AP Biology – Unit Summaries
A free overview of the eight units of AP Biology, written in plain language to help you see what each unit asks you to understand before you start practising.
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1. Chemistry of Life
Living things are built from a small set of elements. Water’s polarity and hydrogen bonding give it cohesion, high specific heat and its role as a solvent. Carbohydrates, lipids, proteins and nucleic acids are polymers made from monomers by dehydration synthesis and broken down by hydrolysis. The structure of each macromolecule, such as the folding levels of a protein or the base pairing of DNA, decides its function.
Key terms: hydrogen bond, monomer, polymer, dehydration synthesis, hydrolysis, primary to quaternary structure, nucleotide
2. Cell Structure and Function
Cells keep a high surface-area-to-volume ratio so exchange stays efficient. Organelles divide labour: ribosomes make proteins, the endomembrane system (ER, Golgi, vesicles, lysosomes) processes and ships them, mitochondria and chloroplasts transform energy. The fluid-mosaic membrane controls transport by diffusion, facilitated diffusion, osmosis and active transport, and water potential predicts which way water moves.
Key terms: prokaryote, eukaryote, endosymbiosis, fluid mosaic model, tonicity, water potential, endocytosis, exocytosis
3. Cellular Energetics
Enzymes lower activation energy; their rate depends on temperature, pH, substrate concentration and inhibitors. Photosynthesis captures light in the thylakoid reactions to make ATP and NADPH, which the Calvin cycle uses to fix carbon dioxide. Cellular respiration releases energy from glucose through glycolysis, pyruvate oxidation, the Krebs cycle and oxidative phosphorylation, where a proton gradient drives ATP synthase.
Key terms: active site, competitive inhibition, electron transport chain, chemiosmosis, fermentation, Calvin cycle
4. Cell Communication and Cell Cycle
Cells signal with ligands that bind receptors, starting a transduction pathway that amplifies the signal and leads to a response. Feedback keeps conditions stable (negative) or drives a process to completion (positive). The cell cycle moves through interphase and mitosis under the control of checkpoints, cyclins and CDKs; losing this control can lead to cancer.
Key terms: ligand, receptor, second messenger, homeostasis, checkpoint, cyclin, apoptosis
5. Heredity
Meiosis halves the chromosome number and creates variation through crossing over and independent assortment. Mendel’s laws of segregation and independent assortment predict ratios in crosses, and probability rules let you combine them. Many traits do not follow simple patterns: linked genes, sex-linked genes, incomplete dominance, multiple alleles and environmental effects all change the outcome. Chi-square tests check whether observed data fit a prediction.
Key terms: allele, genotype, phenotype, homologous chromosomes, linkage, nondisjunction, chi-square
6. Gene Expression and Regulation
DNA replicates semi-conservatively. Genes are transcribed into mRNA, processed in eukaryotes, and translated into protein at the ribosome using the genetic code. Gene expression is regulated by operons in bacteria and by transcription factors, epigenetic changes and RNA processing in eukaryotes, letting cells with the same DNA become different. Mutations change sequences, and biotechnology such as PCR, gel electrophoresis and cloning lets us study and use DNA.
Key terms: replication fork, transcription, translation, codon, operon, epigenetics, PCR
7. Natural Selection
Populations evolve when heritable variation affects survival and reproduction. Evidence for evolution comes from fossils, anatomy, molecular sequences and observed change. Hardy–Weinberg equilibrium is a null model; genetic drift, gene flow, mutation, non-random mating and selection move populations away from it. Speciation happens when reproductive isolation stops gene flow, and phylogenetic trees show inferred relationships.
Key terms: fitness, genetic drift, Hardy–Weinberg, homologous structure, speciation, cladogram
8. Ecology
Organisms respond to their environment through behaviour and physiology. Energy flows through food webs and is lost at each trophic level, while matter cycles. Populations grow exponentially or logistically toward a carrying capacity, and communities are shaped by competition, predation and symbiosis. Biodiversity makes ecosystems more resilient, and human activity, invasive species and climate change can disrupt them.
Key terms: trophic level, carrying capacity, logistic growth, keystone species, biodiversity, succession
These summaries are written by Dabour Hub from the published course outline. Full notes, diagrams, quizzes and question banks are available to registered students.
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