylation inside the citric acid cycle. Succinate dehydrogenase, the next enzyme, oxidizes succinate to fumarate and passes the electrons to FAD. Malate dehydrogenase oxidizes malate to oxaloacetate and reduces NAD+ to NADH. Phosphoglycerate kinase does make ATP by substrate-level phosphorylation, but it works in glycolysis, not in the citric acid cycle.

Correct answer: Facilitated diffusion

Facilitated diffusion: GLUT carriers let glucose cross the membrane down its concentration gradient through a specific protein, with no energy input. Simple diffusion uses no protein at all and suits small nonpolar molecules, not glucose. Primary active transport spends ATP directly to pump a solute against its gradient. Secondary active transport, as in the SGLT sodium-glucose symporter, moves glucose uphill by coupling it to a sodium gradient that ATP-driven pumps maintain, so it is still active transport.

Correct answer: They bind and activate cyclin-dependent kinases, driving phase transitions

They bind and activate cyclin-dependent kinases, driving phase transitions: cyclin levels rise and fall through the cycle, and each cyclin binds its partner CDK and switches on its kinase activity, so the complex phosphorylates the targets that move the cell past G1/S and G2/M. They bind and block cyclin-dependent kinases, arresting cells at checkpoints describes CDK inhibitors such as p21 and p27, which do the opposite of cyclins. They phosphorylate and modify mitotic proteins, acting as enzymes themselves is the common mistake of treating cyclins as kinases; cyclins have no catalytic activity, and the CDK subunit does the phosphorylating. They polymerize and assemble into mitotic spindles, separating chromatids describes tubulin, which forms the microtubules of the spindle.

Correct answer: The proton-motive force across the inner mitochondrial membrane

The proton-motive force across the inner mitochondrial membrane: the electron transport chain pumps protons into the intermembrane space, and ATP synthase lets them flow back into the matrix, using that electrochemical gradient to drive the phosphorylation of ADP. The electrical potential across the outer mitochondrial membrane is wrong because the outer membrane is full of porins and holds no useful gradient. The energy released as electrons pass down the transport chain is the indirect source: electron flow powers the proton pumps, and ATP synthase never receives electrons. The energy released as a phosphate leaves a high-energy molecule describes substrate-level phosphorylation, as at pyruvate kinase, not oxidative phosphorylation.

Correct answer: It is degenerate, so most amino acids possess multiple codons

It is degenerate, so most amino acids possess multiple codons: there are 64 codons but only 20 standard amino acids, so most amino acids are encoded by more than one codon (leucine, serine and arginine have six each). It is ambiguous, so most codons encode multiple amino acids confuses degeneracy with ambiguity; the code is unambiguous, because each codon specifies only one amino acid. It is species-specific, so most organisms keep their own codons is false because the code is nearly universal, with only minor variants in mitochondria and a few microbes. It is overlapping, so most codons share their bases with others is false because codons are read as continuous, non-overlapping triplets.

Correct answer: Cis double bonds put kinks in the chains that prevent tight packing

Cis double bonds put kinks in the chains that prevent tight packing: a saturated chain is straight and lines up closely with its neighbors, maximizing van der Waals (London dispersion) contacts, whereas each cis double bond in an unsaturated chain bends it, so the molecules pack loosely and melt at a lower temperature. Trans double bonds put kinks in the chains that loosen their packing is backward: trans double bonds keep a chain nearly straight, which is why trans fats pack tightly and are solid at room temperature. Saturated chains form ionic bonds between the charged carboxyl heads is false: in a solid fatty acid the carboxyl heads pair up through hydrogen bonds rather than ionic bonds, and the heads are identical in saturated and unsaturated fatty acids, so they cannot explain the difference. Saturated chains form hydrogen bonds along the hydrocarbon tails is false because a hydrocarbon tail has no polar groups and cannot form hydrogen bonds.

Correct answer: Bound ribosomes that make secretory and membrane proteins

Bound ribosomes that make secretory and membrane proteins: ribosomes attached to its cytosolic face give the rough ER its studded look, and they feed proteins destined for secretion, the plasma membrane, or the endomembrane organelles into the ER lumen as they are made. Free ribosomes that make cytoplasmic and nuclear proteins describes ribosomes floating in the cytosol, which are not attached to the ER at all. Enzymes that detoxify drugs and form steroid hormones describes the smooth ER, which has no ribosomes. Enzymes that break down fatty acids and hydrogen peroxide describes peroxisomes, which oxidize fatty acids and use catalase to break down hydrogen peroxide.

Correct answer: It is amphipathic, with a hydrophilic head and hydrophobic tails

It is amphipathic, with a hydrophilic head and hydrophobic tails: the polar, phosphate-containing head faces water while the two nonpolar fatty acid tails hide from it, so phospholipids assemble on their own into a bilayer. It is hydrophobic, with a hydrocarbon head and two fatty tails would leave the molecule with no part that interacts with water, so it could not form a bilayer. It is a steroid molecule, with four fused rings and one short tail describes cholesterol, which sits within membranes but is not a phospholipid. It is a triglyceride molecule, with three fatty acids and glycerol describes a fat (triacylglycerol), which has no polar head and stores energy instead of forming membranes.

Correct answer: Phosphofructokinase-1 (PFK-1)

Phosphofructokinase-1 (PFK-1) phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate, the first step that commits sugar to glycolysis, and it is the main control point, inhibited by ATP and citrate and activated by AMP and fructose-2,6-bisphosphate. Hexokinase (HK) catalyzes an earlier irreversible step, but its product, glucose-6-phosphate, can still enter glycogen synthesis or the pentose phosphate pathway, so that step is not committed. Pyruvate kinase (PK) catalyzes the last irreversible step and is regulated, but it is not the rate-limiting, committed step. Fructose-1,6-bisphosphatase (FBPase-1) works in gluconeogenesis, reversing the PFK-1 step, not in glycolysis.

Correct answer: Specific DNA sequences

Specific DNA sequences: in a Southern blot, DNA fragments are separated by gel electrophoresis, transferred to a membrane, and probed with a labeled complementary sequence. Specific RNA transcripts are what a Northern blot detects. Specific protein bands are what a Western blot detects, using antibodies rather than nucleic acid probes. Specific membrane lipids are not the target of any of these standard blots; lipids are usually analyzed by chromatography or mass spectrometry.

Correct answer: FAD (from riboflavin) accepts electrons as a hydrogen carrier in redox reactions

FAD (from riboflavin) accepts electrons as a hydrogen carrier in redox reactions: flavin adenine dinucleotide, made from vitamin B2, takes up two hydrogen atoms (two electrons and two protons) and becomes FADH2 in steps such as the succinate dehydrogenase reaction of the citric acid cycle. PLP (from pyridoxine) transfers carbon dioxide onto acetyl-CoA in fatty acid synthesis gives PLP the job of biotin, the carboxylation coenzyme; PLP (vitamin B6) works in transamination and amino acid decarboxylation. TPP (from thiamine) transfers amino groups to keto acids in transamination gives TPP the job of PLP; TPP (vitamin B1) serves in oxidative decarboxylations such as pyruvate dehydrogenase. THF (from folate) carries acyl units as thioesters in beta-oxidation gives THF the job of coenzyme A, which carries fatty acyl groups through beta-oxidation; tetrahydrofolate actually carries one-carbon units.

Correct answer: Lipids and proteins both diffuse laterally within the membrane

Lipids and proteins both diffuse laterally within the membrane: the fluid mosaic model pictures a fluid phospholipid bilayer in which lipids and embedded proteins move sideways, forming a shifting mosaic of components. Both leaflets hold matching lipids and proteins within the membrane is false because the two leaflets differ in composition, which is part of membrane asymmetry. Individual lipids flip rapidly between leaflets within the membrane is false because moving from one leaflet to the other (flip-flop) is slow and usually needs enzymes such as flippases. Individual lipids stay locked in fixed spots within the membrane describes a rigid, static membrane, the opposite of what the model emphasizes.

Correct answer: Inhibits an early enzyme in the pathway, often allosterically

Inhibits an early enzyme in the pathway, often allosterically: when the end product builds up, it binds a regulatory site on an early, usually committed-step enzyme and slows the whole pathway, saving resources, and the effect reverses when the product level falls. Activates an early enzyme in the pathway, generally covalently describes activation, the opposite of inhibition. Blocks the final enzyme in the pathway, typically competitively targets the wrong step: inhibiting only the last enzyme would let the intermediates pile up. Cleaves the final enzyme in the pathway, typically irreversibly is wrong because feedback inhibition is a reversible binding effect, not destruction of the enzyme.

Correct answer: Prokaryotic ribosomes are 70S, and eukaryotic cytoplasmic ribosomes are 80S

Prokaryotic ribosomes are 70S, and eukaryotic cytoplasmic ribosomes are 80S: the bacterial ribosome (50S + 30S subunits) is smaller than the eukaryotic cytoplasmic ribosome (60S + 40S subunits), a difference that antibiotics such as tetracyclines and aminoglycosides exploit. Prokaryotic ribosomes are 80S, and eukaryotic cytoplasmic ribosomes are 70S reverses the two values. Prokaryotic ribosomes are 70S, and eukaryotic cytoplasmic ribosomes are 70S is wrong because eukaryotic cytoplasmic ribosomes are larger than bacterial ones; only mitochondria and chloroplasts carry bacteria-like 70S ribosomes. Prokaryotic ribosomes are 80S, and eukaryotic cytoplasmic ribosomes are 80S is wrong because bacterial ribosomes are 70S; note that S values are not additive, so 50S + 30S gives 70S, not 80S.

Correct answer: Discontinuous lagging-strand synthesis in the 5' to 3' direction

Discontinuous lagging-strand synthesis in the 5' to 3' direction: DNA polymerase can add nucleotides only to a 3' end, so the strand whose template runs the wrong way relative to fork movement is made in short pieces, each started from a new primer, and DNA ligase later joins them. Continuous leading-strand synthesis in the 5' to 3' direction describes the other strand: leading-strand synthesis runs toward the fork as one unbroken piece, so it produces no fragments. DNA polymerase extending nascent strands in the 3' to 5' direction is wrong because no DNA polymerase builds a strand 3' to 5'. DNA primase assembling short RNA primers in the 3' to 5' direction is wrong because primase, like polymerase, synthesizes its primers 5' to 3'.

Correct answer: Binding of O2 to one subunit raises the affinity of the others

Binding of O2 to one subunit raises the affinity of the others: this positive cooperativity means the first oxygen binds weakly, but each bound oxygen helps shift the tetramer from the low-affinity tense (T) state toward the high-affinity relaxed (R) state, so the curve rises steeply in the middle and gives the S shape. Binding of O2 moves hemoglobin from the R state to the T state reverses the transition; oxygen binding favors the R state. Binding of O2 oxidizes each heme iron to the ferric state is false because the iron stays ferrous (Fe2+) when it carries oxygen; oxidized, ferric methemoglobin cannot bind oxygen at all. Binding of O2 to each of the four hemes proceeds independently describes non-cooperative binding, which would give a hyperbolic curve like that of single-site myoglobin.

Correct answer: Crossing-over of homologous chromosomes during prophase I

Crossing-over of homologous chromosomes during prophase I: paired homologs exchange segments at chiasmata, so each resulting chromosome carries a new mix of maternal and paternal alleles, which is what recombinant chromosome means. Independent assortment of whole chromosomes during metaphase I also creates variation, but it shuffles whole chromosomes into new combinations without changing the allele content of any single chromosome. Release of sister chromatids as new chromosomes in anaphase II only separates sister chromatids that were formed earlier, without exchanging anything between them. Nondisjunction of the duplicated chromosomes in anaphase II is an error that leaves cells with too many or too few chromosomes, not recombinant ones.

Correct answer: Calculate the pH of a buffer system from the ratio of conjugate base to weak acid

Calculate the pH of a buffer system from the ratio of conjugate base to weak acid. The Henderson-Hasselbalch equation, pH = pKa + log([A-]/[HA]), gives the pH of a buffered solution from the pKa of the weak acid and the ratio of conjugate base to acid, and it shows that pH equals pKa when the two concentrations are equal. The other uses belong to other equations: to calculate the free energy change of a reaction from the ratio of products to reactants you add RT ln Q to the standard free energy change; to calculate the equilibrium potential of a single ion from its concentration gradient you use the Nernst equation; and to calculate the concentration of a colored solution from its measured absorbance you use the Beer-Lambert law.

Correct answer: Addition of a 5' 7-methylguanosine cap

Addition of a 5' 7-methylguanosine cap. Capping attaches a modified guanine nucleotide, 7-methylguanosine, to the 5' end of the pre-mRNA through an unusual 5'-to-5' triphosphate link; the cap protects that end from degradation and is recognized during ribosome binding and translation initiation. Addition of a 3' poly-A tail of roughly 200 nucleotides protects the opposite, 3' end. Cleavage at the 5' splice site and cleavage at the 3' splice site are the two cutting steps of splicing, which removes an intron and joins the exons but adds no protective structure to either end.

Correct answer: Has a sigmoidal velocity curve due to cooperative subunits and regulatory sites

Has a sigmoidal velocity curve due to cooperative subunits and regulatory sites. An allosteric enzyme usually has several subunits that bind substrate cooperatively, plus regulatory sites separate from the active site where effectors bind noncovalently and shift the enzyme between less active and more active conformations, so velocity rises along an S-shaped curve as substrate increases. An enzyme that has a hyperbolic velocity curve due to independent subunits and identical sites is behaving like the simple Michaelis-Menten enzyme. The claim that it is controlled mainly by covalent phosphorylation at its active site confuses allostery with covalent modification, a separate form of regulation, and the claim that it is controlled mainly by competitive inhibitors at its active site describes competitive inhibition, whereas allosteric effectors act away from the active site.

Correct answer: DNA to RNA to protein

DNA to RNA to protein. The central dogma describes the usual flow of genetic information: DNA is transcribed into RNA, and RNA is translated into protein. RNA to DNA to protein starts with reverse transcription, a retroviral exception rather than the general rule. DNA to protein to RNA and RNA to protein to DNA both place protein before a nucleic acid, but proteins are never used as templates to build RNA or DNA.

Correct answer: Fructose-1,6-bisphosphatase

Fructose-1,6-bisphosphatase. It hydrolyzes fructose-1,6-bisphosphate to fructose-6-phosphate, bypassing the irreversible phosphofructokinase-1 step, and it is one of the four enzymes unique to gluconeogenesis. Phosphofructokinase-1 is the glycolytic enzyme that this bypass gets around, not the bypass itself. Phosphofructokinase-2 and fructose-2,6-bisphosphatase are the two activities of one bifunctional enzyme that make and break down fructose-2,6-bisphosphate; they set the level of that regulator but catalyze no step of either pathway.

Correct answer: They cleave DNA at specific recognition sequences

They cleave DNA at specific recognition sequences. Restriction endonucleases recognize short, usually palindromic sequences and cut the sugar-phosphate backbone there, leaving sticky or blunt ends, which makes them central to cloning and recombinant DNA work. The statement that they join adjacent DNA fragments at backbone nicks describes DNA ligase; the statement that they unwind DNA at advancing replication forks describes helicase; and the statement that they lengthen linear DNA at chromosome ends describes telomerase.

Correct answer: Ribose-5-phosphate and NADPH

Ribose-5-phosphate and NADPH. The oxidative phase of the pentose phosphate pathway converts glucose-6-phosphate to ribulose-5-phosphate while reducing NADP+ to NADPH, which supplies reducing power for fatty acid and cholesterol synthesis and for antioxidant defense; the pentose is then isomerized to ribose-5-phosphate for nucleotide synthesis. Ribose-5-phosphate and NADH names the wrong cofactor, because this pathway reduces NADP+, not NAD+. Glucose-6-phosphate and NADPH lists the pathway's starting substrate instead of a product, and glucose-6-phosphate and NADH makes both errors.

Correct answer: Insertion or deletion of a number of nucleotides that is not a multiple of three

Insertion or deletion of a number of nucleotides that is not a multiple of three. Codons are read in consecutive triplets, so adding or removing one, two, four or any count not divisible by three shifts the reading frame downstream, changing every later codon and often creating an early stop codon. Insertion or deletion of a number of nucleotides that is a multiple of three adds or removes whole codons and keeps the frame. Substitution or inversion of a number of nucleotides that is not a multiple of three, like substitution or inversion of a number of nucleotides that is a multiple of three, leaves the length of the sequence unchanged, so it can alter codons but cannot shift the frame.

