AP Biology Review Guide: All 8 Units Simplified

AP Biology has more content than any other AP science, and the exam rewards students who can connect topics rather than just memorize isolated facts. If you understand the four big ideas (evolution, energy transfer, information transmission, and system interactions) and how each unit illustrates them, you can reason through questions on material you do not fully remember.

This guide walks unit by unit through the full CED, then shows the experimental-design and data-analysis patterns the exam keeps using. If you are more than two weeks out, use it as a map. If you are less than a week out, read it in one sitting and then drill FRQs.

What the exam looks like

Exam structure and scoring

• 3 hours total.
• Section I (90 minutes): 60 multiple choice. Worth 50 percent of the score. Calculator allowed throughout.
• Section II (90 minutes): 6 free response: 2 long FRQs (one is always an experimental design / data analysis) and 4 short FRQs. Worth 50 percent.
• Formula sheet is provided (Hardy-Weinberg, chi-square, rates of change, Q10). You do not have to memorize them.
• Graders look for connections across units. An FRQ on photosynthesis can hide an evolution question inside it.

Unit 1: Chemistry of Life

What you need to know

About 8 to 11 percent of the exam. The foundation: water, macromolecules, and enzymes. Every later unit depends on this one.

Water and chemistry basics

• Water is polar because oxygen is more electronegative than hydrogen. This polarity drives cohesion, adhesion, surface tension, high specific heat, and high heat of vaporization.
• Hydrogen bonds form between water molecules and give water its life-supporting properties.
• pH: acidic solutions have more H plus ions, basic solutions have more OH minus ions. Buffers resist change in pH.
• Carbon is the backbone of life because it can form 4 stable covalent bonds, enabling long chains and rings.

The four macromolecules

• Carbohydrates: monomer is monosaccharide (glucose). Polymers include starch (energy storage in plants), glycogen (energy storage in animals), cellulose (plant cell walls).
• Lipids: not true polymers. Include fats (triglycerides), phospholipids (membrane bilayers), and steroids. Hydrophobic.
• Proteins: monomer is amino acid (20 types). Peptide bonds link them. Four levels of structure (primary sequence, secondary helix/sheet, tertiary 3D folding, quaternary multiple subunits).
• Nucleic acids: monomer is nucleotide (phosphate + sugar + base). DNA is double-stranded with deoxyribose and bases A, T, C, G. RNA is single-stranded with ribose and bases A, U, C, G.
• Dehydration synthesis (condensation) joins monomers by removing water. Hydrolysis breaks polymers by adding water.

Enzymes

• Enzymes are proteins (mostly) that lower activation energy, making reactions faster. They do NOT change delta G.
• Active site is the region where substrate binds. Shape complementarity (induced fit) drives specificity.
• Temperature and pH affect enzyme activity. Each enzyme has an optimum. Too hot denatures (unfolds) the enzyme; extreme pH does too.
• Inhibitors: competitive (binds active site, overcome by more substrate) vs non-competitive / allosteric (binds elsewhere, changes shape).

Unit 2: Cell Structure and Function

What you need to know

About 10 to 13 percent. Cell compartments, membranes, and transport. The surface-area-to-volume ratio theme starts here and returns in ecology.

Cells and organelles

• Prokaryotes (bacteria, archaea): no membrane-bound nucleus, no organelles, small, circular DNA, ribosomes.
• Eukaryotes (plants, animals, fungi, protists): membrane-bound nucleus, organelles, linear DNA, larger cells.
• Nucleus: houses DNA. Nucleolus makes ribosomes.
• Mitochondria: cellular respiration, makes ATP. Double membrane (evidence for endosymbiosis).
• Chloroplasts (plants only): photosynthesis. Also double membrane and own DNA.
• Endoplasmic reticulum: rough ER (ribosomes, protein synthesis), smooth ER (lipid synthesis, detoxification).
• Golgi apparatus: modifies, sorts, and ships proteins.
• Lysosomes: digestion. Vacuoles: storage (large central vacuole in plants).
• Cytoskeleton: microfilaments, intermediate filaments, microtubules. Gives shape, enables movement.

