Your FREE AP Biology Flashcards 2026 – 200+ Cards

A weak attraction between a slightly positive hydrogen and a slightly negative atom (like O or N). Holds water molecules and DNA strands together.

Water's bent shape gives it a partially negative oxygen and partially positive hydrogens, making it polar and able to form hydrogen bonds.

Water resists temperature change because hydrogen bonds absorb heat; this buffers organisms and oceans against swings.

In solid water, hydrogen bonds lock molecules into a spacious lattice that is less dense than liquid water, so ice floats and insulates the water below.

Water-loving — polar or charged molecules that dissolve readily in water.

Water-fearing — nonpolar molecules (like lipids) that do not dissolve in water and cluster away from it.

A reaction joining two monomers into a polymer by removing one molecule of water; also called condensation.

A reaction breaking a polymer into monomers by adding one molecule of water across the bond — the reverse of dehydration synthesis.

A macromolecule of monosaccharide monomers; provides quick energy (glucose) and structure (cellulose, starch, glycogen).

A single sugar unit, such as glucose or fructose; the monomer of carbohydrates.

A nonpolar, hydrophobic macromolecule (fats, phospholipids, steroids) used for energy storage, membranes, and signaling. Not a true polymer.

Saturated fats have no carbon–carbon double bonds (solid at room temp); unsaturated fats have one or more double bonds with kinks (liquid).

A lipid with a hydrophilic phosphate head and two hydrophobic fatty-acid tails; forms the cell membrane bilayer.

A macromolecule of amino acid monomers that serves as enzymes, structure, transport, and signals; its shape determines its function.

The monomer of proteins; has an amino group, a carboxyl group, and a variable R group that sets its properties.

The covalent bond linking amino acids in a protein, formed by dehydration synthesis.

Primary (sequence) → secondary (alpha helices and beta sheets) → tertiary (3D fold) → quaternary (multiple subunits).

Loss of a protein's functional shape (and activity) when heat or pH disrupts its weak bonds — its sequence is unchanged.

A macromolecule of nucleotide monomers that stores and transmits genetic information — DNA and RNA.

The monomer of nucleic acids: a phosphate group, a five-carbon sugar, and a nitrogenous base.

Adenine pairs with thymine (A–T) and cytosine pairs with guanine (C–G) by hydrogen bonds.

The two strands of a DNA double helix run in opposite directions — one 5' to 3', the other 3' to 5'.

DNA is double-stranded with deoxyribose and thymine; RNA is single-stranded with ribose and uracil (replacing thymine).

Nucleic-acid strands have a 5' phosphate end and a 3' hydroxyl end; new nucleotides are added only to the 3' end.

A measure of hydrogen-ion concentration; below 7 is acidic, 7 is neutral, above 7 is basic. Enzymes work best near a specific pH.

A large molecule built from repeating monomer subunits joined by covalent bonds.

All living things are made of cells, the cell is the basic unit of life, and all cells come from preexisting cells.

A small cell with no membrane-bound nucleus or organelles; DNA is a single circular molecule in the cytoplasm (bacteria and archaea).

A cell with a membrane-bound nucleus and organelles (plants, animals, fungi, protists).

The two-layer sheet of phospholipids forming the cell membrane: hydrophilic heads face the water, hydrophobic tails face inward.

The membrane is a fluid bilayer with embedded proteins, cholesterol, and carbohydrates that move laterally.

The membrane lets small nonpolar molecules cross freely while controlling or blocking ions and large polar molecules.

Movement of small nonpolar molecules across a membrane down their concentration gradient, requiring no energy.

Passive movement of substances down their gradient through channel or carrier proteins; no energy needed.

The diffusion of water across a selectively permeable membrane toward the side with more solute.

Movement of a substance against its concentration gradient using energy (ATP), e.g., the sodium-potassium pump.

A solution with more solute than the cell; water leaves the cell, which shrinks (crenates).

A solution with less solute than the cell; water enters the cell, which swells and may burst (lyse).

A solution with equal solute concentration to the cell; no net movement of water.

The organelle that stores the cell's DNA and controls gene expression; surrounded by a double-membrane nuclear envelope.

The site of protein synthesis (translation); free in the cytoplasm or bound to the rough ER. Made of rRNA and protein.

ER studded with ribosomes that synthesizes and folds proteins destined for membranes or secretion.

ER without ribosomes that synthesizes lipids and detoxifies drugs and poisons (abundant in liver cells).

The organelle that modifies, sorts, packages, and ships proteins and lipids in vesicles.

The site of aerobic cellular respiration and most ATP production; has a double membrane and its own DNA.