Correct answer: A competitive inhibitor elevates apparent Km and leaves Vmax unchanged

A competitive inhibitor elevates apparent Km and leaves Vmax unchanged. It binds the free enzyme at the active site and competes with substrate, so enough substrate can outcompete it: Vmax is still reached, but more substrate is needed for half-maximal velocity. The statement that an uncompetitive inhibitor elevates apparent Km and leaves Vmax unchanged is false, because an uncompetitive inhibitor binds only the enzyme-substrate complex and lowers both apparent Km and Vmax. The statement that a competitive inhibitor leaves Km unchanged and decreases apparent Vmax describes pure noncompetitive inhibition, and the statement that an uncompetitive inhibitor leaves Km unchanged and decreases apparent Vmax is also wrong, because uncompetitive inhibition lowers Km as well as Vmax.

Correct answer: Fixation of CO2 to ribulose-1,5-bisphosphate

Fixation of CO2 to ribulose-1,5-bisphosphate. RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the first step of the Calvin cycle, attaching carbon dioxide to the five-carbon sugar ribulose-1,5-bisphosphate (RuBP) to form an unstable six-carbon intermediate that splits into two molecules of 3-phosphoglycerate. Reduction of 1,3-bisphosphoglycerate to glyceraldehyde 3-phosphate belongs to the next phase: ATP first phosphorylates 3-phosphoglycerate, and NADPH then reduces the resulting 1,3-bisphosphoglycerate in a step catalyzed by glyceraldehyde 3-phosphate dehydrogenase, not by RuBisCO. Splitting of water by photosystem II and absorption of light by photosystem I are events of the light-dependent reactions in the thylakoid membrane, not of the Calvin cycle in the stroma.

Correct answer: They lower the activation energy and leave the overall free energy change unchanged

They lower the activation energy and leave the overall free energy change unchanged. Enzymes bind and stabilize the transition state and offer an alternative pathway with a smaller energy barrier, which speeds the forward and reverse reactions equally, so the equilibrium position and delta G stay the same and the enzyme is not consumed. The idea that they lower the activation energy and make the overall free energy change more negative is wrong, because a catalyst cannot change the energies of the reactants and products. The idea that they supply energy from ATP and leave the overall free energy change unchanged is wrong because most enzymes use no ATP and do not work by adding energy, and the idea that they supply energy from ATP and make the overall free energy change more negative combines both errors.

Correct answer: Rough ER to Golgi apparatus to secretory vesicles to plasma membrane

Rough ER to Golgi apparatus to secretory vesicles to plasma membrane. Secretory proteins are made by ribosomes on the rough endoplasmic reticulum and enter its lumen, where they fold and begin glycosylation; transport vesicles carry them to the Golgi apparatus for further modification and sorting, and the Golgi packages them into secretory vesicles that fuse with the plasma membrane and release their contents by exocytosis. Rough ER to secretory vesicles to Golgi apparatus to plasma membrane puts the packaging step before the Golgi, which is the wrong order. Smooth ER to Golgi apparatus to secretory vesicles to plasma membrane and smooth ER to secretory vesicles to Golgi apparatus to plasma membrane both start at the smooth ER, which makes lipids and detoxifies drugs but lacks the ribosomes that make secretory proteins.

Correct answer: Nonpolar (hydrophobic) side chains to pack into the core away from water

Nonpolar (hydrophobic) side chains to pack into the core away from water. The hydrophobic effect is the main driving force of globular folding: burying nonpolar side chains frees ordered water molecules and raises the entropy of the surroundings, while polar and charged residues stay on the surface in contact with water. The other three options reverse this pattern. Polar (hydrophilic) side chains to pack into the core away from water would bury groups that hydrogen-bond well with water, and negatively charged (acidic) side chains to pack into the core away from water or positively charged (basic) side chains to pack into the core away from water would bury strongly hydrated ions; such residues usually stay on the surface, where charged ones can also form salt bridges.

Correct answer: It directs RNA polymerase to specific promoter sequences during initiation

It directs RNA polymerase to specific promoter sequences during initiation. In bacteria, sigma factor joins the core RNA polymerase to form the holoenzyme, recognizes the -10 and -35 promoter elements, and positions the enzyme to open the DNA and begin transcription; it is usually released soon after initiation. The claim that it adds new ribonucleotides to the RNA chain during elongation describes the catalytic core enzyme. The claim that it frees the completed RNA from the transcription bubble during termination describes rho factor. The claim that it lays down short RNA primers for new DNA strands during replication describes primase, which works in DNA replication rather than transcription.

Correct answer: Provide alternative fuel for the brain and peripheral tissues when glucose is scarce

Provide alternative fuel for the brain and peripheral tissues when glucose is scarce. During prolonged fasting the liver turns acetyl-CoA from fatty acid oxidation into acetoacetate and beta-hydroxybutyrate, which it cannot use itself; these are exported and oxidized by the brain, heart and skeletal muscle, sparing glucose and muscle protein. Ketone bodies cannot supply carbon for gluconeogenesis in the liver and kidneys when glucose is scarce, because their carbon enters metabolism as acetyl-CoA, which gives no net glucose. The job to carry fatty acids through the blood to muscle and heart cells when glucose is scarce belongs to serum albumin. Nor do they buffer hydrogen ions in the blood and urine when glucose is scarce; they are acids, and their buildup causes ketoacidosis.

Correct answer: Starch uses alpha-glucose units, while cellulose uses beta-glucose units

Both are glucose polymers, and the defining difference is the anomer: "Starch uses alpha-glucose units, while cellulose uses beta-glucose units", so starch has alpha-1,4 links that human enzymes digest and cellulose has beta-1,4 links that they cannot. "Starch forms unbranched strands, while cellulose forms branched networks" is wrong because cellulose is unbranched, while the amylopectin in starch is branched. "Starch contains glucose units, while cellulose contains fructose units" is wrong because cellulose is also made of glucose. "Starch forms rigid straight fibers, while cellulose forms coiled helices" reverses their shapes: amylose coils into a helix, and cellulose forms straight chains that pack into fibers.

Correct answer: Disrupt its tertiary structure

A residue that is positively charged at pH 7.4, placed where a neutral, nonpolar side chain sat, usually puts a charge into the protein's hydrophobic interior, breaking hydrophobic contacts and creating unfavorable charge interactions; that would "Disrupt its tertiary structure" and so impair function. It would not "Tighten its hydrophobic core", because a charged side chain is polar and loosens that packing. It would not "Decrease its isoelectric point"; adding a positive charge raises the pI. "Raise its catalytic efficiency" is not the expected outcome, since substitutions that disturb folding tend to lower activity rather than improve it.

Correct answer: Its activity declines

In feedback inhibition the end product binds the pathway's first committed enzyme, usually at an allosteric site, and inhibits it, so as the end product accumulates "Its activity declines" and the whole pathway slows. "Its activity increases" describes positive feedback (feedback activation), the opposite kind of control. "Its activity stays flat" would mean the end product has no regulatory effect, which contradicts the premise. "Its activity oscillates" is not the direct response; the first enzyme is simply inhibited more as the end-product concentration rises.

Correct answer: Impaired membrane anchoring

Transmembrane segments are held in the lipid bilayer by hydrophobic side chains, so replacing one with a hydrophilic residue makes the segment less compatible with the nonpolar bilayer interior; the most likely result is "Impaired membrane anchoring", which can mislocalize the protein and compromise its function. "Tighter hydrophobic packing" is the opposite of what a polar residue does inside a membrane. "Increased thermal stability" is unlikely, since burying a polar group in a nonpolar environment is destabilizing. "Elevated catalytic activity" does not follow, because the change affects how the protein sits in the membrane, not an active site.

Correct answer: The rate doubles

With substrate in excess the enzyme is saturated, so the rate equals Vmax, and Vmax = kcat x [enzyme]; doubling the enzyme doubles Vmax, so "The rate doubles". "The rate holds steady" confuses substrate saturation with a ceiling on the reaction, but adding enzyme raises Vmax itself. "The rate rises less than twofold" applies the hyperbolic substrate curve to the enzyme, yet the rate is linear in enzyme concentration. "The rate quadruples" would need a second-order dependence on enzyme concentration, which the Michaelis-Menten model does not have.

Correct answer: Serine

Protein kinases transfer a phosphate group to a side-chain hydroxyl, and "Serine" (along with threonine and tyrosine) carries the hydroxyl that is phosphorylated most often. "Asparagine" has an amide side chain; it is the attachment site for N-linked glycosylation, a different modification. "Methionine" has a thioether side chain and "Proline" a nonpolar ring that closes onto the backbone nitrogen, so neither offers a hydroxyl for a kinase to phosphorylate.

Correct answer: Its acidic side chains gain protons, potentially altering the conformation

Aspartate and glutamate side chains carry carboxyl groups with a pKa near 4; as the H+ concentration rises they pick up protons and lose their negative charge, so "Its acidic side chains gain protons, potentially altering the conformation" is correct, because ionic interactions that held the fold can no longer form. "Its acidic side chains become deprotonated, raising the negative charge" reverses the direction; deprotonation happens when the pH rises. "Its acidic side chains bind lysine, strengthening the interior salt bridges" is also backward, because a neutral carboxyl group cannot pair with a lysine ammonium group, so those salt bridges weaken. "Its acidic side chains turn positive, strongly repelling the basic residues" overshoots: a protonated carboxyl group is neutral, not positively charged.

Correct answer: Amino and carboxyl groups

A peptide (amide) bond forms by condensation between the carboxyl group of one amino acid and the amino group of the next, releasing water, so the answer is "Amino and carboxyl groups". "Amino and side-chain groups" is wrong because the side chains (R groups) project from the backbone and take no part in peptide bonds. "Amino and sulfhydryl groups" does not describe a peptide bond; cysteine's sulfhydryl group links to another sulfhydryl group in a disulfide bond. "Amino and ketone groups" react to form an imine (a Schiff base), not an amide.

Correct answer: The point at which extra substrate stops raising the reaction rate

Substrate saturation is the condition in which essentially every active site is occupied, so "The point at which extra substrate stops raising the reaction rate" is correct: the Michaelis-Menten curve has leveled off at Vmax. "The point at which substrate makes the reaction rate half maximal" defines Km, which lies on the rising part of the curve. "The point at which excess substrate starts slowing the reaction rate" describes substrate inhibition, a separate effect seen with some enzymes. "The point at which scarce substrate starts slowing the reaction rate" describes substrate depletion, the opposite situation, when too little substrate is left.

Correct answer: Denaturation

"Denaturation" is the loss of a protein's secondary, tertiary and quaternary structure, usually caused by heat, extremes of pH or chaotropic agents, which leaves an unfolded, random-coil chain whose peptide bonds remain intact. "Precipitation" is a protein coming out of solution, which can happen to a fully folded protein, as in salting out. "Proteolysis" cleaves peptide bonds, destroying the primary structure rather than unfolding the chain. "Isomerization", such as cis-trans isomerization at a proline peptide bond, changes the geometry at one bond rather than unfolding the whole protein.

Correct answer: It will be truncated

A codon changed into a stop codon (a nonsense mutation) makes the ribosome release the chain when it reaches that codon, so "It will be truncated": the polypeptide ends early and is shorter than intended. "It will be elongated" describes the loss of a stop codon, not the gain of one. "It will be unchanged" describes a silent mutation, in which the new codon still specifies the same amino acid. "It will be scrambled" describes a frameshift from an insertion or deletion, which garbles every codon downstream; a new stop codon leaves the sequence before it correct.

Correct answer: Disrupted hydrogen bonding between the protein and water

At high salt concentration the ions bind water so strongly that too little is left to hydrate the protein; the hydrogen bonds between water and the protein surface are lost, protein molecules associate through exposed hydrophobic patches, and the protein precipitates (salting out). That makes "Disrupted hydrogen bonding between the protein and water" the cause. "Enhanced repulsion between the protein and its neighbors" is backward: the added ions screen surface charges and weaken the repulsion between protein molecules. "Stronger ionic bonding between the protein and salt ions" describes salting in, which raises solubility at low salt concentration rather than causing precipitation. "Covalent cross-linking between the protein and salt ions" does not occur; salt ions interact with proteins only noncovalently, which is why salting out can be reversed.

Correct answer: It offers an alternative pathway with reduced activation energy

A catalyst speeds a reaction because "It offers an alternative pathway with reduced activation energy", so a larger fraction of molecular collisions have enough energy to react. "It establishes an altered equilibrium with larger product yield" is a common misconception: a catalyst speeds the forward and reverse reactions equally, so the equilibrium position and the yield are unchanged. "It produces a smaller enthalpy change for the overall reaction" is wrong because the enthalpy change depends only on the reactants and products, not on the path between them. "It is consumed in the reaction and becomes part of the products" is also wrong; a catalyst may take part in intermediate steps, but it is regenerated unchanged at the end.

Correct answer: Replacing a hydrophobic residue with a hydrophilic one

A substitution that changes the chemical character of a side chain is the one most likely to weaken binding, so the answer is "Replacing a hydrophobic residue with a hydrophilic one": it can disturb the local fold and remove contacts that the binding surface relies on. "Replacing a basic residue with a similarly charged one" (for example lysine to arginine) keeps the positive charge that pairs with the negatively charged DNA backbone. "Replacing a nonpolar residue with an aliphatic one" (for example alanine to valine) and "Replacing a polar residue with a hydroxyl-carrying one" (for example asparagine to serine) are conservative swaps that keep the side chain's character, so they seldom change binding affinity much.

Correct answer: By giving bonding electrons to the more electronegative partner

Oxidation states come from electron bookkeeping: each bonding pair is credited entirely to the more electronegative atom, and the result is compared with the free atom's valence electrons, so the method is "By giving bonding electrons to the more electronegative partner". "By giving shared electrons to the more electropositive partner" reverses the rule and would flip the sign of every oxidation state. "By dividing shared electrons evenly between the bonded partners" is the rule for formal charge, not oxidation state. "By counting the electrons lost or gained by the redox partners" tracks the change in oxidation state during a redox step, not the oxidation state an atom holds in a compound.

Correct answer: Nonsense mutation

A single-nucleotide change that turns a codon into a premature stop codon is a "Nonsense mutation"; translation ends early and the enzyme is truncated and usually nonfunctional. A "Synonymous mutation" (a silent mutation) changes a codon to another one for the same amino acid. A "Nonstop mutation" is the reverse case, a stop codon changed into a sense codon so that translation runs past the normal end. A "Frameshift mutation" comes from inserting or deleting a number of nucleotides that is not a multiple of three, which shifts the reading frame instead of substituting one base.

Correct answer: Hydrogen bonds between backbone atoms

Alpha-helices and beta-sheets are secondary structures held together by "Hydrogen bonds between backbone atoms": the carbonyl oxygen of one peptide group bonds to the amide hydrogen of another. "Hydrogen bonds between side chains" help stabilize tertiary structure but do not define helices and sheets. "Ionic bonds between two side chains" (salt bridges) and "Disulfide bonds between two cysteines" also stabilize tertiary and quaternary structure rather than forming secondary structure.

Correct answer: They form micelles with hydrophobic tails in and hydrophilic heads out

In water, amphipathic molecules hide their nonpolar parts from the solvent: "They form micelles with hydrophobic tails in and hydrophilic heads out", so only the polar heads touch the water. "They gather into clusters with charged heads in and nonpolar chains out" describes a reverse (inverted) micelle, which forms in a nonpolar solvent, not in water. "They make hydrogen bonds to the water along their hydrocarbon backbones" is impossible, because C-H bonds are nonpolar and cannot hydrogen-bond with water. "They clump into large solid particles that precipitate from the water" ignores the polar heads, which keep the micelles dispersed in solution.

Correct answer: RNA has uracil, and DNA has thymine

The two nucleic acids differ in one of their bases: "RNA has uracil, and DNA has thymine", which is why adenine pairs with uracil in RNA but with thymine in DNA. "RNA has deoxyribose, and DNA has ribose" reverses the sugars; RNA contains ribose, with a 2'-hydroxyl group, and DNA contains deoxyribose. "RNA has paired strands, and DNA has one" reverses the usual strandedness, since DNA is normally a double helix and most RNA is single-stranded. "RNA has hexoses, and DNA has pentoses" is wrong because ribose and deoxyribose are both five-carbon sugars (pentoses).