Membranes and transport

• Membranes are phospholipid bilayers with embedded proteins. Fluid mosaic model.
• Passive transport: no ATP needed. Diffusion (high to low concentration), osmosis (water), facilitated diffusion (through protein channel).
• Active transport: uses ATP, moves solutes against gradient. Example: sodium-potassium pump (3 Na out, 2 K in).
• Tonicity: hypertonic solution (cell loses water, shrivels), hypotonic (cell gains water, may burst), isotonic (no net movement).
• Bulk transport: endocytosis (in), exocytosis (out). Phagocytosis is cell-eating, pinocytosis is cell-drinking.

Unit 3: Cellular Energetics

What you need to know

About 12 to 16 percent. Photosynthesis and respiration. One of the heaviest and most important units. The reactions are mirror images.

Photosynthesis

• Overall: 6 CO2 + 6 H2O + light energy -> C6H12O6 + 6 O2.
• Light reactions (in thylakoid membrane): chlorophyll absorbs light, water splits (O2 released), NADP+ reduced to NADPH, ATP made via chemiosmosis.
• Calvin cycle (in stroma): CO2 fixed by RuBisCO onto RuBP, making G3P (sugar precursor). Uses ATP and NADPH from light reactions.
• C3, C4, CAM plants: different adaptations to prevent photorespiration in hot or dry climates.

Cellular respiration

• Overall: C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O + ATP energy.
• Glycolysis (cytoplasm): glucose splits into 2 pyruvate. Net 2 ATP and 2 NADH. Anaerobic.
• Pyruvate oxidation (mitochondrial matrix): pyruvate to acetyl-CoA, produces 2 NADH and 2 CO2.
• Krebs cycle (mitochondrial matrix): acetyl-CoA oxidized. Produces 2 ATP, 6 NADH, 2 FADH2, 4 CO2.
• Electron transport chain (inner mitochondrial membrane): NADH and FADH2 donate electrons. Protons pumped across membrane. Oxygen is final electron acceptor (makes water). About 32-34 ATP via chemiosmosis.
• Fermentation (anaerobic): glycolysis only. Lactic acid (animals) or ethanol (yeast) regenerates NAD+.

Unit 4: Cell Communication and Cell Cycle

What you need to know

About 10 to 15 percent. How cells talk to each other and how they reproduce. Cancer shows up when cell cycle control fails.

Cell signaling

• Three steps: reception (ligand binds receptor), transduction (signal cascade inside cell, often phosphorylation), response (gene expression, cell activity).
• G-protein coupled receptors (GPCR) and receptor tyrosine kinases are the main types.
• Second messengers amplify signals: cAMP, Ca2+.
• Signal transduction pathways are conserved across organisms (evidence of evolution).

Cell cycle and division

• Interphase: G1 (growth), S (DNA synthesis), G2 (prep for mitosis). Most of the cell's life.
• Mitosis: prophase, prometaphase, metaphase, anaphase, telophase. Then cytokinesis. Produces 2 identical diploid cells.
• Meiosis: two divisions (meiosis I and II) producing 4 genetically distinct haploid gametes. Crossing over in prophase I increases variation.
• Cell cycle checkpoints: G1/S (is the DNA damaged? are nutrients adequate?), G2/M (is DNA replicated correctly?), M (are chromosomes attached to spindle?). Cyclins and Cdks regulate.
• Cancer: uncontrolled cell division. Tumor suppressor genes (p53, Rb) prevent it; proto-oncogenes drive division. Mutations in either type can cause cancer.

Unit 5: Heredity

What you need to know

About 8 to 11 percent. Mendelian and non-Mendelian inheritance, chromosomes, chi-square. Combines well with Unit 6.

Mendelian genetics

• Law of segregation: alleles separate during gamete formation (one from each parent).
• Law of independent assortment: genes on different chromosomes assort independently.
• Punnett squares predict offspring ratios. 3:1 for monohybrid, 9:3:3:1 for dihybrid with independent genes.
• Test cross: cross unknown genotype with homozygous recessive to determine genotype.