The site of photosynthesis in plants and algae; contains chlorophyll, thylakoids, and stroma, and its own DNA.

A membrane-bound sac of digestive enzymes that breaks down waste, debris, and worn-out organelles at acidic pH.

A storage organelle; the large central vacuole of plant cells maintains turgor pressure and stores water and ions.

A rigid layer outside the membrane (cellulose in plants) that provides support and protection.

Mitochondria and chloroplasts arose from free-living prokaryotes engulfed by a host cell; evidence: own DNA, ribosomes, double membranes.

As a cell grows, volume rises faster than surface area; a high ratio (small cells) speeds exchange across the membrane.

Wedged between phospholipids, cholesterol buffers membrane fluidity — keeping it fluid in cold and stable in heat.

A biological catalyst (usually a protein) that speeds a reaction by lowering its activation energy without being consumed.

The energy barrier that must be overcome for a reaction to start; enzymes lower it.

The region of an enzyme that binds the substrate; its shape gives the enzyme its specificity.

The reactant molecule an enzyme acts on, binding at the active site.

The active site changes shape slightly to grip the substrate more tightly once it binds.

An inhibitor binds the active site and competes with the substrate; can be overcome by adding more substrate.

An inhibitor binds away from the active site, changing the enzyme's shape so it works less well; not reversed by adding substrate.

Each enzyme has an optimal temperature and pH; beyond it, activity drops and the enzyme can denature.

Adenosine triphosphate, the cell's energy currency; energy is released when its terminal phosphate bond is broken to form ADP.

6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂. Converts light energy into chemical energy stored in sugar.

Occur in the thylakoid membranes; split water (releasing O₂) and produce ATP and NADPH using light energy.

The light-independent reactions in the stroma that use ATP and NADPH to fix CO₂ into sugar.

The O₂ released comes from splitting water (H₂O) in the light reactions, not from CO₂.

The green pigment in chloroplasts that absorbs light (mainly red and blue) to drive the light reactions.

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP. Releases the energy stored in glucose.

The first stage of respiration, in the cytoplasm: splits glucose into two pyruvate with a small net gain of ATP and NADH. No oxygen needed.

Occurs in the mitochondrial matrix; oxidizes pyruvate, releases CO₂, and loads NADH and FADH₂.

On the inner mitochondrial membrane; electrons drive proton pumping, and chemiosmosis powers ATP synthase. Oxygen is the final electron acceptor.

In aerobic respiration, oxygen accepts electrons at the end of the chain, forming water.

Using a proton (H⁺) gradient across a membrane to drive ATP synthase and make ATP — in both respiration and photosynthesis.

The enzyme that makes ATP as protons flow through it down their gradient (chemiosmosis).

Anaerobic pathway that regenerates NAD⁺ so glycolysis can continue without oxygen; yields little ATP (lactic acid or alcohol).

Aerobic respiration uses oxygen and yields much ATP; anaerobic processes (fermentation) do not use oxygen and yield little.

Electron carriers that shuttle high-energy electrons from glycolysis and the Krebs cycle to the electron transport chain.

The three-carbon product of glycolysis; enters the mitochondrion for the Krebs cycle when oxygen is present.

Reception (signal binds receptor) → transduction (signal relayed and amplified inside) → response (change in cell activity).

A signaling molecule that binds a specific receptor to trigger a cellular response.

A protein that binds a specific signal molecule; only cells with the right receptor respond to a given signal.

The relay of a signal inside a cell after a ligand binds, often through a cascade that amplifies the message.

A small intracellular molecule (such as cyclic AMP or calcium ions) that spreads a signal rapidly through the cytoplasm.

A chain of protein kinases that activate one another by transferring phosphate groups, amplifying a signal.

Hydrophobic signals (steroids) cross the membrane to intracellular receptors; hydrophilic signals bind surface receptors.

Hormones secreted into the bloodstream act on distant target cells that carry the matching receptor.

A loop that counteracts a change to restore a set point and maintain homeostasis (cooling when overheated).

A loop that amplifies a change to drive a process to completion (blood clotting, childbirth contractions).

The maintenance of a stable internal environment despite external change, mostly by negative feedback.

Programmed cell death — a controlled, signal-driven self-destruction used in development and to remove damaged cells.

The ordered sequence a cell follows to divide: interphase (G₁, S, G₂) followed by mitosis and cytokinesis.

The longest phase of the cell cycle: the cell grows (G₁), copies its DNA (S), and prepares to divide (G₂).

Nuclear division producing two genetically identical diploid daughter cells, used for growth and repair.

Prophase → metaphase → anaphase → telophase, followed by cytokinesis.