Correct answer: +1.95 V

The cell potential is the cathode's reduction potential minus the anode's: E(cell) = E(cathode) - E(anode) = +1.10 V - (-0.85 V) = "+1.95 V". "+0.25 V" comes from adding the two reduction potentials, 1.10 + (-0.85), instead of subtracting the anode's. "-1.95 V" reverses the subtraction, E(anode) - E(cathode) = -0.85 - 1.10. "-0.25 V" subtracts in the reverse order after also dropping the anode's negative sign, 0.85 - 1.10.

Correct answer: Forward and reverse rates are equal

Equilibrium is dynamic: both reactions keep running, and "Forward and reverse rates are equal", so the concentrations stop changing. "Reactant and product amounts are equal" is a common misconception; the amounts settle wherever the equilibrium constant puts them, which is rarely at equal values. "Forward and reverse rate constants match" confuses rates with rate constants; the ratio of the two rate constants equals K, which is 1 only in a special case. "Reactant and product enthalpies coincide" would mean the enthalpy change is zero, which is not a condition for equilibrium.

Correct answer: 1.6

HCl is a strong acid that dissociates completely, so [H+] = 0.025 M and pH = -log(0.025) = 1.60, which rounds to "1.6". "1.3" is -log(0.05), the result of doubling [H+] as if each HCl released two protons. "2.4" comes from a sign slip in the logarithm, 2 + log 2.5 instead of 2 - log 2.5. "3.7" comes from using the natural logarithm, -ln(0.025), instead of the base-10 logarithm.

Correct answer: Raising the partial pressure of the gas

By Henry's law the solubility of a gas is proportional to its partial pressure above the liquid, so "Raising the partial pressure of the gas" increases it. "Raising the temperature of the liquid" does the opposite, because gases become less soluble as temperature rises. "Adding a dissolved salt to the solution" lowers gas solubility through the salting-out effect. "Adding a metal catalyst to the solution" could at most speed up how fast equilibrium is reached; it does not change how much gas can dissolve.

Correct answer: 4.76

With equal concentrations of acetic acid and acetate, the Henderson-Hasselbalch equation gives pH = pKa + log(1) = pKa, and the pKa of acetic acid is about 4.76, so the pH is "4.76". "2.88" is the pH of 0.1 M acetic acid on its own, which ignores the acetate. "7.00" assumes that equal amounts of an acid and its conjugate base give a neutral solution. "9.24" is 14 - 4.76, the pKb of acetate (equivalently the buffer's pOH), not its pH.

Correct answer: Oxidation

In a galvanic cell the anode is where electrons are lost, so it is the site of "Oxidation"; the released electrons flow through the external circuit to the cathode. "Reduction", the gain of electrons, takes place at the cathode. "Diffusion" describes how ions move through the solution and the salt bridge, not the reaction at an electrode. "Catalysis" does not define either electrode; the anode is identified by the oxidation half-reaction that occurs there.

Correct answer: The spatial orientation of the orbital

The magnetic quantum number m_l gives "The spatial orientation of the orbital", with allowed values from -l to +l (the three p orbitals, for example, point along three different axes). "The characteristic shape of the orbital" is set by the angular momentum quantum number l. "The main energy level of the electron" is set by the principal quantum number n. "The magnetic spin state of the electron" is described by the spin quantum number m_s, not by m_l.

Correct answer: Fluorine

Electronegativity increases across a period and decreases down a group, and "Fluorine" (about 4.0 on the Pauling scale) has the highest value of any element. "Oxygen" (about 3.4) is close behind but still lower. "Chlorine" (about 3.2) sits one period below fluorine in the halogens, so it is less electronegative. "Nitrogen" (about 3.0) lies further left in the same period and is lower still.

Correct answer: CO2

In "CO2" each oxygen forms a double bond to carbon and keeps two lone pairs, so carbon (4 valence electrons, 4 bonds) and both oxygens (6 valence electrons: 4 nonbonding plus half of 4 bonding) all have a formal charge of zero. "O3" cannot avoid formal charges: the central oxygen is +1 and the singly bonded end oxygen is -1. "NO3-" and "NH4+" are ions, and the formal charges in a Lewis structure must add up to the ion's charge, so at least one atom carries a charge; nitrogen is +1 in both.

Correct answer: sp^3d^2

Xenon in XeF4 has four bonding pairs and two lone pairs, six electron domains in all, and six domains call for "sp^3d^2" hybridization (octahedral electron geometry, square planar molecular shape). "sp^3" counts only the four bonds and ignores the two lone pairs. "sp^3d^3" corresponds to seven electron domains, as in IF7. "sp" corresponds to just two electron domains, as in a linear molecule such as BeCl2.

Correct answer: +1.10 V

The standard cell potential is E(cell) = E(cathode) - E(anode) = +0.34 V - (-0.76 V) = "+1.10 V". "-0.42 V" comes from adding the two reduction potentials, 0.34 + (-0.76). "+0.34 V" reports only the cathode's reduction potential. "+2.20 V" doubles the answer as if a cell potential scaled with the two electrons transferred, but E(cell) is an intensive property and does not depend on how many electrons the balanced equation shows.

Correct answer: Wavelength is inversely proportional to energy

A photon's energy is E = hc/lambda, so "Wavelength is inversely proportional to energy": doubling the wavelength halves the energy. "Wavelength is in direct proportion to energy" reverses that relationship. "Photon energy tracks intensity, not wavelength" is the classical-wave idea that the photoelectric effect disproved; intensity sets how many photons arrive, not the energy of each one. "Photon energy follows the square of wavelength" has no basis, because the dependence is on 1/lambda, not on lambda squared.

Correct answer: Vaporization

"Vaporization" turns a liquid into a gas at constant temperature, and the heat absorbed goes into breaking the intermolecular hydrogen bonds that hold the liquid together. "Freezing" and "Deposition" both form a solid, and "Condensation" turns a gas back into a liquid; all three form intermolecular attractions and release heat instead of breaking hydrogen bonds.

Correct answer: Evaporation

"Evaporation" absorbs heat to overcome the attractions between liquid molecules, so it is endothermic, which is why evaporating sweat cools the skin. "Combustion" releases heat. "Condensation" is the reverse of evaporation and releases the same amount of heat. "Fermentation" of sugar is also exothermic overall; fermentation tanks warm up and often need cooling.

Correct answer: pH and pKa

The Henderson-Hasselbalch equation, pH = pKa + log([A-]/[HA]), links a buffer's pH to the acid's pKa through the ratio of conjugate base to acid, so the pair is "pH and pKa". "pOH and pH" are linked by pH + pOH = 14 at 25 degrees C, a relation that comes from water's ionization. "Ka and Kb" and "Kb and Kw" belong to the conjugate-pair relation Ka x Kb = Kw, not to the buffer equation.

Correct answer: Increased solubility

Colligative properties depend only on the number of dissolved solute particles; they are "Vapor-pressure lowering", "Boiling-point elevation", freezing-point depression and "Osmotic pressure". "Increased solubility" is not one of them, because how much of a substance dissolves depends on the identities of the solute and solvent and on temperature, not simply on how many particles are already dissolved.

Correct answer: At equilibrium, the products are favored over the reactants.

An equilibrium constant greater than 1 means that at equilibrium the product concentrations, each raised to its coefficient, outweigh the reactant concentrations, so "At equilibrium, the products are favored over the reactants." The reverse statement, "At equilibrium, the reactants are favored over the products.", describes an equilibrium constant less than 1. The standard free energy change equals -RT ln K, so a constant above 1 makes it negative, and "At equilibrium, the standard free energy change is positive." is wrong. "At equilibrium, the standard free energy change equals zero." confuses the standard value, which is fixed for the reaction, with the actual free energy change, which is zero at equilibrium; the standard value is zero only when K equals 1.

Correct answer: Le Chatelier's Principle

Le Chatelier's Principle states that when a system at equilibrium is disturbed by a change in concentration, pressure or temperature, the position of equilibrium shifts in the direction that partly offsets the change. Pascal's Principle says that pressure applied to an enclosed fluid is transmitted undiminished to every part of the fluid, and Bernoulli's Principle relates the pressure in a moving fluid to its flow speed; both describe fluids, not chemical equilibria. The Pauli Exclusion Principle states that no two electrons in an atom can have the same four quantum numbers, which governs electron configurations rather than the position of an equilibrium.

Correct answer: The potential of a cell under non-standard conditions

The Nernst equation corrects the standard cell potential for the actual concentrations, through the reaction quotient, and for temperature, so it gives "The potential of a cell under non-standard conditions". It contains no rate constant or activation energy, so it says nothing about "The rate of a reaction under non-standard conditions", which is a kinetic property. "The current of a circuit under non-standard conditions" also depends on the resistance of the circuit (Ohm's law), and "The resistance of a wire under non-standard conditions" depends on the wire's material, length, cross-sectional area and temperature; the Nernst equation calculates neither.

Correct answer: 1 x 10^6 eV

A charge accelerated from rest through a potential difference V gains kinetic energy KE = qV. The proton's charge is +e, so 1 MV gives it 1 MeV, which is 1 x 10^6 eV; the particle's mass does not enter. 5 x 10^5 eV wrongly brings the factor of one-half from (1/2)mv^2 into qV. 1.6 x 10^-13 eV is the energy in joules, (1.6 x 10^-19 C)(10^6 V), labeled as electron volts. 5.4 x 10^2 eV divides by the proton-to-electron mass ratio of about 1836, but any particle carrying charge e gains the same 1 eV for every volt.

Correct answer: rho L / (2 pi epsilon_0 d)

For a long, uniformly charged rod, Gauss's law applied to a coaxial cylinder of radius d and length l gives E(2 pi d l) = lambda l / epsilon_0, where lambda is the charge per unit length, so the field outside the rod is lambda / (2 pi epsilon_0 d): it falls off as 1/d, with 2 pi epsilon_0 in the denominator. The answer has this form, rho L / (2 pi epsilon_0 d). "rho L / (4 pi epsilon_0 d)" borrows the 4 pi of Coulomb's point-charge law, but the flux from a line of charge passes through a cylindrical surface of area 2 pi d l. "rho L / (2 pi epsilon_0 r)" evaluates the field at the rod's own surface, using its radius r instead of the distance d to point P. "rho L / (4 pi epsilon_0 r)" makes both mistakes.

Correct answer: A concave mirror with a focal length of 8 cm

A convex mirror always forms an image that is smaller than the object, upright and virtual, so it cannot produce an image three times the size of the object; that rules out "A convex mirror with a focal length of -8 cm" and "A convex mirror with a focal length of -3 cm". Only a concave mirror magnifies, which points to "A concave mirror with a focal length of 8 cm". The focal length comes from the magnification, m = -di/do, which fixes the image distance at three times the 12 cm object distance, together with the mirror equation, 1/f = 1/do + 1/di. "A concave mirror with a focal length of 3 cm" inverts the magnification, placing the image at one-third of the object distance, which would give an image smaller than the object.

Correct answer: v = sqrt(2gh)

With no friction, all of the potential energy mgh becomes kinetic energy (1/2)mv^2; the mass cancels, giving v = sqrt(2gh) whatever the angle. "v = 2sqrt(gh)" pulls the 2 out of the square root, but sqrt(2gh) equals sqrt(2) times sqrt(gh), not 2 times sqrt(gh). "v = sqrt(2gh sin theta)" lets the angle change the final speed, but the acceleration along the slope, g sin theta, acts over a slope length of h/sin theta, so the angle cancels and only sets how long the trip takes. "v = 2sqrt(gh sin theta)" makes both mistakes.

Correct answer: a = omega^2 r

Centripetal acceleration is v^2/r, and in circular motion v = omega r, so a = (omega r)^2/r, which simplifies to a = omega^2 r. "a = omega r" is the particle's linear speed, not its acceleration, and has units of meters per second. "a = r / omega" and "a = r / omega^2" turn the relationships upside down: a radius divided by an angular velocity, or by its square, has units of meter-seconds or meter-seconds squared, not the meters per second squared that an acceleration needs.

Correct answer: It cannot be found, because the work function is unknown.

Einstein's photoelectric equation gives eV = hf - W, where W is the work function of the metal. Doubling the frequency doubles the photon energy hf, but W stays the same, so the new stopping potential equals 4 V plus W/e. Because W is not given, "It cannot be found, because the work function is unknown." "It doubles from 2 V to 4 V, because the frequency doubles." ignores the work function, which makes the stopping potential grow by more than a factor of two. "It halves from 2 V to 1 V, because the wavelength shrinks." treats a shorter wavelength as lower energy, when it means higher-energy photons. "It cannot be larger, because the photon flux is reduced." confuses the number of photons, which sets the photocurrent, with the energy of each photon, which sets the stopping potential.

Correct answer: A circular path

The magnetic force on a moving charge is always perpendicular to its velocity, so it changes the direction of motion but not the speed; a force of constant size at right angles to the motion acts as a centripetal force and gives "A circular path". "A helical path" needs a velocity component along the field lines, which this particle does not have. "A parabolic path" is produced by a uniform electric field, whose force keeps one fixed direction. "A cycloidal path" arises only when an electric field acts together with the magnetic field.

Correct answer: 240 V

For an ideal transformer the voltages are in the ratio of the turns, Vs/Vp = Ns/Np, so Vs = 120 V x (200/100) = 240 V. The value 60 V inverts the turns ratio, using Np/Ns, which is how the current rather than the voltage scales. The values 80 V and 360 V both bring in the total of 300 turns: 80 V uses 200/300 and 360 V uses 300/100, but only the ratio of secondary turns to primary turns sets the voltage.

Correct answer: 2 sqrt(2) m

The speed of a wave on a string is v = sqrt(T/mu), so doubling the tension multiplies the speed by sqrt(2). The source keeps the frequency at 500 Hz, so from v = f lambda the wavelength grows by the same factor: 2 m x sqrt(2) = 2 sqrt(2) m. The value 2/sqrt(2) m divides by sqrt(2), as if a faster wave had a shorter wavelength at the same frequency. The value 4 m doubles the wavelength, as if it were proportional to the tension itself rather than to its square root, and 1 m halves it, making that mistake and also reversing the direction of the change.

Correct answer: It increases by a factor of 2.

For a charge moving perpendicular to a uniform magnetic field, the magnetic force supplies the centripetal force, qvB = mv^2/r, so r = mv/(qB). The radius is directly proportional to the speed, so doubling the speed means "It increases by a factor of 2." "It increases by a factor of 4." would follow only if the radius depended on the square of the speed, as kinetic energy does. "It decreases by a factor of 2." and "It decreases by a factor of 4." treat the radius as falling with speed, but a faster electron is harder to turn and sweeps out a wider circle.

Correct answer: It circles at a constant speed.

The magnetic force qv x B is always perpendicular to the velocity, so it does no work: it bends the path into a circle but cannot change the particle's kinetic energy. The motion is therefore "It circles at a constant speed." "It circles at an increasing speed." assumes the magnetic force does work on the particle, which a force at right angles to the motion cannot do. "It spirals at a constant speed." would need the radius to change, but at constant speed in a uniform field the radius r = mv/(qB) stays fixed. "It spirals at an increasing speed." makes both mistakes.

Correct answer: The capacitance doubles, and so does the stored energy.

For a parallel-plate capacitor C = k epsilon_0 A/d, so halving the separation d doubles the capacitance. With the voltage held constant, the stored energy U = (1/2)CV^2 is proportional to C, so it doubles as well: "The capacitance doubles, and so does the stored energy." "The capacitance doubles, while the stored energy halves." applies U = Q^2/(2C), which assumes a fixed charge; here the source holds the voltage fixed and supplies extra charge. "The capacitance halves, and so does the stored energy." and "The capacitance halves, while the stored energy doubles." both treat capacitance as proportional to the plate separation, when it is inversely proportional to it.

Correct answer: 2 m

Wave speed, frequency and wavelength are related by v = f lambda, so lambda = v/f = (340 m/s)/(170 Hz) = 2 m. The value 1 m comes from v = 2fL, the relation for the fundamental mode of a string fixed at both ends, which spans half a wavelength rather than a whole one. The value 0.5 m inverts the ratio, computing f/v = 170/340, a quantity with units of 1/m rather than meters. The value 12.6 m multiplies by 2 pi, treating 170 Hz as an angular frequency; no factor of 2 pi belongs in lambda = v/f when f is in hertz.

