Evolution, energy, information, and systems — the four big ideas that organize modern biology.
Study guide last reviewed September 25, 2026 · see what changed
Format reflects the current published structure — always confirm against AP Central before test day, since formats occasionally change.
| Section | Details | Time | Weight |
|---|---|---|---|
| Section I – Multiple Choice | 60 questions | 1 hr 30 min | 50% |
| Section II – Free Response | 6 questions (2 long, 4 short, including math and data analysis) | 1 hr 30 min | 50% |
Straight answers about the AP Biology exam — format, credit, dates, and this site.
Section I – Multiple Choice: 60 questions (1 hr 30 min, 50% of the total score); Section II – Free Response: 6 questions (2 long, 4 short, including math and data analysis) (1 hr 30 min, 50% of the total score). See the Exam Format tab above for the full breakdown, and always double-check the current year's format on AP Central before test day.
It's multiple-choice questions and written free-response or essay questions.
It depends entirely on the college — most award credit for a 3 or higher, but many selective schools require a 4 or a 5, and some don't grant credit at all for certain scores. Always check your target college's own AP credit policy page rather than assuming a 3 will count everywhere.
AP exams are administered during the College Board's annual testing window in the first two full weeks of May, with the exact date and time for each subject set by the College Board and announced the preceding fall. Check AP Central or your school's AP coordinator for this year's exact date.
Yes. The unit-by-unit study guide, worked examples, practice quiz, and full-length timed practice exam on this page are all free, with no login, signup, or paywall.
Unit-by-unit breakdown of what the course actually covers, in teaching order.
Water's polarity and hydrogen bonding give it cohesion, adhesion, a high specific heat, and its role as a near-universal solvent. Nearly every biological process — from temperature regulation to molecular transport — depends on one of these properties.
Carbon's four bonding sites let it form the long, branching, ring-shaped backbones that make organic diversity possible. Monomers link into polymers via dehydration synthesis (releasing water) and split apart via hydrolysis (consuming water).
Monosaccharides (like glucose) link into disaccharides and polysaccharides; starch and glycogen store energy in plants and animals respectively, while cellulose and chitin provide structural support.
Nonpolar and hydrophobic, lipids include fats (energy storage), phospholipids, and steroids. Phospholipids are amphipathic — a hydrophilic head and hydrophobic tails — which is exactly what allows them to spontaneously form the cell membrane's bilayer.
Built from amino acids linked by peptide bonds, a protein's specific three-dimensional shape (its primary through quaternary structure) determines its function — and that shape is dictated entirely by its amino acid sequence.
DNA and RNA store and transmit genetic information using nucleotides (sugar, phosphate, and a nitrogenous base); strand directionality, labeled 5' to 3', matters for how these molecules are read and copied.
All living things are made of cells, and cells arise only from pre-existing cells. The endosymbiotic theory explains mitochondria and chloroplasts as descendants of free-living prokaryotes engulfed by an ancestral host cell.
Prokaryotes lack a true nucleus and membrane-bound organelles; eukaryotes compartmentalize their functions into organelles, which increases efficiency at the cost of added structural complexity.
The nucleus, rough and smooth ER, Golgi apparatus, mitochondria, chloroplasts, lysosomes, and vacuoles each carry out specialized roles, and in every case their internal structure directly supports that role.
The fluid mosaic model describes a phospholipid bilayer studded with proteins. Passive transport (diffusion, facilitated diffusion, osmosis) moves substances down a concentration gradient at no energy cost; active transport moves them against the gradient and requires ATP.
As a cell grows, its volume increases faster than its surface area, which eventually limits how large a cell can get before it can no longer exchange materials with its environment fast enough.
Dividing cellular processes among distinct membrane-bound organelles lets incompatible reactions occur simultaneously in the same cell and increases the efficiency of specialized functions.
Enzymes lower a reaction's activation energy without being consumed themselves. Their function depends on a precisely shaped active site and is sensitive to temperature, pH, and inhibitors, both competitive and noncompetitive.
ATP stores readily usable energy in its phosphate bonds; hydrolyzing the terminal phosphate releases that energy to power cellular work, from active transport to biosynthesis.
