The eight units at a glance

Unit	Name	Multiple-choice weighting
1	Chemistry of Life	8% to 11%
2	Cells	10% to 13%
3	Cellular Energetics	12% to 16%
4	Cell Communication and Cell Cycle	10% to 15%
5	Heredity	8% to 11%
6	Gene Expression and Regulation	12% to 16%
7	Natural Selection	13% to 20%
8	Ecology	10% to 15%

Unit names and weightings: College Board, AP Biology course page. AP is a trademark registered by the College Board, which is not affiliated with, and does not endorse, this site.

Unit 1: Chemistry of Life

What it is asking. Why water behaves the way it does, and how the four kinds of macromolecule are built from and broken into their monomers.

Sort these out first:

DNA vs RNA: DNA and RNA are both polynucleotides, but DNA uses deoxyribose and the bases A, T, G and C, and forms an antiparallel double helix.

Unit 2: Cells

What it is asking. What each organelle does, why cell size is limited by surface area, and how anything gets across a membrane.

Sort these out first:

Prokaryotic vs eukaryotic cells: All cells share four components: a plasma membrane, cytoplasm, DNA and ribosomes.
Plant cell vs animal cell: Both are eukaryotic, so both have a plasma membrane, nucleus, ribosomes, mitochondria and the endomembrane system.
Osmosis vs diffusion: Diffusion is the passive movement of any substance from a high concentration area to a low concentration area.
Active vs passive transport: Passive transport moves a substance down its concentration gradient and costs the cell nothing, because the gradient is itself potential energy.
Hypertonic vs hypotonic: Both terms describe the solution outside a cell, compared with the cytoplasm inside it.

Unit 3: Cellular Energetics

What it is asking. How enzymes lower the cost of a reaction, and how photosynthesis and respiration move energy through the same few molecules.

Sort these out first:

Photosynthesis equation: The overall equation for photosynthesis is 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂.
Cellular respiration equation: Aerobic cellular respiration is summarized as C₆H₁₂O₆ + 6O₂ → 6H₂O + 6CO₂, the reverse of the photosynthesis summary.
Krebs cycle: The Krebs cycle, also called the citric acid cycle or TCA cycle, is an eight-step closed loop in the mitochondrial matrix.

Unit 4: Cell Communication and Cell Cycle

What it is asking. How a signal outside a cell becomes a response inside it, and how the cell cycle is checked before a cell divides.

Sort these out first:

Cell cycle: The eukaryotic cell cycle has four phases: G₁, S and G₂, which together make up interphase, followed by M phase, the mitotic phase.
Stages of mitosis: OpenStax divides mitosis, or karyokinesis, into five stages: prophase, prometaphase, metaphase, anaphase and telophase.
Mitosis vs meiosis: Mitosis is a single nuclear division that produces two nuclei genetically identical to the original and at the same ploidy level.

Unit 5: Heredity

What it is asking. How meiosis creates variation, and how to predict offspring from parents, including when Mendel's ratios do not hold.

Sort these out first:

Haploid vs diploid: A haploid cell contains a single set of chromosomes.
Punnett square: A Punnett square applies the rules of probability to predict the outcomes of a genetic cross.
Dihybrid cross: A dihybrid cross follows two characteristics at once, starting from true-breeding parents that differ in both.
Incomplete dominance vs codominance: In incomplete dominance the heterozygote shows an intermediate phenotype, as a red and a white snapdragon parent give pink offspring.
Genotype vs phenotype: A genotype is an organism's underlying genetic makeup, including alleles that are not expressed.

Unit 6: Gene Expression and Regulation

What it is asking. How DNA becomes RNA becomes protein, how that is switched on and off, and what a mutation changes.

Sort these out first:

Transcription vs translation: Transcription copies a gene's DNA sequence into messenger RNA, one nucleotide at a time.
Codon chart: A codon chart maps each three-nucleotide mRNA codon to the amino acid it specifies.
DNA vs RNA: DNA and RNA are both polynucleotides, but DNA uses deoxyribose and the bases A, T, G and C, and forms an antiparallel double helix.

Unit 7: Natural Selection

What it is asking. How populations change over generations, the evidence for common ancestry, and how to test whether a population is evolving.

Sort these out first:

Hardy-Weinberg equation: The Hardy-Weinberg equation, p² + 2pq + q² = 1, predicts genotype frequencies in a population that is not evolving, with p and q the frequencies of the two alleles.

Unit 8: Ecology

What it is asking. How energy and matter move through ecosystems, how populations grow and are limited, and how communities respond to disruption.

How to review so it sticks

Retrieve before you reread. Blank page, one topic, everything you can recall. Then check and fix. The struggle is the part that works.
Learn pairs as pairs. Mitosis and meiosis, transcription and translation, osmosis and diffusion. The exam asks for the difference, so study the difference.
Explain the diagram. Cover the labels on a figure and say what is happening and why. Most free-response questions are a figure or a data set you have not seen.
Work the genetics by hand. Punnett squares, dihybrid ratios and Hardy-Weinberg are procedures. They get fast only by doing them.
Space it. Twenty minutes on four days beats eighty minutes on one. Bring back what you missed first.

Questions students ask

How many units are in AP Biology?

Eight: Chemistry of Life, Cells, Cellular Energetics, Cell Communication and Cell Cycle, Heredity, Gene Expression and Regulation, Natural Selection, and Ecology. The College Board publishes a weighting range for each unit on the multiple-choice section, from 8% to 11% for the lightest to 13% to 20% for Natural Selection.

Which AP Biology unit carries the most weight?

Natural Selection, at 13% to 20% of the multiple-choice section, followed by Cellular Energetics and Gene Expression and Regulation at 12% to 16% each. Chemistry of Life and Heredity are the lightest at 8% to 11%.

What is the best way to review?

Retrieval, not rereading. Close the book and write what you remember about a topic, check it, and fix the gaps. Then explain a diagram or a data set out loud as if to someone else, because AP questions ask you to apply a concept to a situation you have not seen rather than to recite it.