Monosaccharides; quick energy and structure (cellulose, starch, glycogen)

Glycerol + fatty acids (not true polymers); energy storage, membranes, hormones

Amino acids; enzymes, structure, transport, signaling — folded into specific shapes

Nucleotides; store and transmit genetic information (DNA, RNA)

Small nonpolar molecules cross directly, down the gradient — no energy

Facilitated diffusion

Channel or carrier proteins move substances down the gradient — no energy

Diffusion of water across a membrane toward the more concentrated (solute) side

Pumps move substances against the gradient using ATP (e.g., sodium-potassium pump)

Endocytosis brings material in; exocytosis sends it out — both use energy

Both species benefit (+/+) — e.g., bees and flowers

One benefits, the other is unaffected (+/0)

One benefits, the other is harmed (+/−) — e.g., a tapeworm and its host

Predator benefits, prey is killed (+/−)

Both are harmed as they vie for the same resource (−/−)

How is the AP Biology exam scored?

AP exams are scored on a 1–5 scale. The multiple-choice and free-response sections each contribute 50% of the composite score, which is then converted to the final 1–5. A score of 3 is generally considered passing, and a 3, 4, or 5 may earn college credit or placement, depending on the institution.

What units are on the AP Biology exam?

Eight College Board units: (1) Chemistry of Life, (2) Cell Structure and Function, (3) Cellular Energetics, (4) Cell Communication and Cell Cycle, (5) Heredity, (6) Gene Expression and Regulation, (7) Natural Selection, and (8) Ecology. Natural Selection is the most heavily weighted unit (13–20%).

What are the science practices on AP Biology?

AP Biology tests six science practices: (1) Concept Explanation, (2) Visual Representations, (3) Question and Method (experimental design), (4) Representing and Describing Data, (5) Statistical Tests and Data Analysis, and (6) Argumentation. These skills, not just content recall, are tested throughout both sections — especially on the free-response questions.

AP Biology is widely considered one of the more challenging AP science courses because it is broad and emphasizes reasoning, data analysis, and experimental design over memorization. Steady study across all eight units, plus practice with graphs, experiments, and free-response writing, is the most reliable way to score a 3 or higher.

Is a calculator allowed on the AP Biology exam?

Yes. A four-function calculator (with square root), scientific, or graphing calculator is permitted on both sections of the AP Biology exam. The College Board also provides a formula and equation sheet, including the Hardy-Weinberg equations, so you do not need to memorize the formulas.

How should I use this AP Biology study guide?

Work through the eight units in order, from Chemistry of Life to Ecology. After each unit, take the checkpoint quiz to find gaps, then drill that unit with our free practice questions and flashcards. Pay extra attention to Natural Selection, Cellular Energetics, and Gene Expression — the three heaviest units.

Is this AP Biology study guide free?

Yes — the full guide, the checkpoints, the glossary, the practice questions, and the flashcards are 100% free, with no account required.

Why is water able to support life so well?

Water's polarity creates hydrogen bonds, which give it cohesion, adhesion, surface tension, a high specific heat, and the ability to dissolve polar and ionic substances. Because ice is less dense than liquid water, it floats and insulates the water below, allowing aquatic life to survive. This is a Unit 1: Chemistry of Life concept. Water's emergent properties all trace back to hydrogen bonding between its partially charged ends. High specific heat lets organisms and oceans resist large temperature swings; evaporative cooling uses water's high heat of vaporization.

What is the difference between dehydration synthesis and hydrolysis?

Dehydration synthesis builds a polymer by joining two monomers and removing one molecule of water at the new bond. Hydrolysis is the reverse: it breaks a polymer into monomers by adding a molecule of water across the bond. Cells use both to build and break down macromolecules. This is a Unit 1 concept. The four biological macromolecules — carbohydrates, lipids, proteins, and nucleic acids — are assembled and disassembled by these reactions. Dehydration synthesis is also called a condensation reaction because water condenses out as the bond forms.

What determines whether a molecule can cross the cell membrane?

The phospholipid bilayer has a hydrophobic interior, so small nonpolar molecules (O₂, CO₂) and small uncharged polar molecules pass freely, while large or charged particles and ions cannot. Those require transport proteins — channels or carriers — and may need energy for active transport against the gradient. This is a Unit 2: Cell Structure and Function concept. Selective permeability lets the cell control its internal environment. Passive transport (diffusion, facilitated diffusion, osmosis) needs no ATP; active transport and bulk transport (endocytosis/exocytosis) do.

What is the endosymbiotic theory?