The division of the cytoplasm that splits one cell into two after mitosis.

Control points (G₁, G₂, M) that verify each stage is complete before the cell proceeds, driven by cyclins and Cdks.

Uncontrolled cell division caused by mutations that disable cell-cycle checkpoints and apoptosis.

The two identical copies of a chromosome joined at the centromere after DNA replication, separated in anaphase.

A common membrane receptor that activates a G protein when a signal binds, triggering second messengers like cyclic AMP.

Mitosis makes 2 identical diploid cells (growth/repair); meiosis makes 4 genetically varied haploid gametes (reproduction).

Fertilization fuses two haploid gametes (n + n) to restore the diploid (2n) chromosome number in the zygote.

Cell division producing four haploid gametes, halving the chromosome number and generating genetic variation.

Diploid (2n) cells have two sets of chromosomes; haploid (n) gametes have one set.

A matched pair of chromosomes (one from each parent) carrying the same genes, that pair up in meiosis I.

The exchange of segments between homologous chromosomes in prophase I, creating new allele combinations.

Homologous pairs line up and separate randomly in meiosis I, mixing alleles of different genes into gametes.

Meiosis I separates homologous chromosomes (reduction to haploid); meiosis II separates sister chromatids.

A haploid reproductive cell (sperm or egg) that fuses with another in fertilization to form a diploid zygote.

The failure of chromosomes to separate properly in meiosis, producing gametes with extra or missing chromosomes.

One of the alternative versions of a gene that can occupy a given locus.

Genotype is the genetic makeup (alleles); phenotype is the observable trait that results.

Homozygous = two identical alleles (TT or tt); heterozygous = two different alleles (Tt).

A dominant allele is expressed in the heterozygote; a recessive allele is masked unless homozygous.

Mendel's principle that the two alleles of a gene separate during gamete formation, one per gamete.

Mendel's principle that alleles of different genes (on different chromosomes) are inherited independently.

A grid that predicts offspring genotypes and phenotypes from the parents' alleles.

Crossing two heterozygotes (Tt × Tt) gives a 3:1 phenotype ratio and a 1:2:1 genotype ratio.

Crossing two double heterozygotes (RrYy × RrYy) gives a 9:3:3:1 phenotype ratio.

Crossing an individual of unknown genotype with a homozygous recessive to reveal whether it is homozygous or heterozygous.

The heterozygote shows a blended, intermediate phenotype (red × white snapdragons → pink).

Both alleles are fully and separately expressed in the heterozygote (AB blood type shows both A and B antigens).

A gene with more than two possible alleles in the population, such as the ABO blood group (Iᴬ, Iᴮ, i).

A trait controlled by many genes, producing continuous variation (skin color, height).

A trait whose gene is on a sex chromosome (usually X); X-linked recessive traits appear more often in males.

Males have one X chromosome, so a single recessive allele is enough to show the trait (e.g., color blindness).

A chart tracking a trait through generations of a family, used to deduce inheritance patterns.

The flow of genetic information in cells: DNA → RNA → protein, via transcription and translation.

DNA replication in which each new molecule keeps one original (template) strand and one newly made strand.

The enzyme that builds a new DNA strand by adding nucleotides to the 3' end of the growing strand.

The enzyme that unwinds and separates the two DNA strands at the replication fork.

Short DNA segments synthesized on the lagging strand and later joined by DNA ligase.

The synthesis of messenger RNA from a DNA template by RNA polymerase, the first step of gene expression.

The enzyme that builds an mRNA strand from a DNA template during transcription.

The assembly of a protein at the ribosome, reading mRNA codons and adding the matching amino acids via tRNA.

A sequence of three mRNA nucleotides that specifies one amino acid or a start/stop signal.

The three-base sequence on a tRNA that pairs with a complementary mRNA codon during translation.

Messenger RNA carries the code; transfer RNA brings amino acids; ribosomal RNA forms part of the ribosome.

AUG (methionine) starts translation; UAA, UAG, and UGA signal it to stop.

In eukaryotes, a primary transcript has introns removed (splicing) and exons joined before leaving the nucleus.

A cluster of bacterial genes controlled together by one promoter and operator, such as the lac operon.

An inducible operon in E. coli; lactose (allolactose) inactivates the repressor, turning on lactose-digesting genes.

The DNA sequence where RNA polymerase binds to begin transcription of a gene.

A protein that binds DNA to turn the transcription of specific genes up or down in eukaryotes.

Heritable changes in gene expression without changes to the DNA sequence — DNA methylation and histone modification.

Adding methyl groups to DNA, which typically silences gene expression by making the gene less accessible.