Correct answer: The fringes spread farther apart.

In Young's experiment the fringe spacing is lambda D/d, where d is the slit separation and D the distance to the screen, so reducing d with lambda and D fixed makes the spacing larger: "The fringes spread farther apart." "The fringes crowd closer together." is what happens when the slits are moved farther apart. "The fringes keep the same spacing." ignores the 1/d dependence. "The fringes shift toward one side." would need an extra path difference between the two slits; changing their separation symmetrically leaves the central bright fringe on the axis.

Correct answer: 6.0 x 10^14 Hz

Frequency and wavelength are related by c = f lambda, so f = c/lambda = (3.0 x 10^8 m/s)/(500 x 10^-9 m) = 6.0 x 10^14 Hz. Dividing 3.0 by 5 gives 0.6 x 10^15; moving the decimal point without lowering the exponent produces 6.0 x 10^15 Hz. The value 1.7 x 10^15 Hz divides 5 by 3 instead of 3 by 5, and 1.7 x 10^14 Hz also lowers the exponent, which only a quotient below 1 requires.

Correct answer: The quantum number increases by a factor of 2.

In the given expression, E_n = n^2 h^2/(8mL^2), the energy is proportional to n^2, so n is proportional to the square root of the energy, and quadrupling the energy multiplies n by sqrt(4) = 2: "The quantum number increases by a factor of 2." "The quantum number increases by a factor of 4." treats the energy as proportional to n rather than to n^2. "The quantum number decreases by a factor of 2." and "The quantum number decreases by a factor of 4." reverse the relationship; higher energy levels have larger quantum numbers.

Correct answer: Diffraction

Diffraction is the spreading of a wave as it passes through an opening or around an edge; a slit comparable in width to the wavelength turns a plane wave front into circular wave fronts, whatever the angle of approach. Refraction is the change in direction when a wave passes into a medium where its speed is different, and no change of medium occurs here. Interference is the superposition of two or more waves, as behind a pair of slits, not the spreading from a single opening. Dispersion is the separation of a wave into its component frequencies because its speed depends on frequency, as when a prism spreads white light into colors; it does not reshape a wave front at an opening.

Correct answer: Dienes reacting with dienophiles to give cyclohexene rings.

The Diels-Alder reaction is a [4+2] cycloaddition in which a conjugated diene and a dienophile form two new carbon-carbon sigma bonds in a single step, so it is best represented by "Dienes reacting with dienophiles to give cyclohexene rings." "Dienes reacting with hydrogen bromide to give allylic bromides." describes electrophilic 1,2- or 1,4-addition to a conjugated diene, not a cycloaddition. "Alkenes reacting with other alkenes to give cyclobutane rings." describes [2+2] cycloaddition, which needs light and builds a four-membered ring rather than a six-membered one. "Alkenes reacting with ozone to give smaller carbonyl compounds." describes ozonolysis, which cleaves the double bond instead of forming a ring.

Correct answer: Benzene

Benzene is aromatic: it is cyclic, planar and fully conjugated, with 6 pi electrons, which fits Hückel's 4n + 2 rule with n = 1. Cyclobutadiene is cyclic, planar and conjugated but has 4 pi electrons (4n), which makes it antiaromatic. Cyclopentadiene and cycloheptatriene each contain an sp3-hybridized CH2 carbon that breaks the ring of overlapping p orbitals, so neither is aromatic; only their ions, the cyclopentadienyl anion and the cycloheptatrienyl (tropylium) cation, are aromatic.

Correct answer: Cis-trans isomerism

Compounds with the same molecular and structural formulas that differ only in how groups are arranged on either side of a double bond, which cannot rotate freely, show cis-trans isomerism, also called geometric isomerism; so the answer is "Cis-trans isomerism". "Conformational isomerism" involves arrangements that interconvert by rotation about single bonds, so it does not depend on a double bond. "Optical isomerism" describes non-superimposable mirror images that arise from chirality, usually a stereocenter, not from restricted rotation. "Constitutional isomerism" requires a different connectivity, which identical structural formulas rule out.

Correct answer: SN2 mechanism

Thionyl chloride first converts the alcohol's OH group into a chlorosulfite, a good leaving group; chloride ion then attacks the carbon from the back side in one concerted step, displacing it with inversion of configuration, which is an SN2 mechanism (the textbook course when pyridine is present). An SN1 mechanism would require a free carbocation, which would scramble the stereochemistry, and an E1 mechanism and an E2 mechanism are eliminations that would give an alkene rather than an alkyl chloride.

Correct answer: Favors one enantiomer over the other.

An enantioselective reaction produces more of one of two mirror-image products, giving an enantiomeric excess, so it "Favors one enantiomer over the other." A reaction that "Gives both enantiomers in equal amounts." produces a racemic mixture and is not enantioselective. A reaction that "Favors one diastereomer over the other." is diastereoselective, and one that "Gives both diastereomers in equal amounts." shows no diastereoselectivity; diastereomers are stereoisomers that are not mirror images, so neither describes enantioselectivity.

Correct answer: It is coupled to two neighboring protons.

By the n + 1 rule, a signal split into n + 1 peaks comes from a proton with n equivalent neighboring protons, so a triplet (three peaks) means "It is coupled to two neighboring protons." "It is coupled to three neighboring protons." takes the number of peaks as the number of neighbors; three neighbors would give a quartet. "It is coupled to four neighboring protons." adds 1 to the number of peaks instead of subtracting it; four neighbors would give a quintet. "It is coupled to no neighboring protons." describes a singlet.

Correct answer: A carbon-metal bond

A Grignard reagent, RMgX, contains a polar covalent bond between carbon and magnesium, so its defining feature is "A carbon-metal bond"; the carbon carries a partial negative charge and acts as a strong nucleophile and base. "A carbon-halogen bond" belongs to the alkyl or aryl halide the reagent is made from; inserting magnesium into that bond is what creates the Grignard reagent. "A carbon-oxygen bond" is found in the ethers used as its solvent and in the carbonyl compounds it attacks, not in the reagent itself. "A carbon-silicon bond" defines organosilanes; silicon is a metalloid, and an organosilane is not a Grignard reagent.

Correct answer: A transition state resembles the stable species closest to it in energy.

The Hammond postulate states that a transition state is closest in structure to whichever species, reactant, intermediate or product, lies closest to it in energy, so "A transition state resembles the stable species closest to it in energy." An exothermic step therefore has an early, reactant-like transition state, and an endothermic step a late, product-like one. "A transition state resembles the most stable species in the reaction." reverses this relationship, since the transition state resembles the higher-energy partner of its step. "A transition state is an intermediate that can be isolated in a flask." and "A transition state is an intermediate that rests in a local energy minimum." confuse a transition state with an intermediate: a transition state is a fleeting energy maximum that cannot be isolated.

Correct answer: Shield one site of a molecule against later reactions.

A protecting group is attached to one functional group so that it will not react while other parts of the molecule are transformed, and it is removed afterward; its job is to "Shield one site of a molecule against later reactions." A group used to "Activate one site of a molecule toward later reactions." does the opposite of protecting it. Turning an alcohol into a tosylate is how chemists "Convert one site of a molecule into a leaving group.", which makes that site more reactive, not less. A bulky anchoring group, such as a tert-butyl group on a cyclohexane ring, is used to "Lock one site of a molecule into a single conformation."; it controls shape rather than blocking a reaction.

Correct answer: An alkene is formed.

In the Wittig reaction a phosphonium ylide adds to an aldehyde or ketone, and the oxaphosphetane intermediate breaks apart into triphenylphosphine oxide and a new carbon-carbon double bond: "An alkene is formed." "An alcohol is formed." when a Grignard or organolithium reagent adds to a carbonyl group. "An epoxide is formed." when a sulfur ylide is used instead (the Corey-Chaykovsky reaction). "An imine is formed." when a primary amine condenses with an aldehyde or ketone.

Correct answer: Glyceraldehyde-3-phosphate dehydrogenase

In the sixth step of glycolysis, glyceraldehyde-3-phosphate is oxidized and phosphorylated, using NAD+ and inorganic phosphate, to 1,3-bisphosphoglycerate; the enzyme is glyceraldehyde-3-phosphate dehydrogenase. Glucose-6-phosphate dehydrogenase catalyzes the first step of the pentose phosphate pathway, producing NADPH. The alpha-ketoglutarate dehydrogenase complex works in the citric acid cycle, and the pyruvate dehydrogenase complex converts pyruvate to acetyl-CoA after glycolysis; neither acts on a three-carbon sugar phosphate.

Correct answer: DNA helicase

DNA helicase uses ATP to break the hydrogen bonds between base pairs, unwinding the double helix ahead of the replication fork so that each strand can serve as a template. DNA gyrase, a topoisomerase, relieves the supercoiling that builds up ahead of the fork but does not separate the strands. DNA primase lays down the short RNA primers that DNA polymerase extends, and DNA polymerase synthesizes the new strands; neither unwinds the helix.

Correct answer: The maximum velocity (Vmax) and the Michaelis constant (Km)

A Lineweaver-Burk plot graphs 1/v against 1/[S] for an enzyme-catalyzed reaction; the y-intercept is 1/Vmax and the x-intercept is -1/Km, so it yields "The maximum velocity (Vmax) and the Michaelis constant (Km)". "The activation energy (Ea) and the Arrhenius constant (A)" come from an Arrhenius plot of ln k against 1/T. "The Hill coefficient (nH) and the dissociation constant (Kd)" come from a Hill plot, which measures cooperative binding. "The overall reaction order (n) and the rate constant (k)" come from integrated rate-law plots of concentration against time.

Correct answer: Using ATP hydrolysis, it moves sodium out of the cell and potassium into it.

The sodium-potassium pump is an ATPase: each cycle hydrolyzes one ATP and moves three Na+ ions out of the cell and two K+ ions in, against both concentration gradients, so "Using ATP hydrolysis, it moves sodium out of the cell and potassium into it." "Using ATP hydrolysis, it moves potassium out of the cell and sodium into it." reverses both directions; the pump keeps sodium low and potassium high inside the cell. "Using a proton gradient, it moves sodium out of the cell and potassium into it." describes secondary active transport, but this pump is driven directly by ATP, and "Using a proton gradient, it moves potassium out of the cell and sodium into it." makes both mistakes.

Correct answer: Catalyze reactions in place of protein enzymes.

Ribozymes are RNA molecules with catalytic activity, such as the peptidyl transferase center of the ribosome and self-splicing introns, so they "Catalyze reactions in place of protein enzymes." Molecules that "Deliver activated amino acids to the ribosomes." are transfer RNAs, which act as adapters rather than catalysts. Molecules that "Carry coded instructions from DNA to ribosomes." are messenger RNAs, which serve as templates. RNA genomes, such as those of RNA viruses, "Store genetic information in place of DNA."; storing information is not what defines a ribozyme.

Correct answer: Nucleus

In eukaryotic cells the chromosomal DNA is enclosed in the nucleus, where RNA polymerases transcribe genes into pre-mRNA that is processed before it is exported through the nuclear pores. The cytosol is where most mRNA is translated, and transcription takes place there only in prokaryotes, which lack a nucleus. A ribosome carries out translation, not transcription, and a vacuole stores water, ions and wastes; neither holds the chromosomes that are transcribed.

Correct answer: Non-covalent forces such as hydrogen bonds, salt bridges, and hydrophobic interactions

Tertiary structure, the overall three-dimensional fold of a single polypeptide, is held mainly by interactions among side chains: "Non-covalent forces such as hydrogen bonds, salt bridges, and hydrophobic interactions", with disulfide bonds adding covalent cross-links in some proteins. "Backbone hydrogen bonds such as those in alpha helices, beta sheets, and reverse turns" stabilize secondary structure. "Covalent peptide bonds, which join the amino acids in order along the protein backbone" make up the primary structure. "Bonds between separate protein subunits, which hold two or more polypeptides together" describe quaternary structure, which requires more than one polypeptide.

Correct answer: Transcription

Transcription is the process in which RNA polymerase reads a DNA template strand and builds a complementary RNA, such as messenger RNA. Translation uses mRNA to build a polypeptide at the ribosome; DNA replication copies DNA into new DNA before cell division; and RNA splicing removes introns from a pre-mRNA that has already been transcribed, so none of them makes mRNA from DNA.

Correct answer: Active transport

Active transport is correct because it moves ions or molecules across a membrane against their concentration gradient, which requires an input of energy, usually from ATP hydrolysis by a pump such as the sodium-potassium ATPase. Osmosis is the diffusion of water across a selectively permeable membrane toward the side with the higher solute concentration, a passive process that uses no metabolic energy. Chemiosmosis is the flow of hydrogen ions down their gradient through ATP synthase, which releases energy to make ATP rather than spending energy to move a substance uphill. Facilitated diffusion uses channel or carrier proteins to move ions and polar molecules across the membrane, but only down their concentration gradient and with no energy input, so the help of a membrane protein does not make it active transport.

Correct answer: 30 m/s

30 m/s. An object falling freely from rest gains speed at the rate g, so v = g t = (10 m/s^2)(3.0 s) = 30 m/s. The 2.0 kg mass does not matter, because every freely falling object has the same acceleration. 45 m/s is the number for the distance fallen, one-half g t^2 = 45 m, reported as if it were a speed. 60 m/s comes from also multiplying by the 2.0 kg mass. 90 m/s comes from squaring the time, g t^2, as if speed grew with the square of time.

Correct answer: Work performed by the gas

Work performed by the gas. Pressure-volume work equals pressure times the change in volume, and at constant volume that change is zero, so no work is done. By the first law, all the heat absorbed by the gas then goes into internal energy, so that heat is not zero. The change in pressure of the gas is positive, because at fixed volume pressure rises in proportion to absolute temperature, and the change in entropy of the gas is positive too, since heat flows into the gas as it warms.

Correct answer: +6

+6. Oxygen is assigned -2, so the four oxygens total -8; because the whole ion carries a charge of -2, sulfur must be +6, since +6 plus -8 gives -2. The value +4 is sulfur's state in sulfite, SO3 2-, which has one oxygen fewer. The value -2 is sulfur's state in sulfide, and it repeats the ion's overall charge instead of solving for sulfur. The value +2 comes from counting each oxygen as -1 instead of -2, so that the four oxygens total only -4.

Correct answer: It rises by a factor of two

It rises by a factor of two. Kinetic molecular theory gives the average translational kinetic energy of a gas molecule as (3/2)kT, which is directly proportional to absolute temperature, so doubling the kelvin temperature doubles the average kinetic energy. Saying it rises by a factor of four would need the energy to depend on the square of temperature; it is the rms speed that changes by a square-root factor, rising by sqrt(2). Saying it decreases by a factor of two reverses the direct proportion, and saying it decreases by a factor of four combines that reversal with the squaring error.

Correct answer: 15 cm

15 cm. The thin-lens equation 1/f = 1/do + 1/di gives 1/di = 1/10 - 1/30 = 2/30, so di = 15 cm; the positive value means a real, inverted image on the far side of the lens. An answer of 20 cm comes from subtracting the focal length from the object distance, or the denominators 30 and 10, instead of the reciprocals. An answer of 40 cm comes from adding the focal length to the object distance, which also combines the distances instead of their reciprocals. An answer of 7.5 cm comes from adding the reciprocals, 1/10 + 1/30 = 4/30, instead of subtracting them.

Correct answer: A carbonyl and a hydroxyl attached to the same carbon

A carbonyl and a hydroxyl attached to the same carbon. The carboxyl group, -COOH, places a C=O and an -OH on one carbon, and the O-H is unusually acidic because the carboxylate anion left behind is resonance-stabilized. A carbonyl and a hydrogen attached to the same carbon defines an aldehyde, a carbonyl and a nitrogen attached to the same carbon defines an amide, and a carbonyl and a chlorine attached to the same carbon defines an acid chloride; the last two are carboxylic acid derivatives, not acids.

Correct answer: 18

18. A principal energy level n holds at most 2n^2 electrons, so n = 3 holds 2 x 3^2 = 18, filling the 3s (2), 3p (6) and 3d (10) subshells. The value 8 counts only the 3s and 3p subshells, which is why the third period has only eight elements, but the level also includes 3d. The value 6 comes from 2n, leaving out the square, and 36 comes from squaring 2n instead of n.