Endosymbiotic theory proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by a larger host cell. Evidence: both have their own circular DNA, their own ribosomes that resemble bacterial ribosomes, a double membrane, and they reproduce independently by binary fission inside the cell. This is a Unit 2 concept that links cell structure to evolution. The engulfed prokaryote and host formed a mutually beneficial relationship. Mitochondrial ribosomes are sensitive to antibiotics that target bacteria but not the host's own ribosomes — strong supporting evidence.

How does photosynthesis work?

Photosynthesis converts light energy into chemical energy: 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂. The light-dependent reactions in the thylakoid membranes split water, release O₂, and make ATP and NADPH; the Calvin cycle in the stroma uses that ATP and NADPH to fix CO₂ into sugar. This is a Unit 3: Cellular Energetics concept. The oxygen you breathe comes from splitting water in the light reactions, not from CO₂. The Calvin cycle is light-independent but still requires the ATP and NADPH produced by the light reactions.

What are the stages of cellular respiration?

Aerobic respiration breaks glucose down in stages: glycolysis (in the cytoplasm) splits glucose into pyruvate; the Krebs cycle (in the mitochondrial matrix) releases CO₂ and loads electron carriers; and the electron transport chain (on the inner mitochondrial membrane) uses oxygen as the final electron acceptor to drive ATP synthesis. This is a Unit 3 concept. Most ATP is made by the electron transport chain through chemiosmosis — a proton gradient powering ATP synthase. Without oxygen, cells switch to fermentation, which regenerates NAD⁺ so glycolysis can continue but yields far less ATP.

How do cells communicate through signal transduction?

Cell signaling has three stages: reception (a signal molecule binds a specific receptor), transduction (the signal is relayed and often amplified inside the cell, frequently through a phosphorylation cascade and second messengers like cyclic AMP), and response (a change in cell activity, such as gene expression or an enzyme being switched on). This is a Unit 4: Cell Communication and Cell Cycle concept. Only target cells with the correct receptor respond to a given signal. Hydrophobic signals like steroid hormones cross the membrane and bind intracellular receptors; hydrophilic signals bind surface receptors.

What is the difference between negative and positive feedback?

Negative feedback counteracts a change to restore a set point and maintain homeostasis — for example, the body cooling itself when it gets too hot. Positive feedback amplifies a change to push a process to completion, such as the cascade of platelet activation during blood clotting or contractions during childbirth. This is a Unit 4 concept. Most homeostatic systems (temperature, blood glucose, pH) use negative feedback to stay near a stable value. Positive feedback is far less common because, left unchecked, it drives a system away from stability.

How do you solve a dihybrid cross?

For a dihybrid cross of two heterozygotes (RrYy × RrYy) where the genes assort independently, the offspring phenotype ratio is 9:3:3:1 — nine showing both dominant traits, three and three showing one dominant trait each, and one showing both recessive traits. A 16-box Punnett square or the rules of probability gives the same result. This is a Unit 5: Heredity concept built on Mendel's law of independent assortment. The 9:3:3:1 ratio holds only when the two genes are on different chromosomes (or far apart) so they assort independently.

What is the difference between incomplete dominance and codominance?

In incomplete dominance, the heterozygote shows a blended, intermediate phenotype — a red and a white snapdragon produce pink offspring. In codominance, both alleles are fully and separately expressed in the heterozygote — a person with both A and B blood-group alleles expresses both A and B antigens (type AB). This is a Unit 5 concept. Both are non-Mendelian inheritance patterns that go beyond simple dominant/recessive. Codominance shows both phenotypes at once (AB blood); incomplete dominance shows a mix (pink flowers).

What is the central dogma of molecular biology?

The central dogma describes the flow of genetic information: DNA → RNA → protein. Transcription copies a gene's DNA into messenger RNA; translation reads that mRNA in three-base codons at the ribosome to assemble a chain of amino acids into a protein. Each codon specifies one amino acid via the genetic code. This is a Unit 6: Gene Expression and Regulation concept. In eukaryotes, transcription occurs in the nucleus and translation in the cytoplasm. Retroviruses are an exception — reverse transcriptase makes DNA from an RNA template.

How is gene expression regulated?

Cells control which genes are expressed and when. In bacteria, operons such as the lac operon switch gene clusters on or off in response to the environment. In eukaryotes, regulation occurs at many levels — DNA methylation and histone modification (epigenetics), transcription factors, RNA processing, and the stability of the mRNA. This is a Unit 6 concept that explains how cells with identical DNA become specialized (differentiation). The lac operon is induced by lactose: the inducer inactivates the repressor so the genes for digesting lactose can be transcribed.

How does natural selection drive evolution?