Glycolysis (cytoplasm), the Krebs/citric acid cycle (mitochondrial matrix), and oxidative phosphorylation (inner mitochondrial membrane) together break down glucose into ATP, using oxygen as the final electron acceptor.
Electrons passed along the electron transport chain power proton pumps that build a gradient across the inner mitochondrial membrane; ATP synthase then uses that gradient to generate ATP — the same chemiosmotic principle photosynthesis uses.
Without oxygen, cells regenerate NAD+ through lactic acid or alcoholic fermentation so glycolysis can keep running, though at a far lower ATP yield than full aerobic respiration.
The light reactions (thylakoid membrane) convert light energy into ATP and NADPH while splitting water and releasing oxygen; the Calvin cycle (stroma) then uses that ATP and NADPH to fix CO2 into sugar.
The two processes are essentially mirror images in their inputs and outputs, and both rely on an electron transport chain and chemiosmosis to generate ATP.
A signal transduction pathway starts with a ligand binding a receptor, which triggers a cascade of intracellular events ending in a cellular response. Signals can act locally (paracrine) or at a distance (endocrine).
G-protein coupled receptors, receptor tyrosine kinases, and ligand-gated ion channels each convert an extracellular signal into an intracellular response through a different mechanism.
A single signal molecule can trigger a cascade that amplifies the response many-fold, typically through a sequential chain of protein phosphorylation events.
The cycle proceeds through G1, S (DNA synthesis), G2, and M (mitosis) phases; checkpoints regulate progression and stop damaged or incomplete cells from dividing further.
A single round of nuclear division produces two genetically identical daughter cells, moving through prophase, metaphase, anaphase, and telophase in order.
Cyclins and cyclin-dependent kinases (CDKs) drive the cell cycle forward. Mutations in checkpoint genes like p53 can disable that regulation and lead to uncontrolled cell division — cancer.
Meiosis produces four genetically unique haploid gametes through two rounds of division. Crossing over during prophase I and independent assortment both generate genetic variation beyond what mitosis could produce.
The law of segregation and the law of independent assortment predict how alleles pass to offspring; Punnett squares model both monohybrid and dihybrid crosses to predict offspring ratios.
Incomplete dominance, codominance, multiple alleles, polygenic traits, and sex-linked inheritance all produce inheritance patterns that don't follow simple dominant/recessive ratios.
Genes on the same chromosome are linked and don't assort independently unless crossing over separates them — the further apart two genes sit, the more likely they are to be separated by a crossover.
The multiplication rule (independent events) and addition rule (mutually exclusive events) predict offspring ratios, and the chi-square test evaluates whether observed data significantly differs from what those ratios predict.
Replication is semiconservative — each original strand serves as a template for a new one. Helicase unwinds the DNA, DNA polymerase synthesizes the new strand 5' to 3', and ligase seals any remaining gaps.
RNA polymerase synthesizes mRNA from a DNA template inside the nucleus. In eukaryotes, the resulting pre-mRNA is processed — a 5' cap and poly-A tail are added, and introns are spliced out — before it leaves the nucleus.
Ribosomes in the cytoplasm read mRNA codons three at a time, and tRNA molecules deliver the matching amino acids to build a polypeptide according to the genetic code.
Operons, like the lac operon, let bacteria switch genes on or off in response to environmental conditions, conserving energy by producing only the proteins currently needed.
Transcription factors, epigenetic modifications (DNA methylation, histone acetylation), and post-transcriptional controls all determine which genes get expressed, when, and how much.
Point mutations, insertions, and deletions can be silent, missense, or nonsense; the effect on the resulting protein depends heavily on where the mutation occurs and whether it shifts the reading frame.
Gel electrophoresis separates DNA fragments by size, PCR amplifies specific DNA sequences, and CRISPR allows direct, targeted editing of a genome — tools that turned much of modern genetics from descriptive to experimental.
Comparative anatomy (homologous vs. analogous structures), molecular biology (DNA and protein similarity), the fossil record, and biogeography each independently support common descent.
Individuals with heritable traits better suited to their environment survive and reproduce at higher rates, gradually shifting allele frequencies in the population across generations.