How cells with identical DNA become specialized by expressing different subsets of genes.

A change in a single DNA base; may be silent, missense (changes an amino acid), or nonsense (creates a stop).

An insertion or deletion that shifts the reading frame, usually altering every codon downstream.

Mutations are the ultimate source of new alleles and genetic variation on which selection acts.

Polymerase chain reaction — a technique that amplifies (copies) a target DNA sequence through repeated heating and cooling.

Nearly all organisms use the same codons for the same amino acids — strong evidence of common ancestry.

The process by which individuals with heritable advantageous traits survive and reproduce more, shifting allele frequencies.

An individual's reproductive success — how many offspring it leaves relative to others in the population.

A heritable trait that improves survival or reproduction in a given environment, shaped by natural selection.

A change in the heritable allele frequencies of a population over generations (descent with modification).

Selection favoring one extreme phenotype, shifting the population's trait distribution toward it.

Selection favoring the intermediate phenotype and against extremes (e.g., human birth weight).

Selection favoring both extremes over the intermediate, which can split a population.

Selection driven by differences in mating success, producing traits like elaborate plumage or large antlers.

A random change in allele frequencies due to chance, with the greatest effect in small populations.

A form of genetic drift in which a population is sharply reduced by a chance event, narrowing its gene pool.

A form of genetic drift in which a few individuals start a new population, carrying only a subset of the alleles.

The movement of alleles between populations through migration; tends to make populations more similar.

A non-evolving state where p + q = 1 and p² + 2pq + q² = 1, given no mutation, no selection, random mating, no gene flow, and a large population.

p is the frequency of the dominant allele and q the recessive; p² is homozygous dominant, 2pq heterozygous, q² homozygous recessive.

No mutation, no natural selection, random mating, no gene flow, and a large population.

The formation of new species, usually when populations become reproductively isolated and diverge genetically.

Barriers (prezygotic or postzygotic) that prevent populations from interbreeding, driving speciation.

Structures shared by related species due to common ancestry (the forelimb bones of mammals); evidence for evolution.

A branching diagram showing evolutionary relationships and common ancestry among species.

A diagram grouping organisms by shared derived characters to show their branching evolutionary relationships.

Similarities in DNA and protein sequences across species reveal shared ancestry; more similarity means closer relationship.

Unrelated species independently evolve similar traits because they face similar environments (analogous structures).

The principle that all life descends from a shared ancestor, supported by the universal genetic code and shared biochemistry.

An example of natural selection: resistant bacteria survive antibiotics and reproduce, so resistance spreads.

Inherited differences among individuals — the raw material natural selection acts on; without it, no evolution.

An organism that makes its own food, usually by photosynthesis; the base of the food chain (plants, algae).

An organism that gets energy by eating other organisms — primary (herbivores), secondary, and tertiary consumers.

An organism (fungi, bacteria) that breaks down dead matter and waste, returning nutrients to the environment.

A feeding position in a food chain — producers, primary consumers, secondary consumers, and so on.

Only about 10% of the energy at one trophic level is passed to the next; the rest is lost as heat and metabolism.

A network of interconnected food chains showing the many feeding relationships in an ecosystem.

Energy flows one way through an ecosystem and is lost as heat; matter (carbon, nitrogen) is recycled repeatedly.

Carbon moves from the atmosphere into organisms by photosynthesis and back out by respiration and combustion.

The conversion of atmospheric nitrogen gas into usable compounds (ammonia) by bacteria, including those in legume root nodules.

The movement of nitrogen through fixation, nitrification, assimilation, and denitrification among air, soil, and organisms.

Accelerating population growth with unlimited resources, producing a J-shaped curve.

Population growth that slows as it nears the carrying capacity, producing an S-shaped curve.

The maximum population size an environment can sustain given its resources; logistic growth levels off here.

Limits that intensify as a population grows — competition, predation, and disease.

Limits that affect a population regardless of its size — weather, natural disasters, fire.

A symbiosis in which both species benefit (+/+), such as bees pollinating flowers.

A symbiosis in which one species benefits and the other is unaffected (+/0).

A symbiosis in which one species benefits at the expense of the host (+/−), such as a tapeworm.

An interaction in which organisms vie for the same limited resource, harming both (−/−).

An interaction in which one organism (predator) kills and eats another (prey).

The gradual, predictable change in a community over time, from a disturbance toward a stable climax community.

The variety of life in an ecosystem; greater biodiversity generally increases stability and resilience.

All the individuals of one species living in the same area and interbreeding.

All the populations of different species living and interacting in the same area.

A community of organisms together with the nonliving (abiotic) factors of their environment.