Natural selection requires heritable variation in traits that affect survival and reproduction. Individuals with advantageous traits leave more offspring, so over generations the favorable alleles become more common in the population. The result is a population better adapted to its environment — evolution by descent with modification. This is a Unit 7: Natural Selection concept, the largest unit on the exam (13–20%). Selection acts on phenotypes but changes allele frequencies; it can be directional, stabilizing, or disruptive.

What is genetic drift and how does it differ from natural selection?

Genetic drift is a random change in allele frequencies caused by chance events, not by fitness. It has the biggest effect in small populations, where chance can quickly fix or eliminate alleles. Unlike natural selection — which is non-random and favors advantageous traits — drift is random and can fix harmful or neutral alleles. This is a Unit 7 concept. The bottleneck effect and the founder effect are two forms of genetic drift. Gene flow (migration of alleles between populations) tends to make populations more genetically similar.

Why is only about 10% of energy passed between trophic levels?

Energy flows one way through an ecosystem, and only about 10% of the energy stored at one trophic level is captured by the next. The rest is lost as heat through metabolism, used in life processes, or never consumed. This 10% rule limits food chains to a few links and explains why top predators are rare. This is a Unit 8: Ecology concept. Producers capture the most energy; energy available shrinks at each higher level. Matter, unlike energy, is recycled — carbon, nitrogen, and other elements cycle repeatedly through organisms and the environment.

What is the difference between exponential and logistic growth?

Exponential growth produces a J-shaped curve: the population grows at an accelerating rate with unlimited resources. Logistic growth produces an S-shaped curve: growth slows as the population nears the carrying capacity (K), the maximum the environment can sustain, where birth and death rates balance and the population levels off. This is a Unit 8 concept. Real populations show exponential growth only briefly before resource limits impose logistic growth. Density-dependent factors (competition, predation, disease) intensify as a population approaches carrying capacity.

A weak attraction between a slightly positive hydrogen and a slightly negative atom (like oxygen). Hydrogen bonds between water molecules give water its cohesion, high specific heat, and the fact that ice floats.

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

Dehydration synthesis

A reaction that joins two monomers into a polymer by removing one molecule of water. Also called a condensation reaction.

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

The two-layer sheet of phospholipids that forms cell membranes, with hydrophilic heads facing the watery inside and outside and hydrophobic tails in the middle.

Selective permeability

The property of a membrane that lets some substances cross freely while controlling or blocking others.

Movement of a substance across a membrane down its concentration gradient with no energy input (diffusion, facilitated diffusion, osmosis).

Movement of a substance across a membrane against its concentration gradient, requiring energy (usually ATP).

The theory that mitochondria and chloroplasts arose from free-living prokaryotes engulfed by a host cell; supported by their own DNA, ribosomes, and double membranes.

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

The energy barrier that must be overcome for a chemical reaction to proceed. Enzymes lower it.

The process that converts light energy into chemical energy: 6 CO₂ + 6 H₂O + light → glucose + 6 O₂. Light reactions occur in the thylakoids; the Calvin cycle in the stroma.

FREE AP Biology Study Guide 2026 + Quizzes & Readiness Score

This free AP Biology study guide teaches to the current College Board AP Biology exam — all eight units, organized the way the course is built.^[1] AP Biology is a college-level introductory biology course assessed by a single end-of-year exam scored on a 1–5 scale, where a 3 or higher generally earns college credit or placement.^[3]

The exam rewards reasoning as much as recall: half your score comes from free-response questions that ask you to design experiments, analyze data, and argue from evidence. So this guide is interactive, not a wall of text — every unit has a built-in checkpoint quiz, hover-able glossary terms, worked genetics problems, labeled diagrams, and concept questions, so you learn by doing.

Read the guide unit by unit, test yourself at each checkpoint, then round out your free AP Biology prep with our practice questions and flashcards.

AP Biology is one of the 17 AP exams — explore our AP study guides to compare and prep across the whole family.

AP Biology Exam Snapshot

AP Biology exam at a glance (2026)
Detail	AP Biology
Sections	Section I — 60 multiple choice; Section II — 6 free response
Section weights	Each section is 50% of the exam score
Total time	3 hours (90 min multiple choice + 90 min free response)
Free response	2 long FRQs (incl. experimental design) + 4 short FRQs
Score scale	1–5; a 3 or higher generally earns college credit
Units	8 College Board units (Chemistry of Life → Ecology)
Science practices	6 practices: concept explanation, models, experimental design, data, statistics, argumentation
Calculator	Four-function, scientific, or graphing calculator allowed; formula sheet provided
Publisher	College Board

How the AP Biology exam is built — two sections, each worth 50%

Section I — Multiple Choice60 questions · 90 minutes · 50% of the scoreDiscrete questions and question sets, many with data, graphs, or experiments.