Correct answer: 200 J

200 J. Work is force times displacement times the cosine of the angle between them; here the force points along the motion, so cos 0 = 1 and W = (50 N)(4.0 m) = 200 J. An answer of 54 J comes from adding the force and the distance instead of multiplying them. An answer of 100 J brings in a factor of one-half that belongs to formulas such as kinetic energy, not to work done by a constant force. An answer of 800 J comes from squaring the 4.0 m distance.

Correct answer: It follows first-order kinetics and forms a carbocation intermediate

It follows first-order kinetics and forms a carbocation intermediate. In an SN1 reaction the slow, rate-determining step is ionization of the substrate to a carbocation, so the rate depends only on substrate concentration; the nucleophile then attacks the planar carbocation from either face. The option saying it follows second-order kinetics and forms a carbocation intermediate gets the rate law wrong, because the nucleophile is not involved in the rate-determining step. The option saying it follows first-order kinetics and occurs in one concerted step gets the pathway wrong, and the option saying it follows second-order kinetics and occurs in one concerted step describes the SN2 mechanism, which strong nucleophiles favor.

Correct answer: 0.5 m

0.5 m. Wavelength equals wave speed divided by frequency, so 340 m/s divided by 680 Hz gives 0.5 m. An answer of 2.0 m comes from dividing the frequency by the speed instead. An answer of 5.0 m comes from dropping a zero and dividing 340 by 68. An answer of 0.0015 m is 1/680, the period of the wave in seconds, reported as if it were a length.

Correct answer: 20 minutes

20 minutes. For a first-order reaction the half-life is ln 2 divided by k, which depends only on the rate constant, so doubling the starting concentration leaves it at 20 minutes; the reaction simply takes one half-life to fall from the doubled value back to the original one. An answer of 40 minutes treats the half-life as proportional to the starting concentration, which is true only of zero-order reactions. An answer of 14 minutes comes from taking k as 1/20 per minute, leaving out ln 2, and then dividing ln 2 by that k. An answer of 7 minutes first halves the half-life, as if the reaction were second order, and then applies the same ln 2 slip.

Correct answer: Hydrogen bonds

Hydrogen bonds. In water, hydrogen is bonded to highly electronegative oxygen, so each molecule can both donate and accept hydrogen bonds, and breaking that network takes far more energy than separating molecules of similar mass that lack it. Polar covalent bonds join the oxygen and hydrogen atoms within each water molecule; they make the molecule polar but are not broken when water boils. Dipole-dipole forces act between all polar molecules, such as hydrogen sulfide, but are much weaker than the hydrogen bonds that set water apart, and London dispersion forces are weak for a molecule as small as water.

Correct answer: 3 ohms

3 ohms. For resistors in parallel, 1/Req = 1/4 + 1/12 = 3/12 + 1/12 = 4/12, so Req = 3 ohms, which is smaller than the smaller single resistor, as a parallel combination must be. An answer of 6 ohms comes from subtracting the reciprocals, 1/4 - 1/12 = 2/12, instead of adding them, and then inverting. An answer of 0.33 ohms is the correct sum of the reciprocals, 1/3, left without being inverted. An answer of 0.17 ohms makes both errors: it subtracts the reciprocals and then leaves 2/12 uninverted.

Correct answer: Enantiomers

Enantiomers. Enantiomers are stereoisomers that are nonsuperimposable mirror images, with the opposite configuration at every stereocenter. Diastereomers are stereoisomers that are not mirror images of each other, and epimers are diastereomers that differ at only one of several stereocenters. Conformers are not separate stereoisomers at all; they are shapes of a single molecule that interconvert by rotation about single bonds.

Correct answer: Basic, with a pH of 9

Basic, with a pH of 9. pH is the negative base-10 logarithm of the hydrogen ion concentration, and -log(1 x 10^-9) = 9; at 25 degrees C any pH above 7 means a basic solution. Acidic, with a pH of 9 gets the number right but the classification backward, since acidic solutions have a pH below 7. Basic, with a pH of 5 and acidic, with a pH of 5 both come from subtracting 9 from 14, which gives the pOH, not the pH.

Correct answer: K falls

K falls. For an exothermic reaction heat behaves like a product, so raising the temperature shifts the equilibrium toward the reactants, and by the van 't Hoff relation the equilibrium constant itself becomes smaller when delta H is negative. K rises is the result for an endothermic reaction. K is constant would hold only for a reaction with zero enthalpy change; changes in concentration or pressure leave K alone, but a change in temperature does not. K is inverted confuses heating with reversing the reaction, which is what turns K into 1/K.

Correct answer: 30000 kg·m/s

30000 kg·m/s. Momentum is mass times velocity, so p = (1500 kg)(20 m/s) = 30000 kg·m/s. An answer of 15000 kg·m/s carries over the factor of one-half from the kinetic energy formula, and 600000 kg·m/s comes from multiplying by the speed twice, as in m v^2. An answer of 300000 kg·m/s makes both changes and is really the kinetic energy, one-half m v^2 = 300000 J, given the units of momentum.

Correct answer: Sodium borohydride (NaBH4)

Sodium borohydride (NaBH4). It is a mild hydride donor: hydride adds to the carbonyl carbon of the ketone, and protonation of the resulting alkoxide gives a secondary alcohol. Methylmagnesium bromide (CH3MgBr) also adds to the carbonyl carbon, but it delivers a methyl group, so the product is a tertiary alcohol. Chromic acid (H2CrO4) is an oxidizing agent that turns secondary alcohols into ketones, the reverse of what is wanted, and osmium tetroxide (OsO4) dihydroxylates alkenes and leaves ketones alone.

Correct answer: 1.0 J

1.0 J. The elastic potential energy stored in a spring is one-half k x^2, so U = (1/2)(200 N/m)(0.10 m)^2 = (1/2)(200)(0.010) = 1.0 J. An answer of 2.0 J leaves out the factor of one-half. An answer of 10 J comes from one-half k x, which forgets to square the displacement. An answer of 20 J makes both slips: k x = (200 N/m)(0.10 m) = 20 N is the force the compressed spring exerts, not the energy it stores.

Correct answer: 40 g

40 g. Moles of solute equal molarity times volume in liters: 2.0 mol/L x 0.500 L = 1.0 mol, and 1.0 mol x 40 g/mol = 40 g. An answer of 20 g multiplies the volume by the molar mass, 0.500 x 40, and leaves out the 2.0 M concentration. An answer of 10 g divides the volume by the molarity, 0.500/2.0 = 0.25 mol, instead of multiplying. An answer of 25 g leaves the volume as 500 mL and then divides by the molar mass instead of multiplying: 2.0 x 500 / 40 = 25.

Correct answer: 2.0 x 10^8 m/s

2.0 x 10^8 m/s. The index of refraction is n = c/v, so v = c/n = (3.0 x 10^8 m/s)/1.5 = 2.0 x 10^8 m/s; light always travels more slowly in a medium than in a vacuum. An answer of 4.5 x 10^8 m/s multiplies by the index instead of dividing, which would make light faster than in a vacuum. An answer of 1.0 x 10^8 m/s is the amount by which the light slows down, not its speed in the medium. An answer of 1.5 x 10^8 m/s subtracts the index from the vacuum speed, 3.0 - 1.5, instead of dividing by it.

Correct answer: Exactly 7

Exactly 7. At the equivalence point of a strong acid-strong base titration the acid and base have neutralized each other completely, leaving water and a salt such as NaCl whose ions do not hydrolyze, so at 25 degrees C the pH equals 7. A pH between 8 and 9 is typical of a weak acid titrated with a strong base, where the conjugate base hydrolyzes, and a pH between 5 and 6 is typical of a weak base titrated with a strong acid. A pH of roughly 1 is the starting pH of a 0.1 M strong acid, before any base has been added, not the pH at equivalence.

Correct answer: The carbonyl carbon is bonded to at least one hydrogen

The carbonyl carbon is bonded to at least one hydrogen. In an aldehyde the carbonyl carbon carries a hydrogen (two in formaldehyde), so the group sits at the end of a carbon chain, whereas in a ketone that carbon is bonded to two other carbons. If the carbonyl carbon is bonded to at least one hydroxyl, the compound is a carboxylic acid; if the carbonyl carbon is bonded to at least one nitrogen, it is an amide; and if the carbonyl carbon is bonded to at least one chlorine, it is an acid chloride.

Correct answer: 2.5 m/s^2

Newton's second law gives acceleration as net force divided by mass: 25 N divided by 10 kg is 2.5 m/s^2. Dividing the mass by the force instead gives 0.4 m/s^2. Dividing the force by the object's weight (10 kg times 9.8 m/s^2, or 98 N) rather than by its mass gives 0.26 m/s^2. Adding the acceleration due to gravity to 2.5 gives 12.3 m/s^2, but a net force already accounts for every force acting on the object, gravity included, so nothing more is added.

Correct answer: Size-exclusion chromatography

Size-exclusion chromatography, also called gel filtration, separates molecules by size: the column is packed with porous beads, small molecules enter the pores and take a longer path, and large molecules pass around the beads and elute first. Ion-exchange chromatography separates molecules by net charge, affinity chromatography by specific binding to a ligand fixed to the beads, and hydrophobic interaction chromatography by how hydrophobic a molecule's surface is.

Correct answer: 1.0 x 10^15 Hz

Photon energy is Planck's constant times frequency (E = hf), so f = E/h: dividing 6.63 x 10^-19 J by 6.63 x 10^-34 J·s gives 1.0 x 10^15 Hz. Dividing h by E instead gives 1.0 x 10^-15 Hz, which is numerically the period in seconds, not a frequency. Multiplying the correct result by 2 pi gives 6.3 x 10^15 Hz, the angular frequency in radians per second rather than the frequency. 6.3 x 10^-15 Hz makes both slips at once.

Correct answer: Azimuthal quantum number (l)

The azimuthal quantum number (l), also called the angular momentum quantum number, fixes the subshell and therefore the orbital's shape: l = 0 gives a spherical s orbital, l = 1 a dumbbell-shaped p orbital, and l = 2 the d orbitals. The principal quantum number (n) sets the shell, its energy level and the orbital's size; the magnetic quantum number (m_l) sets the orbital's orientation in space; and the spin quantum number (m_s) gives the electron's spin state, +1/2 or -1/2.

Correct answer: 3.72 °C

Freezing-point depression is delta Tf = i x Kf x m. NaCl dissociates completely into Na+ and Cl-, so the van't Hoff factor i is 2, and 1 mol in 1 kg of water is 1 m: 2 x 1.86 x 1 = 3.72, so the depression is 3.72 °C. Leaving out the van't Hoff factor gives 1.86 °C, and dividing by it instead of multiplying, in effect taking i as 1/2, gives 0.93 °C. Dividing by Kf instead of multiplying by it, 2 x 1 / 1.86, gives 1.08 °C.

Correct answer: Transfer of phosphate from a high-energy intermediate directly to ADP

Substrate-level phosphorylation is the transfer of phosphate from a high-energy intermediate directly to ADP: an enzyme moves the phosphoryl group of a metabolite such as 1,3-bisphosphoglycerate or phosphoenolpyruvate straight onto ADP, making ATP with no membrane or proton gradient involved. Synthesis of ATP from ADP and phosphate driven by a membrane proton gradient is oxidative phosphorylation. Transfer of phosphate to glucose from ATP in the hexokinase reaction spends ATP rather than making it. Release of glucose 1-phosphate from glycogen by phosphorolysis attaches inorganic phosphate to a sugar but makes no ATP.

Correct answer: 1.0 L

At constant temperature Boyle's law holds: P₁V₁ = P₂V₂. So (2.0 atm)(4.0 L) = (8.0 atm)(V₂), and the new volume is 8.0 / 8.0 = 1.0 L; raising the pressure fourfold cuts the volume to a quarter. Halving the volume, as if the pressure had only doubled, gives 2.0 L. Treating volume as directly proportional to pressure gives 4.0 × (8.0 / 2.0) = 16.0 L, and 4.0 L is simply the starting volume, as if raising the pressure had no effect.

Correct answer: sp

Each carbon of a triple bond forms two sigma bonds, one to the other carbon and one to its substituent, so it mixes one s and one p orbital into two sp hybrids pointing 180 degrees apart (a linear geometry) and keeps two unhybridized p orbitals for the two pi bonds. sp^2 describes trigonal planar carbon in a double bond, sp^3 describes tetrahedral carbon with four single bonds, and sp^3d is a trigonal bipyramidal, expanded-octet hybridization that a second-period carbon atom, with no d orbitals in its valence shell, cannot adopt.

Correct answer: 4 kg·m/s

Momentum is a vector, so the rebound reverses its sign. Taking the initial direction as positive, the ball starts with 0.5 x 4 = +2 kg·m/s and ends with -2 kg·m/s; the change is -2 - (+2) = -4 kg·m/s, a magnitude of 4 kg·m/s. Reasoning that the speed is unchanged and so nothing changed gives 0 kg·m/s. Stopping at the ball's momentum, 0.5 x 4, without accounting for the reversal gives 2 kg·m/s, and taking the 8 m/s change in velocity without multiplying by the mass gives 8 kg·m/s.

Correct answer: Infrared spectroscopy (IR)

Infrared spectroscopy (IR) measures the absorption of infrared light at the frequencies that make particular bonds stretch and bend, so each functional group gives characteristic peaks, such as a strong C=O stretch near 1700 cm^-1 or a broad O–H stretch near 3300 cm^-1. Nuclear magnetic resonance spectroscopy (NMR) probes the magnetic environments of nuclei such as hydrogen-1 and carbon-13 to map how atoms are connected, mass spectrometry (MS) measures the mass-to-charge ratios of a molecule's ions and fragments, and atomic absorption spectrometry (AAS) measures light absorbed by free atoms to quantify metals.

Correct answer: -200 kJ

The enthalpy change equals the energy absorbed to break bonds minus the energy released when new bonds form: 600 kJ minus 800 kJ is -200 kJ. The negative sign means the reaction is exothermic, because more energy is released than absorbed. Subtracting in the reverse order (bonds formed minus bonds broken) gives +200 kJ. Counting only the 800 kJ released by bond formation gives -800 kJ, and also treating released energy as positive gives +800 kJ.

Correct answer: Its frequency

Its frequency stays the same, because the frequency is set by the source: wave crests reach the boundary and leave it at the same rate. Its speed falls from about c in air to c/n in glass, so its wavelength shortens by the same factor (wavelength = speed / frequency). Its power also drops, because part of the wave is reflected at the surface and only the rest is transmitted into the glass.

Correct answer: A nonpolar, uncharged side chain

A nonpolar, uncharged side chain, such as those of leucine, isoleucine, valine and phenylalanine, cannot hydrogen-bond with water, so the hydrophobic effect drives it into the protein's interior. A negatively charged, acidic side chain (aspartate, glutamate), a positively charged, basic side chain (lysine, arginine) and a polar, uncharged side chain (serine, threonine) all interact favorably with water and usually stay on the solvent-exposed surface.

Correct answer: 60 C

Current is the rate at which charge passes a point (I = Q/t), so the charge is Q = It = (2.0 A)(30 s) = 60 C. Rearranging the definition wrongly as Q = I/t gives 0.067 C, and dividing the time by the current, Q = t/I, gives 15 C. Taking the 2.0-A current itself as the charge and ignoring the 30 seconds gives 2 C.

Correct answer: Fischer esterification (a condensation reaction)

Fischer esterification (a condensation reaction) is the acid-catalyzed joining of a carboxylic acid and an alcohol into an ester, with water released as the second product. Base-promoted saponification (a hydrolysis reaction) runs the other way, splitting an ester into a carboxylate salt and an alcohol. Alkene hydration (an addition reaction) adds water across a carbon-carbon double bond to give an alcohol, and alcohol dehydration (an elimination reaction) removes water from an alcohol to give an alkene rather than an ester.

Correct answer: 3 microfarads

For capacitors in series the reciprocals add: 1/C = 1/6 + 1/6 = 2/6 = 1/3, so the equivalent capacitance is 3 microfarads, smaller than either capacitor. Adding the capacitances directly, the rule for capacitors in parallel (and for resistors in series), gives 12 microfarads. Adding the reciprocals but forgetting to invert the sum leaves 1/3, read as 0.33 microfarads, and inverting the parallel sum instead gives 1/12, or 0.083 microfarads.