Non-Mendelian patterns

• Incomplete dominance: heterozygotes show blended phenotype (red + white = pink).
• Codominance: both alleles expressed simultaneously (AB blood type).
• Multiple alleles: more than 2 alleles for a gene (ABO blood groups).
• Sex-linked: X-linked recessive traits (color blindness, hemophilia) more common in males.
• Polygenic: multiple genes affect one trait (skin color, height).
• Pleiotropy: one gene affects multiple traits (sickle cell).
• Epistasis: one gene masks another (coat color in mice).
• Linked genes: genes on same chromosome don't assort independently. Recombination frequency measures distance.

Chi-square analysis

• Test if observed ratios match expected (null hypothesis).
• Formula: sum of (observed minus expected) squared divided by expected.
• Degrees of freedom: number of categories minus 1.
• Critical value typically at p = 0.05. If chi-square is greater than critical value, reject null hypothesis.

Unit 6: Gene Expression and Regulation

What you need to know

About 12 to 16 percent. The central dogma (DNA to RNA to protein), regulation, mutations, and biotechnology. Combines with Unit 5 on many FRQs.

DNA replication

• Semiconservative: each new DNA molecule has one old strand and one new strand.
• Helicase unwinds DNA. DNA polymerase adds nucleotides 5' to 3'.
• Leading strand synthesized continuously, lagging strand synthesized in Okazaki fragments (then joined by ligase).
• Primers (RNA) start replication. Telomeres protect chromosome ends.

Transcription and translation

• Transcription (in nucleus): DNA to RNA. RNA polymerase reads DNA 3' to 5', builds mRNA 5' to 3'.
• mRNA processing: 5' cap and poly-A tail added. Introns spliced out, exons joined.
• Translation (at ribosomes): mRNA read in codons (3 bases = 1 amino acid). tRNA brings amino acids matching codons. Ribosome joins them into polypeptide.
• Genetic code: 64 codons for 20 amino acids. Start codon AUG (methionine). Stop codons UAA, UAG, UGA. Code is redundant (multiple codons per amino acid) but not ambiguous.

Regulation of gene expression

• Prokaryotes: operons. lac operon (inducible, turned ON in presence of lactose). trp operon (repressible, turned OFF in presence of tryptophan).
• Eukaryotes: regulation at multiple levels. Transcription factors bind promoter/enhancer regions. DNA methylation silences genes. Histone modification opens or closes chromatin.
• Post-transcriptional: alternative splicing (different exon combinations make different proteins from same gene).
• Post-translational: phosphorylation, ubiquitination modify protein activity.

Mutations and biotechnology

• Point mutations: substitutions can be silent (same amino acid), missense (different amino acid), or nonsense (premature stop).
• Frameshift mutations: insertions or deletions that shift reading frame, disrupting all downstream codons.
• Biotechnology: PCR amplifies DNA. Gel electrophoresis separates DNA by size. CRISPR edits genes. Restriction enzymes cut at specific sequences.

Unit 7: Natural Selection

What you need to know

About 13 to 20 percent. Evolution is the unifying idea of biology. Darwin, Hardy-Weinberg, speciation, phylogenetics. This is the heaviest unit along with Units 3 and 6.

Evolution basics

• Darwin: species change over time through natural selection. Variation exists, organisms compete for resources, fittest reproduce.
• Evidence for evolution: fossil record, anatomical homology (similar structures), molecular homology (similar DNA), embryology, biogeography, direct observation.
• Types of selection: directional (favors one extreme), stabilizing (favors average), disruptive (favors extremes).
• Genetic drift: random changes in allele frequency, more pronounced in small populations. Bottleneck and founder effects.
• Gene flow: migration moves alleles between populations.
• Mutation: introduces new alleles.

Hardy-Weinberg equilibrium

• Formulas: p + q = 1 (allele frequencies). p squared + 2pq + q squared = 1 (genotype frequencies).
• Five assumptions: no mutation, random mating, no selection, no migration, large population (no genetic drift). If ANY are violated, evolution is occurring.
• Use H-W to calculate expected allele and genotype frequencies, then test if population is evolving by comparing with observed.

Speciation and phylogenetics

• Biological species concept: groups that can interbreed and produce fertile offspring.
• Reproductive isolation: prezygotic (habitat, behavior, temporal, mechanical) or postzygotic (hybrid inviability, sterility).
• Allopatric speciation: geographic separation. Sympatric: without geographic separation (often in plants via polyploidy).
• Phylogenetic trees (cladograms): show evolutionary relationships. Shared derived characters (synapomorphies) group clades. Node = common ancestor.