Section II — Free Response6 questions · 90 minutes · 50% of the score2 long (incl. an experimental design + an analysis question) and 4 short FRQs.

3 hours total. The two sections are weighted equally, so written reasoning and data analysis matter as much as content recall.

Because the multiple-choice and free-response sections are weighted equally, spread your prep across content and skills.^[3] Spend your study time across all eight units, but know that Natural Selection, Cellular Energetics, and Gene Expression are the three heaviest units — together they can be nearly half the exam:

AP Biology units by exam weight (2026 College Board ranges)

Unit 7 · Natural Selection17% · 13–20%

Unit 3 · Cellular Energetics14% · 12–16%

Unit 6 · Gene Expression14% · 12–16%

Unit 4 · Cell Comm. & Cycle13% · 10–15%

Unit 8 · Ecology13% · 10–15%

Unit 2 · Cell Structure12% · 10–13%

Unit 1 · Chemistry of Life10% · 8–11%

Unit 5 · Heredity10% · 8–11%

AP Biology by unit (2026 College Board exam-weight ranges)

Unit 7 · Natural Selection

13–20%

Unit 3 · Cellular Energetics

12–16%

Unit 6 · Gene Expression

12–16%

Unit 4 · Cell Communication & Cycle

10–15%

Unit 8 · Ecology

10–15%

Unit 2 · Cell Structure & Function

10–13%

Unit 1 · Chemistry of Life

8–11%

Unit 5 · Heredity

8–11%

Natural Selection (Unit 7) is the single heaviest unit; energetics and gene expression follow. Together these three are roughly 40–50% of the exam.

College Board reports unit weights as approximate ranges, so the exact mix shifts slightly each year.^[4] This guide teaches all eight units in the official order, as eight study modules.

1 · Chemistry of Life

8–11% of the exam.The chemical foundation of life: water’s special properties, the four biological macromolecules, and the structure of nucleic acids.^[1]

Water & Hydrogen Bonding

Water is polar, so its molecules form with one another. These weak bonds give water its life-supporting properties: cohesion and surface tension, adhesion, a high specific heat that buffers temperature, evaporative cooling, and the fact that ice is less dense than liquid water (so it floats and insulates the water below). Water is also the universal solvent for polar and ionic substances, while nonpolar molecules cluster away from it.

Biological Macromolecules

The four macromolecules are built from monomers by and broken down by :

The four biological macromolecules
Macromolecule	Monomer & function
Carbohydrates	Monosaccharides; quick energy and structure (cellulose, starch, glycogen)
Lipids	Glycerol + fatty acids (not true polymers); energy storage, membranes, hormones
Proteins	Amino acids; enzymes, structure, transport, signaling — folded into specific shapes
Nucleic acids	Nucleotides; store and transmit genetic information (DNA, RNA)

Nucleic Acids (DNA & RNA)

DNA stores genetic information; RNA helps express it. In DNA, complementary base pairing joins adenine with thymine and cytosine with guanine; in RNA, uracil replaces thymine. The two DNA strands are antiparallel— one runs 5′ to 3′ and the other 3′ to 5′.

Checkpoint · Unit 1 · Chemistry of Life

Question 1 of 10

Ice floats on liquid water, an unusual property among substances. Which characteristic of water best accounts for solid water being less dense than its liquid form?

2 · Cell Structure & Function

10–13% of the exam. Cell membranes and transport, the organelles and their jobs, the surface-area-to-volume problem, and the origin of mitochondria and chloroplasts.^[1]

Membranes & Transport

The is : its hydrophobic interior lets small nonpolar molecules (O₂, CO₂) pass freely, while charged ions and large polar molecules need transport proteins.

Membrane transport — energy and direction
Type	How it works
Simple diffusion	Small nonpolar molecules cross directly, down the gradient — no energy
Facilitated diffusion	Channel or carrier proteins move substances down the gradient — no energy
Osmosis	Diffusion of water across a membrane toward the more concentrated (solute) side
Active transport	Pumps move substances against the gradient using ATP (e.g., sodium-potassium pump)
Bulk transport	Endocytosis brings material in; exocytosis sends it out — both use energy

Organelles & Compartments

Eukaryotic cells use membrane-bound compartments to separate jobs: the nucleus stores DNA, the rough ER and ribosomes build proteins, the smooth ER makes lipids and detoxifies, the Golgi packages and ships, lysosomes digest, and mitochondria and chloroplasts handle energy. Smaller cells have a higher surface-area-to-volume ratio, which speeds exchange across the membrane — a key reason cells stay small.