Correct answer: From the anode to the cathode

Electrons travel through the external circuit from the anode to the cathode: oxidation at the anode releases them, they pass through the wire, and reduction at the cathode consumes them. From the cathode to the anode reverses that flow. From the anode to the electrolyte and from the electrolyte to the anode both have electrons crossing the solution, but inside the cell (in the electrolyte and the salt bridge) charge is carried by ions; electrons move only through the external wire.

Correct answer: Carbonyl (C=O) group, as in ketones or aldehydes

The answer is the carbonyl (C=O) group, as in ketones or aldehydes: a C=O stretch gives a strong, sharp band, near 1715 cm^-1 for a simple ketone and within about 1680-1750 cm^-1 for carbonyls generally, and the polar, stiff double bond makes it intense. The hydroxyl (O–H) group, as in alcohols or phenols, gives a broad band near 3200-3550 cm^-1; the amino (N–H) group, as in amines or anilines, absorbs near 3300-3500 cm^-1; and the cyano (C≡N) group, as in nitriles or cyanohydrins, gives a sharp band near 2200-2260 cm^-1.

Correct answer: Piaget's stages of cognitive development

The theory described is Piaget's stages of cognitive development: Piaget saw children as active learners who build, test, and revise mental schemas through assimilation and accommodation as they explore and interact with their world, advancing through the sensorimotor, preoperational, concrete operational, and formal operational stages. Freud's stages of psychosexual development trace how unconscious drives center on different body zones, not how children construct knowledge. Bronfenbrenner's ecological systems theory maps the nested environments, from family to culture, that shape development rather than the child's own construction of understanding. Bandura's observational learning theory explains how children acquire behaviors by watching and imitating models, which is learning from others rather than building knowledge through exploration.

Correct answer: Social loafing

Social loafing is correct because people tend to put less effort into a collective task when their individual contributions are pooled and cannot be identified, so responsibility for the outcome feels shared. Social facilitation is improved performance on simple or well-practiced tasks when others are present, an increase rather than a drop in output. Social inhibition is poorer performance on complex or unfamiliar tasks in front of an audience, which reflects heightened arousal rather than reduced effort on a pooled task. Social striving is the opposite pattern, in which some people, especially in collectivist cultures, work harder in a group than they do alone.

Correct answer: An exceptionally vivid, highly detailed snapshot of the moment and circumstances of hearing surprising and consequential or emotionally arousing news

Flashbulb memory is an exceptionally vivid, highly detailed snapshot of the moment and circumstances of hearing surprising and consequential or emotionally arousing news, such as where a person was and what the person was doing on learning of a national tragedy; such memories feel permanent and are held with great confidence, although research shows their details drift over time. An unusually precise, persistent visual image of a briefly viewed picture or scene, retained in such detail that it can be scanned as if still present, describes eidetic imagery, which preserves visual detail rather than the setting in which news was learned. A strikingly short-lived, photograph-like sensory impression of a briefly flashed visual display, holding the outlines and colors for well under a second, is iconic memory, the visual form of sensory memory. A largely unconscious, defensive exclusion from awareness of distressing memories, painful inner conflicts, or intensely shameful impulses and wishes is repression, a defense mechanism that keeps material out of awareness rather than preserving it vividly.

Correct answer: Belongingness and love needs

The answer is belongingness and love needs: in Maslow's hierarchy, the level directly beneath esteem is the need for affection, friendship, intimacy, and acceptance by a group, and esteem becomes the dominant motive once those social needs are largely met. The safety and security needs, for protection, stability, and freedom from fear, sit two levels below esteem and are met before belonging becomes the focus. The basic physiological needs, such as food, water, and sleep, form the base of the pyramid, the first level to be satisfied rather than the one immediately before esteem. Self-actualization needs sit above esteem at the top of the hierarchy, so they are pursued after esteem needs, not before.

Correct answer: Dopamine

Dopamine is correct because dopamine neurons projecting from the ventral tegmental area to the nucleus accumbens form the mesolimbic reward pathway, and addictive drugs raise dopamine signaling there, which reinforces continued use. Endorphin is an endogenous opioid peptide best known for relieving pain and producing a sense of well-being after stress or exercise, rather than serving as the main transmitter of the reward circuit. Glutamate is the brain's chief excitatory neurotransmitter, involved broadly in learning and synaptic plasticity, not the defining signal of reward and pleasure. GABA is the main inhibitory neurotransmitter; alcohol and benzodiazepines enhance its effects, but it dampens neural activity rather than driving the reward response.

Correct answer: Negative reinforcement

Negative reinforcement is correct because removing an aversive stimulus after a behavior, such as a headache easing after a pill is taken, makes that behavior more likely to recur. Positive reinforcement also strengthens behavior, but by adding a desirable consequence rather than removing an unpleasant one. Positive punishment adds an aversive consequence, such as a reprimand, to make a behavior less likely. Negative punishment removes a desirable stimulus, such as a privilege, to make a behavior less likely, so it weakens rather than strengthens the response.

Correct answer: Social facilitation

Social facilitation is correct because the mere presence of others raises arousal, which strengthens dominant responses and so improves performance on simple or well-practiced tasks. Social inhibition is the other side of the same effect, poorer performance on complex or unfamiliar tasks when others are watching, so it describes a decline rather than an improvement. Audience inhibition is a reluctance to act, for example to help in an emergency, because of fear of embarrassment in front of onlookers. The Yerkes-Dodson law relates arousal to performance in general, with performance peaking at a moderate level of arousal, and it does not name the effect of other people being present.

Correct answer: The psychodynamic perspective

The psychodynamic perspective is correct because, building on Freud's psychoanalytic theory, it explains behavior and personality through unconscious motives, internal conflicts, and the lasting influence of early childhood experiences. The humanistic perspective, associated with Rogers and Maslow, stresses conscious choice, free will, and the drive toward personal growth. The behaviorist perspective explains behavior through observable learning from reinforcement and conditioning and sets aside unconscious mental processes. The biopsychosocial perspective integrates biological, psychological, and social influences on health and behavior rather than singling out unconscious conflict and childhood experience.

Correct answer: The Cannon-Bard theory

The Cannon-Bard theory is correct because it holds that an emotional stimulus is relayed through the thalamus to the cortex and to the body at the same time, so physiological arousal and the felt emotion arise simultaneously and independently. The Schachter-Singer theory, the two-factor theory, says arousal comes first and is experienced as a specific emotion only after the person gives it a cognitive label. The opponent-process theory holds that an emotional reaction is followed by an opposing reaction, such as relief after fear, which concerns a sequence of opposite feelings rather than arousal and feeling occurring together. The James-Lange theory holds that physiological arousal comes first and that the felt emotion is the perception of those bodily changes, so the feeling follows arousal rather than arising at the same time.

Correct answer: When the emergency is unambiguous and the bystanders are few

The bystander effect is the reduced likelihood that any one person will help when other people are present, so it is least likely when the emergency is unambiguous and the bystanders are few: a clear need for help leaves little room to misread the situation, and with few others present responsibility is diffused less. When the setting is crowded and the bystanders are strangers, the large number of onlookers spreads responsibility thin and the lack of ties among them further lowers helping. When the cry is ambiguous and the bystanders are unconcerned, people read the unconcerned reactions of others as a sign that nothing is wrong, a pattern known as pluralistic ignorance, so each person holds back. When the victim is unfamiliar and the bystanders are hurried, both conditions reduce helping, as shown in studies of people rushing to an appointment who passed by a person in distress.

Correct answer: The serial-position effect

The serial-position effect is correct because recall of a list is best for the first items, which receive extra rehearsal and reach long-term memory, and for the last items, which are still held in short-term memory, producing a U-shaped recall curve. The primacy effect accounts only for the advantage of the first items, not the last ones. The recency effect accounts only for the advantage of the last items, not the first ones. The picture-superiority effect is the finding that pictures are remembered better than words, which concerns the type of material rather than its position in a series.

Correct answer: The self-serving bias

The self-serving bias is correct because people protect their self-esteem by crediting their successes to internal causes, such as ability and effort, while blaming their failures on external causes, such as bad luck or an unfair test. The actor-observer bias contrasts how people explain their own behavior, mostly by the situation, with how they explain the behavior of others, mostly by disposition, whether the outcome was a success or a failure. The correspondence bias, another name for the fundamental attribution error, is the tendency to explain other people's behavior by their dispositions while underestimating situational pressures. The anchoring bias is a judgment error in which an initial value or piece of information unduly sways later estimates.

Correct answer: Cognitive dissonance

Cognitive dissonance is correct because Festinger described it as the uncomfortable tension that arises when a person holds inconsistent beliefs or meets information that conflicts with existing beliefs, which motivates the person to reduce the inconsistency. Cognitive accommodation is the Piagetian process of revising an existing schema so that new information fits, which resolves a mismatch rather than naming the discomfort itself. Cognitive restructuring is a therapy technique in which a person deliberately identifies distorted thoughts and replaces them with more accurate ones. Cognitive appraisal is the evaluation of a situation's meaning and threat that shapes emotional and stress responses.

Correct answer: Conformity

Conformity is correct because it is the adjustment of one's behavior, attitudes, or beliefs to match those of a group as a result of real or imagined group pressure, even when no one makes a direct request. Compliance is a change in behavior in response to a direct request from another person, such as agreeing to sign a petition. Groupthink is a pattern of faulty decision-making in a cohesive group, in which the desire for consensus overrides a realistic appraisal of alternatives. Reactance is the motivation to resist pressure and reassert one's freedom, which pushes behavior away from what others urge rather than toward it.

Correct answer: Procedural memory

Procedural memory is correct because it is the implicit long-term memory for motor skills and routines, such as riding a bicycle or typing, which are carried out without conscious recall of how they were learned. Episodic memory is the explicit memory of specific personal events tied to a time and place, such as a birthday party. Declarative memory is the broader explicit system for facts and events that can be consciously recalled and described, the counterpart to memory for skills. Prospective memory is remembering to carry out an intended action in the future, such as taking a medication at noon.

Correct answer: Social facilitation

Social facilitation is correct because the presence of others increases arousal, which improves performance on simple or well-learned tasks while it can hinder performance on difficult ones. The spotlight effect is the tendency to overestimate how much others notice one's appearance or behavior, which concerns self-consciousness rather than performance. Social loafing is the tendency to put less effort into a group task when individual contributions are pooled and cannot be identified, a drop in effort rather than an improvement in performance. The mere exposure effect is the tendency to come to like a stimulus, such as a face or a song, simply because one has been exposed to it repeatedly, which concerns liking rather than performance.

Correct answer: Priming

Priming is correct because earlier exposure to a stimulus activates related associations in memory, so the response to a later stimulus changes without the person being aware of the influence, as when seeing the word yellow speeds recognition of the word banana. Framing is the effect that the wording of a choice, such as describing outcomes as gains or as losses, has on the decision made about that same choice. Masking is a perception effect in which a second stimulus presented right after the first blocks conscious perception of the first. Habituation is a decline in response to a stimulus that is presented repeatedly, a weakening response to the same stimulus rather than an influence on a different, later one.

Correct answer: Identity versus role confusion

Identity versus role confusion is correct because Erikson placed this crisis in adolescence, when young people explore roles, values, and goals to build a coherent sense of who they are, and failure to do so leaves them confused about their place in the world. Industry versus inferiority is the school-age stage just before adolescence, centered on mastering skills and schoolwork. Basic trust versus mistrust is the first stage, in infancy, when reliable care builds a sense of security. Generativity versus self-absorption, also called generativity versus stagnation, is the middle-adulthood stage of contributing to the next generation through work, parenting, and mentoring.

Correct answer: The weapons effect

The weapons effect is correct because Berkowitz and LePage found that the mere sight of a gun, even when no one uses it, primes aggressive thoughts and can increase aggressive behavior. The weapon focus effect is an eyewitness memory finding: attention narrows onto a weapon during a crime, so the witness recalls other details, such as the culprit's face, less accurately. The catharsis theory claims that venting aggression reduces later aggression, a prediction research has generally failed to support and the opposite of a cue that raises aggression. The instinct theory, associated with Freud and Lorenz, treats aggression as an innate drive that builds up internally rather than as a response triggered by situational cues such as a weapon.

Correct answer: The fundamental attribution error

The fundamental attribution error is correct because observers tend to explain other people's behavior by personality or disposition while underestimating the pressure of the situation. The ultimate attribution error extends this pattern to intergroup judgments, attributing an out-group's negative acts to its members' nature and its positive acts to circumstance, so it concerns group membership rather than the general tendency. The correspondent inference theory, from Jones and Davis, describes when an observer is justified in inferring a disposition from an action, such as when the action is freely chosen and unexpected, rather than naming a bias. The implicit personality theory is a person's set of assumptions about which traits go together, such as expecting a warm person to also be generous.

Correct answer: The health belief model

The health belief model is correct because it predicts that people take a health action when they believe they are susceptible to a serious illness, that the action will reduce the threat, and that its benefits outweigh its barriers, often prompted by a cue to action and supported by self-efficacy. The health promotion model, developed by Pender, also weighs perceived benefits, barriers, and self-efficacy, but it deliberately leaves out perceived threat of illness as a motivator. The social cognitive model explains behavior through self-efficacy, outcome expectations, and observational learning within the reciprocal influence of person, behavior, and environment. The social norms model holds that people adjust their behavior to what they believe their peers do, so correcting misperceived norms can reduce risky behavior.

Correct answer: Proactive interference

Proactive interference is correct because previously learned information disrupts the recall of newer information, as when an old phone number keeps coming to mind when someone tries to recall a new one. Retroactive interference works in the opposite direction, with new learning disrupting the recall of older information. Motivated forgetting is the forgetting of painful or threatening material because of a wish not to remember it, as in suppression or repression. Cue-dependent forgetting is a failure to retrieve information because the cues present during learning are missing at the time of recall.

Correct answer: The Schachter-Singer theory

The Schachter-Singer theory, also called the two-factor theory, is correct because it holds that emotion requires both physiological arousal and a cognitive label that explains the arousal, so the same arousal can be felt as fear or as excitement depending on how the situation is interpreted. The James-Lange theory holds that an emotion is simply the perception of bodily changes, with no cognitive step that interprets them. The Cannon-Bard theory holds that arousal and the emotional experience occur at the same time and independently of each other. The Lazarus appraisal theory puts a cognitive appraisal of the situation first, with arousal and emotion following from that appraisal, rather than an interpretation of arousal that is already present.

Correct answer: Normative social influence

Normative social influence is the best fit because it is conformity driven by group pressure: people adjust their behavior or stated views to match a group standard in order to be liked and accepted and to avoid rejection. Informational social influence also produces conformity, but because people accept others' judgments as evidence about reality when they are uncertain, not because of social pressure. Social representations theory, from Moscovici, describes the shared beliefs and images through which a society makes sense of the world, not an individual yielding to pressure. Social penetration theory describes how relationships deepen as people gradually disclose more personal information to each other.

Correct answer: A stereotype

A stereotype is correct because it is a schema for a social group, a set of generalized beliefs and expectations about the traits of its members that can shape how individuals from that group are perceived and treated. A prejudice is an evaluative, usually negative attitude toward a group and its members, the emotional component rather than the set of beliefs. A role schema is a set of expectations about how people in a particular position, such as a doctor or a teacher, should behave. A social norm is a shared rule for acceptable behavior within a group, which prescribes conduct rather than describing the characteristics of a group's members.

Correct answer: Social cognition

Social cognition is correct because it is the study of how people perceive, interpret, remember, and think about others and social situations, the mental processes that guide how they influence and relate to one another. Social neuroscience studies the brain and biological mechanisms that underlie social behavior. Social anthropology studies the structures, customs, and institutions of whole societies and cultures rather than individual social thinking. Social learning is the acquisition of behavior by observing and imitating others, a learning process rather than a field that studies social thought and relationships.

Correct answer: Personality

Personality is correct because it is a person's characteristic and relatively enduring pattern of thinking, feeling, and acting, which distinguishes one individual from another and remains fairly stable across time and situations. Temperament is the biologically based emotional and activity style visible from infancy, such as how reactive or easygoing a baby is, which forms only part of the foundation of personality. Self-esteem is a person's overall evaluation of his or her own worth, an attitude toward the self rather than a pattern of thought, feeling, and behavior. Identity is a person's sense of who he or she is, including roles, values, and group memberships, a part of the self-concept rather than the whole enduring pattern.