Unit 8: Ecology

What you need to know

About 10 to 15 percent. Populations, communities, ecosystems. Human impact is heavily tested on FRQs.

Population and community ecology

• Exponential growth: dN/dt = rN. Growth accelerates indefinitely. Happens when resources are unlimited.
• Logistic growth: dN/dt = rN(K-N)/K, where K is carrying capacity. Growth slows as population approaches K.
• Life history: r-selected (many offspring, low care) vs K-selected (few offspring, high care).
• Community interactions: predation, competition, symbiosis (mutualism +/+, commensalism +/0, parasitism +/-).
• Keystone species: small in number but outsized impact (sea otters, wolves).
• Ecological succession: primary (bare rock) or secondary (after disturbance like fire).

Ecosystem ecology

• Energy flow: one-way. Only ~10 percent of energy transferred between trophic levels (10 percent rule).
• Trophic pyramid: producers (plants) -> primary consumers (herbivores) -> secondary consumers -> tertiary consumers.
• Biogeochemical cycles: carbon (photosynthesis/respiration/combustion), nitrogen (fixation/nitrification/denitrification), water (evaporation/precipitation), phosphorus (no atmospheric phase).
• Human impact: carbon dioxide and global warming, ocean acidification, nitrogen runoff causing dead zones, habitat loss, invasive species.

The four big ideas (the framework for every FRQ)

• Evolution: natural selection drives species change. Every biology phenomenon can be viewed through this lens.
• Energy and matter: cells, organisms, and ecosystems transform energy and cycle matter. Photosynthesis, respiration, food webs all illustrate this.
• Information: DNA stores genetic information. Signaling transmits information between cells and organisms.
• Systems interactions: molecules, cells, tissues, organisms, ecosystems all exhibit emergent properties from interactions.

The experimental-design FRQ

One long FRQ always asks you to design or interpret an experiment. The grader is looking for:

1. A clear hypothesis that makes a testable prediction.
2. Independent variable (what you change) and dependent variable (what you measure) clearly identified.
3. Controls: control group (no treatment) and controlled variables (held constant).
4. Adequate replication: multiple trials to reduce random variation.
5. Data analysis: graphs, statistics (chi-square if appropriate), error bars.
6. Conclusion linked back to hypothesis. Explain what the data show and whether they support the hypothesis.

How to score a 5 on AP Biology

1. Master the four big ideas as a framework. Every FRQ connects to at least one. When you feel stuck, ask which big idea is being tested.
2. Focus on the three heaviest units: photosynthesis / respiration (Unit 3), gene expression (Unit 6), and evolution (Unit 7). Together these are ~40 percent of the exam.
3. Learn the experimental-design template cold. One FRQ is always experimental, and the rubric rewards the same moves every year.
4. Practice chi-square and Hardy-Weinberg calculations. These appear on almost every exam.
5. Take at least two timed practice exams. Bio has a LOT of content, and pacing is often what separates a 4 from a 5.

Common mistakes

• Confusing mitosis and meiosis. Mitosis: 1 division, 2 identical diploid cells, for growth/repair. Meiosis: 2 divisions, 4 genetically unique haploid gametes, for reproduction.
• Forgetting that enzymes do not change delta G, only activation energy. Enzymes speed up reactions without changing equilibrium.
• Mixing up inducible (lac, turns ON with lactose) and repressible (trp, turns OFF with tryptophan).
• Treating evolution as 'survival of the fittest' at the individual level. Populations evolve, not individuals. An individual organism does not change its DNA; the allele frequencies in a population shift.
• Forgetting Hardy-Weinberg assumptions. If a population is evolving, it violates at least one of the five assumptions.
• On experimental design FRQs, forgetting to include a control group. The control is what makes the experiment interpretable.
• Confusing primary (photosynthesis fixes CO2) and secondary (decomposition) productivity in ecosystems.
• Writing 'DNA makes RNA makes protein' without identifying where each step happens (nucleus vs cytoplasm) or the enzymes involved.

AP Biology rewards pattern recognition over rote memorization. Learn the patterns, apply the big ideas, and the course shrinks to something manageable.