Endosymbiotic Theory

The explains where mitochondria and chloroplasts came from: a host cell engulfed free-living prokaryotes that became permanent partners. The evidence is striking — both organelles have their own circular DNA, their own ribosomes resembling bacterial ribosomes, a double membrane, and they divide by binary fission.

Checkpoint · Unit 2 · Cell Structure & Function

Question 1 of 10

Cholesterol molecules are wedged between the phospholipids of an animal cell membrane. At high temperatures, what is the primary role of this cholesterol?

3 · Cellular Energetics

12–16% of the exam — one of the three heaviest units. Enzymes, photosynthesis, and cellular respiration: how cells capture, convert, and release energy.^[1]

Enzymes & Activation Energy

An is a biological catalyst that speeds a reaction by lowering its without being consumed. Each enzyme has an active site shaped for a specific substrate, and its activity depends on temperature, pH, and substrate concentration. In competitive inhibition, a molecule blocks the active site (reversible by adding substrate); in noncompetitive (allosteric) inhibition, an inhibitor binds elsewhere and changes the enzyme’s shape.

Photosynthesis

stores light energy as sugar. The overall reaction is $6\,CO_2 + 6\,H_2O + \text{light} \rightarrow C_6H_{12}O_6 + 6\,O_2$ . The light-dependent reactions in the thylakoids split water (releasing O₂) and make ATP and NADPH; the in the stroma uses that ATP and NADPH to fix CO₂ into sugar.

Photosynthesis — light reactions power the Calvin cycle

Light-dependent reactionsIn the thylakoid membranes. Light splits H₂O (releasing O₂), and the energy makes ATP and NADPH.

Calvin cycle (light-independent)In the stroma. Uses that ATP and NADPH to fix CO₂ into sugar (glucose). Needs no light directly.

Inputs: 6 CO₂ + 6 H₂O + light energy → Outputs: C₆H₁₂O₆ + 6 O₂

The O₂ you breathe comes from splitting water in the light reactions — not from CO₂.

Cellular Respiration

releases the energy in glucose to make ATP, the reverse summary of photosynthesis: $C_6H_{12}O_6 + 6\,O_2 \rightarrow 6\,CO_2 + 6\,H_2O + \text{ATP}$ . It runs in three stages, and most ATP is produced by at the electron transport chain.

Aerobic cellular respiration — the three stages

1 · GlycolysisIn the cytoplasm. Splits one glucose into two pyruvate, making a small net of ATP and NADH. No oxygen required.
2 · Krebs (Citric Acid) CycleIn the mitochondrial matrix. Oxidizes pyruvate, releases CO₂, and loads the electron carriers NADH and FADH₂.
3 · Electron Transport ChainOn the inner mitochondrial membrane. Electrons power proton pumping; chemiosmosis drives ATP synthase. Oxygen is the final electron acceptor, forming H₂O.

Overall: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP. Most ATP comes from the electron transport chain via chemiosmosis, not from glycolysis.

Checkpoint · Unit 3 · Cellular Energetics

Question 1 of 10

What is the primary role of an enzyme in a chemical reaction within a cell?

4 · Cell Communication & Cell Cycle

10–15% of the exam. How cells send and receive signals, how feedback loops keep the body stable, and how the cell cycle is controlled.^[1]

Cell Signaling & Transduction

has three stages: reception (a signal molecule binds a specific receptor), transduction (the signal is relayed and amplified inside the cell, often through a phosphorylation cascade and second messengers like cyclic AMP), and response (a change in cell activity). Hydrophobic signals like steroid hormones cross the membrane to intracellular receptors; hydrophilic signals bind surface receptors.

Feedback Mechanisms

counteracts a change to maintain homeostasis (cooling off when overheated, lowering blood glucose after a meal). amplifies a change to drive a process to completion (blood clotting, childbirth contractions). Most homeostatic regulation is negative feedback.

The Cell Cycle & Mitosis

The cell cycle runs interphase (G₁, S, G₂) then and cytokinesis, producing two genetically identical diploid daughter cells for growth and repair. Checkpoints (driven by cyclins and Cdks) make sure each stage is complete before the next begins; losing that control can lead to uncontrolled division (cancer).

Checkpoint · Unit 4 · Cell Communication & Cell Cycle

Question 1 of 10

A nerve cell releases a chemical that diffuses across a tiny gap and acts on an immediately adjacent target cell. This short-range communication, in which a secreting cell affects nearby cells, is best classified as which type of signaling?

5 · Heredity

8–11% of the exam.Meiosis and how it creates variation, Mendel’s laws and Punnett squares, and the non-Mendelian patterns that go beyond simple dominance.^[1]

Meiosis & Genetic Variation

halves the chromosome number to make four haploid gametes and is the main source of genetic variation, through in prophase I and the independent assortment of homologous pairs. A failure of chromosomes to separate (nondisjunction) produces gametes with an extra or missing chromosome.