The equation p² + 2pq + q² = 1 models a population that is NOT evolving. Comparing a real population's actual allele frequencies to this prediction reveals whether an evolutionary force is acting on it.
Genetic drift (random chance, strongest in small populations), gene flow (migration), mutation, and non-random mating all shift allele frequencies alongside natural selection.
Reproductive isolation — behavioral, geographic, temporal, or mechanical — blocks gene flow between populations, letting them diverge into separate species over time.
Cladograms and phylogenetic trees represent hypothesized evolutionary relationships, built from shared derived characteristics rather than overall similarity.
Exponential growth describes a population with unlimited resources; logistic growth describes how growth slows as the population approaches its environment's carrying capacity.
Competition, predation, mutualism, commensalism, and parasitism all shape which species coexist in a community and in what numbers.
Energy enters an ecosystem through producers and flows through trophic levels, with roughly only 10% transferred to each successive level — the rest lost as heat — which limits how many trophic levels an ecosystem can support.
The carbon, nitrogen, phosphorus, and water cycles describe how matter, unlike energy, is recycled through an ecosystem rather than lost from it.
Primary succession begins on bare substrate with no existing soil; secondary succession follows a disturbance where soil already exists. Both proceed through predictable stages toward a climax community.
Habitat destruction, invasive species, climate change, and pollution all disrupt the cycles and species relationships described throughout this unit, often with cascading effects.
Concrete, fully worked examples — this is what separates a page reviewers see as genuinely useful from one that just lists topics.
ATP yield: roughly how much ATP does one glucose molecule yield through complete aerobic respiration, and where does most of it come from?
Glycolysis nets 2 ATP directly; the Krebs cycle nets 2 more directly; but oxidative phosphorylation (the electron transport chain + chemiosmosis, using the NADH and FADH₂ generated earlier) produces the large majority — roughly ~30–32 of the ~34 total ATP. This is why an FRQ asking "which stage produces the most ATP" is really asking whether you understand chemiosmosis, not glycolysis.
Dihybrid cross: cross two heterozygotes (AaBb × AaBb) for two independently assorting genes. What fraction of offspring show both recessive traits?
Each gene independently gives a 3:1 dominant:recessive ratio, so both-recessive requires (1/4) × (1/4) = 1/16. The multiplication rule for independent events, applied gene-by-gene, replaces drawing a full 16-box Punnett square once you trust the logic behind it.
lac operon: predict what happens to lac operon transcription when lactose is present but glucose is also abundant.
Lactose presence inactivates the repressor (removing the block), but high glucose keeps cAMP levels low, so CAP can't bind and strongly activate the promoter. Result: the operon is transcribed only at a low, baseline level — the cell preferentially uses glucose and only ramps up lactose metabolism once glucose runs out. This dual-control logic (repressor AND activator) is a frequent FRQ scenario.
Hardy-Weinberg: in a population, 16% show a recessive phenotype. Find the allele and genotype frequencies, assuming equilibrium.
q² = 0.16, so q = 0.4 and p = 0.6. Genotype frequencies: p² (homozygous dominant) = 0.36, 2pq (heterozygous) = 0.48, q² (homozygous recessive) = 0.16 — note these three sum to 1, a quick check worth doing on any Hardy-Weinberg answer before finalizing it.
Energy transfer: if producers fix 10,000 kcal of energy, roughly how much energy is available to secondary consumers?
Applying the 10% rule twice (producer→primary consumer, then primary→secondary consumer): 10,000 × 0.10 × 0.10 = 100 kcal. This steep loss at each trophic level is exactly why food chains rarely extend past 4-5 levels and why top predators are always rare relative to producers.
The errors that cost the most points across the whole exam, not just one unit.
Unofficial practice questions written for this study guide — not official AP content and not from College Board. Use them to self-check, then use the real past free-response questions below for authentic exam practice.
Unofficial practice, timed to feel like the real thing. Section I pulls from this page's full multiple-choice bank; Section II gives you free-response prompts to write out and self-score against a rubric. Bank size is still growing toward full exam length — timing is paced per-question, not pulled from this subject's official duration.