Correct answer: Modeling

Modeling is correct because it is the process of observing another person's behavior and then reproducing it, the core of Bandura's account of observational learning, as when children copied adults who hit a Bobo doll. Shaping is an operant procedure that reinforces successive approximations of a target behavior, so learning comes from one's own reinforced responses rather than from watching others. Imprinting is a rapid early form of learning in which young animals such as goslings attach to and follow the first moving object they see during a critical period. Extinction is the weakening of a learned response when it is no longer reinforced or, in classical conditioning, when the conditioned stimulus is no longer paired with the unconditioned stimulus.

Correct answer: Ethnocentrism

Ethnocentrism is correct because it is the belief that one's own culture, society, or nation is superior to others, along with the habit of judging other cultures by its standards. Xenocentrism is the reverse, a preference for other cultures and the belief that they are superior to one's own. Cultural relativism is the practice of understanding and judging a culture by its own values and standards rather than by those of another culture, so it rejects ranking one culture above the others. Xenophobia is fear of or hostility toward foreigners and people seen as outsiders, an attitude toward other groups rather than a belief that one's own culture is superior to all others.

Correct answer: Long-term memory

Long-term memory is correct because it is the relatively permanent store whose capacity is thought to be practically unlimited and which can hold information for a lifetime, from facts and skills to personal experiences. Short-term memory holds only a small amount of information, roughly seven items, for about 20 to 30 seconds unless it is rehearsed. Iconic memory is the visual form of sensory memory, a fleeting image that lasts well under a second. Echoic memory is the auditory form of sensory memory, which holds sounds for only a few seconds.

Correct answer: The foot-in-the-door technique

The foot-in-the-door technique is correct because agreeing to a small request makes people more willing to grant a larger, related request later, partly because they come to see themselves as helpful and want to remain consistent. The door-in-the-face technique works in the opposite order: a large request that is refused is followed by a smaller one, which seems like a concession and is more readily accepted. The fear-then-relief technique raises anxiety and then suddenly removes it, leaving people briefly more compliant with a request made during the relief. The disrupt-then-reframe technique briefly confuses people with an odd phrase, such as a price stated in pennies, and then reframes the offer, so the request is accepted before the confusion clears.

Correct answer: A case study

A case study is correct because it is an in-depth examination of a single person, group, event, or situation, often combining interviews, observation, and records to describe it in rich detail. A lab study is research carried out in a controlled laboratory setting, usually with many participants and manipulated conditions. A panel study surveys the same sample of people repeatedly over time to track change. A field study is research conducted in a natural, real-world setting, which describes where data are gathered rather than a focus on one case.

Correct answer: The tendency of group discussion to make prevailing attitudes stronger and more extreme

Group polarization is the tendency of group discussion to make prevailing attitudes stronger and more extreme: when like-minded members talk an issue over, they hear new arguments for the side they already favor and compare themselves with one another, so the group's average view shifts further in its original direction. The tendency of group cohesiveness to make shared decisions quicker and more uncritical describes groupthink, in which the wish for harmony suppresses dissent and realistic appraisal. The tendency of group anonymity to make law-abiding people bolder and more unrestrained describes deindividuation. The tendency of group assignments to make individual effort weaker and more halfhearted describes social loafing.

Correct answer: Public compliance

Public compliance is correct because it is outward conformity, going along with a group in what one says or does to avoid rejection or gain approval while privately continuing to disagree. Private acceptance is the opposite outcome, in which a person genuinely comes to believe what the group believes, so private beliefs and public behavior match. Public self-awareness is attention to oneself as others see one, such as concern about appearance or reputation, rather than a gap between belief and behavior. Private self-awareness is attention to one's own inner thoughts, feelings, and standards.

Correct answer: Self-efficacy

Self-efficacy is correct because Bandura defined it as a person's belief in his or her capability to organize and carry out the actions needed to handle particular situations, a belief that shapes which goals people choose and how long they persist. Self-esteem is a person's overall sense of self-worth, an evaluation of oneself rather than a belief about the ability to perform particular tasks. Self-regulation is the process of controlling one's own behavior, emotions, and impulses in pursuit of goals. Self-monitoring is the tendency to watch social cues and adjust one's self-presentation to fit the situation.

Correct answer: Anthropomorphism

Anthropomorphism is correct because it is the tendency to attribute human traits, emotions, and intentions to animals, objects, or other non-human entities, such as believing that a car is angry or that a pet feels guilty. Zoomorphism is the reverse attribution, giving animal traits or forms to humans, gods, or objects. Animistic thinking is the young child's belief, described by Piaget, that inanimate objects are alive and conscious, a feature of preoperational thought rather than the general habit of humanizing non-human things. Egocentrism is a young child's difficulty in taking any point of view other than his or her own.

Correct answer: A drop in self-awareness and personal identity among people immersed in a group

Deindividuation is a drop in self-awareness and personal identity among people immersed in a group: anonymity and arousal in a crowd weaken self-monitoring and a person's sense of being an individual, which can release impulsive behavior the person would normally restrain. A drop in effort and individual accountability among laborers pooled in a group describes social loafing. A drop in helping and felt responsibility among bystanders assembled in a group describes the bystander effect and diffusion of responsibility. A drop in disagreement and independent judgment among members united in a group describes groupthink.

Correct answer: Mood-congruent memory

Mood-congruent memory is the tendency to recall events whose emotional tone matches one's current mood: a person who feels low right now retrieves more unhappy memories, and a cheerful mood brings happier ones to mind. State-dependent memory is a different effect: recall is best when a person's internal state at retrieval matches the state during learning, whatever the emotional content of the memory. Context-dependent memory is the same matching effect for the external setting, such as studying and being tested in the same room. Autobiographical memory is simply memory for the events of one's own life, a kind of memory rather than an effect of present emotion on recall.

Correct answer: The von Restorff effect

The von Restorff effect, also called the isolation effect, is the finding that an item that stands out from the similar items around it is remembered better, which is why unusual or distinctive information is recalled more often than common information. The serial position effect is better recall for items at the start and end of a list, an effect of where an item sits rather than how distinctive it is. The Zeigarnik effect is better memory for unfinished or interrupted tasks than for completed ones. The self-reference effect is better memory for information that relates to oneself.

Correct answer: Obedience

Obedience is a change in behavior made in response to a direct order from an authority figure; in Milgram's studies, participants kept following the experimenter's instructions even against their own conscience. Compliance is going along with a direct request from someone who holds no authority over the person. Conformity is adjusting one's behavior or beliefs to match the norms of a group, with no one giving an order. Groupthink is the drive for consensus in a cohesive group that suppresses dissent while the group makes decisions.

Correct answer: Cultural capital

Cultural capital is Pierre Bourdieu's term for the non-financial assets passed down within a family, such as knowledge, tastes, ways of speaking and educational credentials. It explains why children of wealthy families gain easier access to prestigious universities: they arrive already fluent in what those institutions reward, so admission is not based on merit alone. The personal networks the question mentions are what Bourdieu called social capital, a companion form of inherited advantage. The hidden curriculum is the set of unwritten lessons schools teach, such as punctuality and deference to authority. Credential inflation is the rising level of schooling employers demand for the same jobs. Educational tracking sorts students into ability groups within a school. None of these three describes advantages that students carry in from a privileged background.

Correct answer: Conflict theory

Conflict theory holds that a society's structures, including the institutions that make authority look legitimate, serve the groups that hold power, and that public behavior is shaped by this unequal distribution of power and resources. Functionalist theory treats the same structures as parts that work together to keep society stable, not as tools of dominant groups. Modernization theory explains how societies develop from traditional to industrial forms. Labeling theory explains deviance as the result of others attaching a deviant label to a person.

Correct answer: Discrimination

Discrimination is the unequal treatment of people because of their membership in a group: it is the behavior that turns prejudiced beliefs into unequal treatment and unequal opportunities. Prejudice is the negative attitude or feeling toward a group, which may never be acted on. Stereotyping is the generalized belief about what members of a group are like, the thinking that can underlie prejudice. Dehumanization is perceiving members of a group as less than fully human, a way of thinking rather than the unequal treatment itself.

Correct answer: Social dynamics

Social dynamics, a term introduced by Auguste Comte, refers to the processes by which a society changes and develops over time, so it covers the evolution of norms and values, including change that grows out of conflict between groups. Social statics was Comte's companion term for the study of social order and stability, the opposite focus. Social mobility is the movement of individuals or groups between positions in a stratification system. A social movement is an organized collective effort to promote or resist a particular change; it is one of the agents of change, not the process by which norms and values evolve.

Correct answer: Rational choice theory

Rational choice theory explains behavior as a weighing of costs against benefits. Because a single vote almost never decides an election, a citizen who judges the time and effort of voting to outweigh the tiny chance of changing the result may stay home, which lowers turnout. Social exchange theory applies the same cost-benefit logic to personal relationships rather than to political participation. The bystander effect is the reduced likelihood of helping someone in an emergency when other people are present. The bandwagon effect is the tendency to adopt a view or back a candidate because many others already do, which would not explain lower turnout.

Correct answer: Differential association theory

Differential association theory, Edwin Sutherland's account, holds that deviance is learned through interaction with others, mainly close associates: a person exposed to more attitudes favorable to breaking the law than unfavorable ones becomes more likely to break it. Differential opportunity theory, from Richard Cloward and Lloyd Ohlin, argues instead that deviance depends on unequal access to legitimate and illegitimate means of success. Social control theory explains conformity through a person's bonds to society, so that deviance follows when those bonds weaken. Social conflict theory treats deviance as whatever powerful groups choose to define and punish as deviant.

Correct answer: Digital divide

The digital divide is the gap between people who have access to modern information technology, and the skills to use it, and those who do not, a gap that limits full participation in work, education and civic life. The global village is Marshall McLuhan's image of electronic media linking the whole world into one community, which describes connection rather than unequal access. Cultural lag is the delay before a society's norms and values catch up with technological change. The knowledge gap is the hypothesis that new information from mass media widens differences in knowledge between groups of higher and lower socioeconomic status.

Correct answer: Post-materialism theory

Post-materialism theory, developed by Ronald Inglehart, holds that once a society becomes economically secure, generations raised without scarcity give priority to values such as self-expression, environmental protection and human rights over survival and material gain. Post-industrial theory, associated with Daniel Bell, describes the economic shift from manufacturing toward services and information, not a change in values. Secularization theory explains the declining social influence of religion as societies modernize. Self-determination theory is a psychological account of individual motivation built on needs for autonomy, competence and relatedness, not a theory of how whole societies change.

Correct answer: Caste system

A caste system is a closed form of stratification in which status is fixed at birth by family lineage and kept for life, with little or no movement between ranks. A class system is an open form of stratification in which position depends partly on personal achievement and social mobility is possible. Ascribed status is the kind of status a person is born into, not a system of stratification built on it. Achieved status is the opposite kind: a position earned through effort and accomplishment.

Correct answer: Collective behavior theory

Collective behavior theory explains social movements as relatively spontaneous, loosely structured action by people who share a grievance or sense of injustice but lack the formal organization of established groups, so the movement grows out of shared discontent rather than planning. Resource mobilization theory takes the opposite view: movements arise and succeed when they can gather money, members, leaders and formal organization. Political opportunity theory ties a movement's rise to openings in the political system, such as divided elites or new allies. Rational choice theory treats joining a movement as an individual cost-benefit calculation, which raises the free-rider problem.

Correct answer: Gatekeeping

Gatekeeping is the process by which a small number of people or institutions, such as editors, producers and platform moderators, decide which information passes on to the public and which is held back, so they control the flow of information and shape public opinion and behavior. Agenda-setting is the media's power to influence which issues the public considers important through how much coverage each receives. Framing is the way a story is presented, which shapes how audiences interpret it. Priming is the way earlier coverage makes certain issues the standards people later use to judge leaders and events.

Correct answer: Structural functionalism

Structural functionalism, the perspective of Emile Durkheim and Talcott Parsons, sees society as a system of interrelated parts and holds that order and stability come mainly from socializing members into a shared set of values and norms rather than from force. The social-conflict approach argues the reverse: order is maintained through the power and coercion of dominant groups. Symbolic interactionism is a micro-level perspective on how people create meaning through everyday interaction. Social contract theory, from Hobbes, Locke and Rousseau, grounds political order in an agreement to accept a common authority, not in the internalization of shared norms.

Correct answer: Globalization

Globalization is the growing connectedness and interdependence of the world's economies, cultures and populations, driven by trade, investment, migration and communication technology. Urbanization is the growth in the share of a population living in cities. Westernization is the adoption of Western European and North American culture by other societies, a one-way spread of influence rather than mutual interdependence. Secularization is the decline in the social influence of religion as societies modernize.

Correct answer: Altruistic suicide

Altruistic suicide is Emile Durkheim's term for suicide that occurs when social integration is too strong: the individual is so absorbed into the group that giving up one's life for it, or because its norms demand it, comes to seem a duty. Egoistic suicide is the opposite case, rooted in too little integration and the isolation that follows. Mechanical solidarity is Durkheim's term for the cohesion of small traditional societies built on shared beliefs and likeness; it names a kind of social bond, not a type of suicide. Organic solidarity is the cohesion of complex modern societies built on interdependent, specialized roles.

Correct answer: Social inequality

Social inequality is the umbrella term for the systematic, unequal distribution of resources and opportunities, including health, wealth, education and power, among countries and among groups within them, produced and sustained by past and present policies and practices. Health disparities and income disparities are each one strand of that inequality, so each is too narrow for differences that span health, wealth, education and power together. Social mobility is the movement of people between positions in a stratification system, not the disparities between those positions.

Correct answer: Symbolic interactionism

Symbolic interactionism, rooted in the work of George Herbert Mead and Herbert Blumer, studies how people create and share meaning through symbols, above all language, in everyday interaction, and treats that interaction as the foundation of social life. Structural functionalism looks at how large social structures work together to keep society stable. The social-conflict approach focuses on inequality and the struggle for power between groups. Social exchange theory explains interaction as a trade of costs and rewards rather than as the shared creation of meaning through symbols.

Correct answer: The glass ceiling

The glass ceiling is the unseen barrier that stops qualified women and members of minority groups from rising to the top levels of an organization. The glass cliff is the tendency for women and minorities to be promoted into leadership mainly when an organization is in crisis and failure is likely. The glass escalator is the faster promotion men often receive in female-dominated professions such as nursing and teaching. The glass walls are barriers that confine women to particular functions, such as support and staff roles, limiting movement across an organization rather than up it.

Correct answer: The activity theory of aging

The activity theory of aging holds that older adults stay satisfied by remaining active and by taking on new roles, such as volunteering or new friendships, to replace the work and family roles lost with age. The continuity theory of aging holds instead that people adapt best by keeping the habits, relationships and roles they already had. The disengagement theory of aging claims that older adults and society gradually withdraw from each other and that this mutual withdrawal is natural. The modernization theory of aging argues that older people lose status as societies industrialize.

Correct answer: Genocide

Genocide is the deliberate and systematic destruction of a group of people; the 1948 UN Genocide Convention names national, ethnic, racial and religious groups, and broader dictionary definitions, like the one in the question, also include political and cultural groups. Assimilation is the process by which a minority group gradually adopts the culture of the dominant group. Amalgamation is the blending of groups into a new, combined culture through intermarriage and mixing. Segregation is the enforced physical and social separation of groups, which keeps a group apart rather than destroying it.

Correct answer: Social stratification

Social stratification is the ranking of individuals and groups into layers, such as classes or castes, according to how much wealth, power and prestige they hold. Social differentiation is the broader process of distinguishing people by roles, traits or statuses without necessarily ranking them; stratification is differentiation that has become a hierarchy. Social reproduction is the process by which an existing hierarchy is passed from one generation to the next. Social mobility is the movement of people up or down between those layers.

Correct answer: It degrades mRNA molecules, blocking their translation

MicroRNAs are short non-coding RNAs that pair with complementary sequences in target messenger RNAs and, working through the RNA-induced silencing complex, mark those messages for destruction or keep ribosomes from using them, so "It degrades mRNA molecules, blocking their translation" describes their role: they lower gene expression after transcription. "It stabilizes mRNA molecules, enhancing their translation" reverses that effect. "It methylates DNA sequences, blocking their transcription" describes DNA methyltransferases, which silence genes at the level of DNA. "It binds DNA sequences, enhancing their transcription" describes a transcriptional activator; miRNA acts on messenger RNA, not on DNA.