Mendelian Genetics

The says the two alleles of a gene separate into different gametes; the law of says alleles of different genes (on different chromosomes) sort independently. A dihybrid cross of two heterozygotes (RrYy × RrYy) gives the classic 9:3:3:1 phenotype ratio:

Dihybrid cross RrYy × RrYy — the 9 : 3 : 3 : 1 ratio

	RY	Ry	rY	ry
RY	RRYY	RRYy	RrYY	RrYy
Ry	RRYy	RRyy	RrYy	Rryy
rY	RrYY	RrYy	rrYY	rrYy
ry	RrYy	Rryy	rrYy	rryy

9 both dominant3 dominant/recessive3 recessive/dominant1 both recessive

Two heterozygous traits assorting independently give a 9:3:3:1 phenotype ratio across 16 boxes.

Non-Mendelian Inheritance

Not every trait follows simple dominance. blends the phenotypes (red × white snapdragons make pink); expresses both alleles fully (AB blood). Other patterns include multiple alleles, polygenic traits, and X-linked inheritance — where recessive traits like red-green color blindness appear more often in males, who have only one X.

Checkpoint · Unit 5 · Heredity

Question 1 of 10

During which process is a diploid cell's chromosome number reduced by half to produce haploid gametes, ensuring that fertilization restores the species' normal chromosome count?

6 · Gene Expression & Regulation

12–16% of the exam — one of the three heaviest units. DNA replication, the central dogma (transcription and translation), how gene expression is regulated, and mutations.^[1]

Replication & the Central Dogma

DNA replication is semiconservative: each new molecule keeps one original strand and one new strand. The describes how that information becomes a protein — transcription copies DNA into mRNA, and translation reads the mRNA in three-base at the ribosome to build a protein.

The central dogma — DNA to RNA to protein

DNAThe gene's nucleotide sequence stores the genetic information in the nucleus.
Transcription → mRNARNA polymerase copies the gene into messenger RNA, which carries the message out of the nucleus.
Translation → ProteinAt the ribosome, the mRNA is read in three-base codons; tRNA brings the matching amino acids to build the protein.

Each three-base codon specifies one amino acid. In eukaryotes, transcription happens in the nucleus and translation in the cytoplasm.

Regulation of Gene Expression

Cells control which genes are on. In bacteria, an like the lac operon switches a gene cluster on or off in response to the environment — lactose induces the operon by inactivating the repressor.

In eukaryotes, regulation happens at many levels: epigenetic DNA methylation and histone modification, transcription factors, RNA processing, and mRNA stability. This is how cells with identical DNA differentiate into distinct types.

Mutations & Biotechnology

Mutations are changes in the DNA sequence — point mutations (substitutions) and frameshifts (insertions/deletions) — that can be silent, harmful, or beneficial, and are the ultimate source of new genetic variation. Biotechnology tools tested on the exam include PCR (amplifying DNA), gel electrophoresis (separating fragments by size), and the use of restriction enzymes.

Checkpoint · Unit 6 · Gene Expression & Regulation

Question 1 of 10

DNA replication is described as semiconservative. What does this term mean about the two molecules produced from one original DNA molecule?

7 · Natural Selection

13–20% of the exam — the single heaviest unit. How natural selection drives evolution, the Hardy-Weinberg model of a non-evolving population, and the evidence for common ancestry.^[1]

Natural Selection & Fitness

needs heritable variation in traits that affect survival and reproduction. Individuals with advantageous traits leave more offspring, so favorable alleles become more common over generations.

Selection comes in three patterns: directional (favors one extreme), stabilizing (favors the intermediate, like human birth weight), and disruptive (favors both extremes). Other mechanisms also change allele frequencies: (random, strongest in small populations) and (migration between populations).

Hardy-Weinberg Equilibrium

The model describes the allele and genotype frequencies of a population that is not evolving: $p + q = 1$ and $p^2 + 2pq + q^2 = 1$ , where $p^2$ is homozygous dominant, $2pq$ heterozygous, and $q^2$ homozygous recessive. If real frequencies differ from these, the population is evolving.

Hardy-Weinberg equilibrium — allele and genotype frequencies

Allele frequenciesp + q = 1p = dominant allele, q = recessive allele

Genotype frequenciesp² + 2pq + q² = 1p² homozygous dominant · 2pq heterozygous · q² homozygous recessive

Worked example: if 16% show the recessive phenotype, q² = 0.16 → q = 0.4, p = 0.6. Heterozygotes 2pq = 2(0.6)(0.4) = 0.48 (48%).

Equilibrium needs five conditions: no mutation, no selection, random mating, no gene flow, and a large population. If the numbers don't match, the population is evolving.

Evidence & Phylogeny

Evidence for evolution and common ancestry includes the fossil record, homologous structures, shared developmental patterns, and — most powerfully — molecular similarities in DNA and proteins (like the near-universal genetic code). Phylogenetic trees and cladograms map these relationships, grouping organisms by shared, derived characteristics.

Checkpoint · Unit 7 · Natural Selection

Question 1 of 10

Charles Darwin proposed natural selection as a mechanism of evolution. Which condition is required for natural selection to act on a population?

8 · Ecology

10–15% of the exam. Energy flow through ecosystems, the cycling of matter, and how populations and communities behave.^[1]

Energy Flow & Trophic Levels

Energy flows one way through an ecosystem, entering through producers (photosynthesis) and passing up through . Only about 10% of the energy stored at one level is captured by the next — the rest is lost as heat and used in metabolism. That 10% rule keeps food chains short and top predators rare.

Trophic energy pyramid — the 10% rule

Tertiary consumers · ~10 units

Secondary consumers · ~100 units

Primary consumers (herbivores) · ~1,000 units

Producers (plants, algae) · ~10,000 units

Only about 10% of energy passes to the next level — the rest is lost as heat and through metabolism. That limit keeps food chains short and top predators rare.

Biogeochemical Cycles

Unlike energy, matter is recycled. Carbon cycles between the atmosphere and organisms through photosynthesis (removing CO₂) and respiration (releasing it); burning fossil fuels adds extra CO₂.

Nitrogen, abundant as gas but unusable directly, is made available by nitrogen-fixing bacteria (including those in legume root nodules). Decomposers return nutrients to the soil.

Population & Community Ecology

Populations grow exponentially (a J-curve) only when resources are unlimited; otherwise they show logistic growth (an S-curve) that levels off at the (K). is checked by density-dependent factors (competition, predation, disease) and density-independent ones (weather, disasters). Communities are shaped by interactions like competition, predation, and the three kinds of symbiosis — mutualism, commensalism, and parasitism.

Symbiotic relationships in communities
Relationship	Effect on each partner
Mutualism	Both species benefit (+/+) — e.g., bees and flowers
Commensalism	One benefits, the other is unaffected (+/0)
Parasitism	One benefits, the other is harmed (+/−) — e.g., a tapeworm and its host
Predation	Predator benefits, prey is killed (+/−)
Competition	Both are harmed as they vie for the same resource (−/−)

Checkpoint · Unit 8 · Ecology

Question 1 of 10

In a typical food chain, which group of organisms occupies the trophic level that captures energy directly from sunlight and forms the base of the energy flow through an ecosystem?

How to Use This Study Guide

A study guide is a map, not the whole territory — use it alongside official College Board practice and the AP Classroom resources. Because half the AP Biology score comes from free response, don’t just memorize: practice explaining mechanisms, analyzing graphs and data, and designing experiments. Spaced, mixed practice across units beats one long cram, and the three heaviest units — Natural Selection, Cellular Energetics, and Gene Expression — deserve the most attention.

A study loop that actually works

1
Read a unit here
Work through one unit at a time, in the official order from Chemistry of Life to Ecology.
2
Take the checkpoint
The quick check at the end of each unit exposes what didn't stick.
3
Drill the gaps
Send your weak unit straight into the free practice questions and flashcards.
4
Practice free response
Work official FRQs — explain, analyze data, and design experiments — then review every miss.

AP Biology Concept Questions

Common AP Biology concepts the exam actually measures — at least one per College Board unit. Tap any card for a short, exam-ready answer backed by an official source (College Board), then test yourself on them as flashcards.

AP Biology Glossary

Quick definitions for the terms you’ll see most across all eight AP Biology units:

Activation energy: The energy barrier that must be overcome for a chemical reaction to proceed. Enzymes lower it.
Active transport: Movement of a substance across a membrane against its concentration gradient, requiring energy (usually ATP).
Calvin cycle: The light-independent reactions in the chloroplast stroma that use ATP and NADPH to fix CO₂ into sugar.
Carrying capacity: The maximum population size (K) an environment can sustain given its resources; logistic growth levels off here.
Cellular respiration: The breakdown of glucose to release energy as ATP: glycolysis (cytoplasm), the Krebs cycle (matrix), and the electron transport chain (inner mitochondrial membrane).
Central dogma: The flow of genetic information in cells: DNA → RNA → protein, via transcription and translation.
Chemiosmosis: The use of a proton (H⁺) gradient across a membrane to drive ATP synthase and produce ATP, in both respiration and photosynthesis.
Codominance: An inheritance pattern in which both alleles are fully expressed in the heterozygote (AB blood type expresses both A and B antigens).
Codon: A sequence of three mRNA nucleotides that specifies one amino acid (or a start/stop signal) during translation.
Crossing over: The exchange of segments between homologous chromosomes during prophase I of meiosis, producing new allele combinations.
Dehydration synthesis: A reaction that joins two monomers into a polymer by removing one molecule of water. Also called a condensation reaction.
Endosymbiotic theory: The theory that mitochondria and chloroplasts arose from free-living prokaryotes engulfed by a host cell; supported by their own DNA, ribosomes, and double membranes.
Enzyme: A biological catalyst (usually a protein) that speeds up a reaction by lowering its activation energy without being consumed.
Exponential growth: Accelerating population growth with unlimited resources, producing a J-shaped curve.
Gene flow: The movement of alleles between populations through migration, which tends to make populations more genetically similar.
Genetic drift: A random change in allele frequencies due to chance, strongest in small populations; includes the bottleneck and founder effects.
Hardy-Weinberg equilibrium: A non-evolving state where allele frequencies follow p + q = 1 and genotype frequencies follow p² + 2pq + q² = 1, given no mutation, no selection, random mating, no gene flow, and a large population.
Hydrogen bond: A weak attraction between a slightly positive hydrogen and a slightly negative atom (like oxygen). Hydrogen bonds between water molecules give water its cohesion, high specific heat, and the fact that ice floats.
Hydrolysis: A reaction that breaks a polymer into monomers by adding one molecule of water across the bond — the reverse of dehydration synthesis.
Hydrophobic: Water-fearing — describing nonpolar molecules (like lipids) that do not dissolve in water and cluster away from it.
Incomplete dominance: An inheritance pattern in which the heterozygote shows a blended, intermediate phenotype (red × white snapdragons → pink).
Independent assortment: Mendel's principle that alleles of different genes (on different chromosomes) are distributed to gametes independently of one another.
Law of segregation: Mendel's principle that the two alleles for a gene separate during gamete formation, so each gamete carries only one.
Meiosis: Cell division that produces four haploid gametes, halving the chromosome number and creating genetic variation through crossing over and independent assortment.
Mitosis: Nuclear division that produces two genetically identical diploid daughter cells, used for growth and repair.
Natural selection: The process by which individuals with heritable advantageous traits survive and reproduce more, shifting allele frequencies over generations.
Negative feedback: A control loop that counteracts a change to keep a system near its set point (most homeostatic regulation).
Operon: A cluster of bacterial genes controlled together by a single promoter and operator, such as the lac operon.
Passive transport: Movement of a substance across a membrane down its concentration gradient with no energy input (diffusion, facilitated diffusion, osmosis).
Phospholipid bilayer: The two-layer sheet of phospholipids that forms cell membranes, with hydrophilic heads facing the watery inside and outside and hydrophobic tails in the middle.
Photosynthesis: The process that converts light energy into chemical energy: 6 CO₂ + 6 H₂O + light → glucose + 6 O₂. Light reactions occur in the thylakoids; the Calvin cycle in the stroma.
Positive feedback: A control loop that amplifies a change to drive a process to completion (e.g., blood clotting, childbirth contractions).
Selective permeability: The property of a membrane that lets some substances cross freely while controlling or blocking others.
Signal transduction: The relay of a signal inside a cell after a signaling molecule binds a receptor — reception, transduction, then response.
Trophic level: A feeding position in a food chain — producers, primary consumers, secondary consumers, and so on.

Free AP Biology Study Materials & Resources

Everything you need to prepare for AP Biology is free here — no paywall, no sign-up. This guide is the foundation; pair it with the rest of our free AP Biology study materials for active recall, timed practice, and last-minute review:

AP Biology Practice Test — exam-style questions across all eight units, with explanations.
AP Biology Flashcards — active-recall decks for the high-yield processes, structures, and terms.

AP Biology Study Guide FAQ

The AP Biology exam has two sections. Section I is 60 multiple-choice questions in 90 minutes (50% of the score). Section II is 6 free-response questions in 90 minutes (50%): two long FRQs (including an experimental-design and an analysis question) and four short FRQs. Total testing time is 3 hours.

References

1.College Board. “AP Biology — Course and Exam Description.” College Board. ↑
2.College Board. “AP Biology — AP Students.” College Board. ↑
3.College Board. “AP Biology Exam — AP Students.” College Board. ↑
4.College Board. “AP Biology Course at a Glance.” College Board. ↑

Sources for the concept answers

Every answer in the AP Biology concept questions above is drawn from an official primary source:

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