Correct answer: It forms as lateral folds of ectoderm fuse together

During neurulation the notochord signals the overlying ectoderm to thicken into the neural plate; the edges of the plate rise as neural folds, and "It forms as lateral folds of ectoderm fuse together" describes what follows: the folds meet and fuse at the midline, closing the neural tube that becomes the brain and spinal cord. "It forms as lateral folds of mesoderm fuse together" names the wrong germ layer; mesoderm gives rise to the notochord, somites, muscle and bone. "It forms as the endoderm rolls into a hollow canal" describes the origin of the gut tube, not the neural tube. "It forms as the notochord rolls into a hollow canal" confuses the structure that induces the neural tube with the tube itself; the notochord is a solid mesodermal rod that later contributes to the intervertebral discs.

Correct answer: To pump sodium ions out and potassium ions in, maintaining the resting membrane potential

The sodium-potassium pump uses the energy of ATP to move three sodium ions out of the neuron and two potassium ions in during each cycle, so its primary function is "To pump sodium ions out and potassium ions in, maintaining the resting membrane potential": the concentration gradients it builds underlie the resting potential and every later action potential. "To pump sodium ions in and potassium ions out, maintaining the resting membrane potential" reverses the direction of transport. "To pump sodium ions out and potassium ions in, repolarizing the membrane after each spike" credits the pump with repolarization, which is produced by potassium leaving through voltage-gated potassium channels; the pump works far too slowly to reset the membrane spike by spike. "To pump sodium ions in and potassium ions out, repolarizing the membrane after each spike" describes the ion movements of the action potential itself, which pass through voltage-gated channels rather than through the pump.

Correct answer: It binds to an allosteric site, decreasing Vmax while leaving Km unchanged

A noncompetitive inhibitor binds a site other than the active site and can attach whether or not substrate is already bound, so the correct description is "It binds to an allosteric site, decreasing Vmax while leaving Km unchanged": it lowers the amount of working enzyme, which lowers Vmax, the maximum reaction rate, but it does not change the enzyme's affinity for its substrate, so Km stays the same. "It binds to the active site, increasing Km while leaving Vmax unchanged" describes a competitive inhibitor, whose effect can be overcome by adding more substrate. "It binds to the active site, decreasing Vmax while leaving Km unchanged" puts the inhibitor in the wrong place, since an inhibitor that occupies the active site competes with substrate. "It binds to an allosteric site, increasing Km while leaving Vmax unchanged" pairs the right binding site with the kinetic pattern of competitive inhibition.

Correct answer: Plasma cells

Plasma cells are fully differentiated B lymphocytes whose main job is to synthesize and secrete large amounts of antibody, which makes them the effector cells of the humoral immune response. Helper T cells release cytokines that help activate B cells, but they do not make antibodies themselves. Mast cells carry IgE antibodies on their surface and release histamine when an antigen binds them; they display antibodies made by plasma cells rather than producing them. Dendritic cells capture antigens and present them to T cells, starting the adaptive response rather than secreting antibodies.

Correct answer: Acetyl-CoA

Acetyl-CoA carries a two-carbon acetyl group on coenzyme A; in the first step of the citric acid cycle, citrate synthase transfers that acetyl group to the four-carbon oxaloacetate, forming the six-carbon citrate. Succinyl-CoA is a later intermediate of the cycle, made from alpha-ketoglutarate and converted to succinate, and it donates no acetyl group to oxaloacetate. Malonyl-CoA is the building block of fatty acid synthesis, adding two carbons at a time to a growing fatty acid chain. Stearoyl-CoA is the activated form of the 18-carbon stearic acid; beta-oxidation breaks fatty acyl-CoAs like it down into acetyl-CoA, which only then enters the cycle.

Correct answer: To signal satiety and regulate energy balance by curbing hunger

Leptin is released by fat cells in proportion to the body's fat stores and acts on the hypothalamus to report that energy reserves are adequate, so its primary role is "To signal satiety and regulate energy balance by curbing hunger". "To stimulate appetite and slow the metabolism by signaling fasting" is closer to the role of ghrelin, the hunger hormone released by an empty stomach, which works against leptin. "To raise blood glucose and mobilize fuel by breaking down glycogen" describes glucagon. "To aid digestion of fats and proteins by triggering enzyme release" describes cholecystokinin, the gut hormone that stimulates pancreatic enzyme secretion after a meal.

Correct answer: Entering the cell and directly activating specific genes

Steroid hormones are lipid-soluble, so they diffuse through the plasma membrane and bind receptors inside the cell; the hormone-receptor complex then binds specific DNA sequences and acts as a transcription factor. That is why "Entering the cell and directly activating specific genes" describes their action. "Binding membrane receptors and creating second messengers" is the mechanism of water-soluble peptide and amine hormones such as glucagon and epinephrine, which cannot cross the membrane. "Binding membrane receptors and triggering kinase cascades" describes receptor tyrosine kinases, the route used by insulin and many growth factors. "Entering the cell and directly speeding up translation" misplaces the effect: steroid hormones change which genes are transcribed, and any rise in protein synthesis follows from that.

Correct answer: Oxygen

Oxygen is a small, nonpolar gas that dissolves readily in the lipid membranes of the capillary endothelium, so it crosses the blood-brain barrier by simple passive diffusion. Glucose is polar and crosses only by facilitated transport through GLUT1 carriers. Dopamine does not cross the barrier, which is why Parkinson disease is treated with its precursor levodopa, which an amino acid transporter carries across. Mannitol is a polar sugar alcohol that stays out of brain tissue, which is why it can draw water out of the brain when it is used to lower intracranial pressure.

Correct answer: It allows flexible base pairing at the third position of the mRNA codon, so one tRNA can recognize several codons

In Francis Crick's wobble hypothesis, the first base of the anticodon pairs loosely with the third base of the codon, so "It allows flexible base pairing at the third position of the mRNA codon, so one tRNA can recognize several codons" is correct; this is why a cell needs fewer than 61 kinds of tRNA to read every sense codon. "It allows flexible base pairing at the first position of the mRNA codon, so one tRNA can recognize several codons" puts the wobble at the wrong end of the codon, where pairing must be exact. "It enforces exact base pairing at the third position of the mRNA codon, so each tRNA can recognize a single codon" reverses the effect of wobble. "It enforces exact base pairing at the first position of the mRNA codon, so each tRNA can recognize a single codon" is wrong on both counts.

Correct answer: The tendency to favor one answer, independent of sensitivity

Response bias is the tendency to favor one answer, independent of sensitivity: a liberal participant leans toward saying the light is there and a conservative one toward saying it is not, whatever their actual ability to see it. The ability to distinguish signal from noise, independent of bias, is sensitivity (d′), the other quantity signal detection theory separates out. The weakest stimulus detected on half the trials is the absolute threshold, and the smallest change in a stimulus that can be detected is the difference threshold, or just-noticeable difference.

Correct answer: It is a constant proportion of the starting stimulus intensity

Under Weber's law the just-noticeable difference scales with the stimulus: it is a constant proportion of the starting stimulus intensity, so the ratio of the change to the baseline (the Weber fraction) stays the same, and a heavier weight or a brighter light needs a proportionally larger change before a difference is noticed. Saying it is a fixed amount independent of the starting stimulus intensity is the misconception the law corrects. Saying it grows with the logarithm of the starting stimulus intensity borrows Fechner's law, which describes perceived magnitude rather than the threshold, and saying it grows with the square of the starting stimulus intensity confuses it with a power function such as Stevens' law.

Correct answer: Perceptual learning, in which practice sharpens the ability to spot features

This is perceptual learning, in which practice sharpens the ability to spot features: with exposure to the new language, the listener learns to separate sounds that the first language treated as the same. Sensory adaptation, in which steady stimulation dulls later responses, would make sounds harder to notice, not easier. Sensitization, in which one strong stimulus heightens later responses, raises responsiveness in general rather than teaching a fine discrimination. Response bias, in which the listener grows more willing to report a difference, changes only the decision criterion, not the ability to hear the distinction.

Correct answer: The fovea centralis

The fovea centralis, the small pit at the center of the retina, is packed almost entirely with cones, each with a nearly private line to the brain, so it gives the sharpest acuity wherever the eye is pointed. The optic disc, where the optic nerve leaves the eye, has no photoreceptors at all and forms the blind spot. The peripheral retina is dominated by rods, whose signals converge heavily onto shared ganglion cells, so its acuity is low, and the crystalline lens focuses light onto the retina but contains no photoreceptors.

Correct answer: Through three cone types tuned to distinct wavelength bands

The trichromatic theory holds that color is encoded through three cone types tuned to distinct wavelength bands (short, medium and long, roughly blue, green and red), each most sensitive to its own range; the brain reads a hue from the ratio of their activity. Coding through opponent channels tuned to complementary color pairs is the opponent-process theory, which describes a later stage of processing. Coding through a separate receptor matched to every hue would need a different receptor for each of the many hues people can tell apart, rather than three cone types whose signals combine, and coding through neural firing rates matched to light frequency is impossible, because light oscillates hundreds of trillions of times per second, far faster than any neuron can fire.

Correct answer: Opponent-process theory

Opponent-process theory explains it: color is coded in opposing pairs (red-green, blue-yellow, black-white), so staring at green fatigues the green side of the red-green channel, and when the gaze moves to a white wall the rebound of the opposing side produces a red afterimage. Trichromatic theory describes the three cone types but cannot explain why the afterimage takes the complementary color. Feature integration theory concerns how attention binds features such as color and shape into objects, and retinex theory explains color constancy across changes in lighting, not afterimages.

Correct answer: Law of proximity

The law of proximity says that elements close together in space are grouped into one unit, so dots spaced in pairs are seen as pairs. The law of similarity groups elements that share features such as color or shape, the law of closure fills in gaps so that an incomplete outline is seen as a whole figure, and the law of symmetry groups mirror-image elements into a single object.

Correct answer: Assimilation

Assimilation is fitting new information into an existing schema without changing the schema, as when a child who knows only dogs calls a new four-legged animal a dog. Accommodation changes the schema to fit the new information, equilibration is the balancing of assimilation and accommodation that drives cognitive growth, and organization is the combining of simple schemas into larger, coordinated ones.

Correct answer: Conservation

The child has not yet mastered conservation, the understanding that a quantity stays the same when only its shape or arrangement changes; a preoperational child centers on the water's height and ignores the narrower width. Object permanence, deferred imitation and symbolic thought are all earlier achievements (the first two belong to the sensorimotor stage, the last marks the start of the preoperational stage), so a child doing this task already shows them; none is what the task tests.

Correct answer: The punishment-obedience orientation of the preconventional level

Reasoning built on avoiding punishment and obeying authority marks the punishment-obedience orientation of the preconventional level, Kohlberg's first stage, in which an act is wrong simply because it is punished. The instrumental-exchange orientation of the preconventional level (stage 2) judges acts by what they gain the individual. The interpersonal-conformity orientation of the conventional level (stage 3) seeks approval and wants to be seen as good, and the social-order-maintaining orientation of the conventional level (stage 4) obeys laws to keep society running rather than to escape punishment.

Correct answer: The gap between unaided and guided performance

The zone of proximal development is the gap between unaided and guided performance: the tasks a learner cannot yet manage alone but can manage with help from a more knowledgeable other, which is where instruction does the most good. The gap between mental and chronological age is Binet's mental-age comparison, the basis of the old ratio IQ, not a Vygotskian idea. The point at which private speech turns into thought is Vygotsky's account of inner speech, and the point at which scaffolding is withdrawn is the end of support within the zone, not the zone itself.

Correct answer: Avoidant attachment

Avoidant attachment is marked by little distress at separation and by avoiding or ignoring the caregiver at reunion; these children seem indifferent to the caregiver's presence. Children with secure attachment are upset at separation and quickly comforted at reunion; resistant attachment (also called anxious-ambivalent) shows intense distress and an angry, clinging resistance to comfort; and disorganized attachment shows contradictory, confused behavior, often linked to frightening or inconsistent care.

Correct answer: Chomsky's nativist theory of a language acquisition device

Chomsky's nativist theory of a language acquisition device holds that children are born with a mental capacity for grammar (a universal grammar), which explains how quickly they master complex rules from limited and imperfect input. Skinner's behaviorist theory of language acquisition by reinforcement attributes speech to imitation and reinforcement; Bruner's theory of a language acquisition support system stresses the caregiver routines that scaffold early talk; and Vygotsky's sociocultural theory of language acquisition by interaction treats language as internalized from social exchange. None of the three posits an inborn grammar faculty.

Correct answer: Language influences thought and perception

Linguistic relativity holds that language influences thought and perception: the categories and grammar a language provides shape how its speakers attend to, sort and remember experience, for example in how they divide up colors or describe space and time. The view that thought forms first, and language follows it, is closer to Piaget's account of cognitive development. The claim that language and thought develop apart, then merge, is Vygotsky's, and the claim that thought and inner language are identical (thinking as silent speech) is the behaviorist position of John B. Watson.

Correct answer: Fast, automatic and intuitive thinking

System 1 thinking is fast, automatic and intuitive thinking: it runs with little effort or awareness and produces quick impressions and gut judgments. Deliberate, effortful and slow thinking describes System 2, which also carries out conscious, step-by-step reasoning and leans on working memory. Reflective, self-monitoring and strategic thinking describes metacognition, the monitoring of one's own thought, and original, flexible and divergent thinking describes creative idea generation; neither defines System 1.

Correct answer: Functional fixedness

Being unable to see any use for an object beyond its usual one is functional fixedness, as when someone fails to think of using a coin as a screwdriver. Confirmation bias is seeking out evidence that supports what one already believes, belief perseverance is clinging to a belief after the evidence for it has been discredited, and the representativeness heuristic judges likelihood by resemblance to a prototype; none describes being stuck on an object's familiar function.

Correct answer: The availability heuristic

Judging how likely an event is by how easily examples come to mind is the availability heuristic, which is why vivid or recent events such as plane crashes seem more common than they are. The representativeness heuristic judges likelihood by similarity to a prototype, the affect heuristic lets one's feelings about something drive judgments of its risks and benefits, and the recognition heuristic simply favors whichever option one recognizes.

Correct answer: REM sleep

REM sleep, sometimes called paradoxical sleep, is the stage with vivid dreaming, rapid eye movements and muscle atonia: brain activity looks almost like waking, while most skeletal muscles are temporarily paralyzed, which keeps the sleeper from acting out dreams. Stage 1 NREM sleep is the brief transition into sleep, stage 2 NREM sleep is marked by sleep spindles and K-complexes, and slow-wave sleep (stage 3, or N3) is the deep stage from which a sleeper is hardest to wake.

Correct answer: The suprachiasmatic nucleus of the hypothalamus

The suprachiasmatic nucleus of the hypothalamus is the master circadian clock: it receives light information directly from the retina through the retinohypothalamic tract and sets the roughly 24-hour rhythms of the body's other clocks. The pineal gland of the epithalamus secretes melatonin, but on a schedule the suprachiasmatic nucleus sets, rather than acting as the clock itself. The dorsal lateral geniculate nucleus of the thalamus relays visual images to the cortex, and the anterior lobe of the pituitary gland releases hormones under hypothalamic control; neither keeps circadian time.

Correct answer: Interpretations of random neural activity during REM sleep

Under the activation-synthesis hypothesis of Hobson and McCarley, dreams are interpretations of random neural activity during REM sleep: the brainstem fires more or less at random, and the cortex synthesizes those signals into the most coherent story it can. Disguised expressions of repressed wishes surfacing while asleep is Freud's psychoanalytic view; replays of daytime memories stored during NREM sleep reflects the memory-consolidation view; and rehearsals of threatening events staged while asleep is the threat-simulation theory.

Correct answer: Cocaine

Cocaine is a stimulant: by blocking the reuptake of dopamine and norepinephrine it raises heart rate, alertness and arousal, speeding up activity in the central nervous system. Alcohol is a depressant that slows neural activity, barbiturates are sedative-hypnotic depressants, and codeine is an opioid that depresses the central nervous system while relieving pain, so none of the three increases CNS activity the way a stimulant does.