AP Biology Key Terms: The 129 Concepts You Need to Know

By Brian Stewart, Barron’s SAT, ACT & PSAT Author & Perfect SAT/ACT Scorer

The AP® Biology exam is less about memorizing isolated facts and more about understanding how living systems actually work. The course is organized around four Big Ideas — Evolution, Energetics, Information Storage and Transmission, and Systems Interactions — and eight units that run from the chemistry of a single water molecule to whole ecosystems. The exam is heavily skills-based: you will interpret unfamiliar experiments, read graphs and data tables, analyze models and diagrams, and construct arguments from evidence. But you still need a strong grip on the vocabulary that comes up again and again, because almost every question assumes you can recognize what is being described and connect it to broader patterns in the course.

Below are the most important AP Biology terms — the structures, molecules, processes, experiments, and concepts that appear most often on multiple-choice questions, short answer questions, and long free-response questions. They are organized by the same eight units the College Board uses in the official Course and Exam Description. This is a review list aligned to the AP Biology course framework — not an official College Board canon — so treat it as a solid study foundation rather than the last word. For the broader strategic picture of the exam, including how to tackle stimulus-based questions and the biggest multiple-choice mistakes students make, see the companion guide on the AP Biology practice page.

One note before you start: Units 3, 6, and 7 are among the most heavily weighted on the exam, while Units 1 and 5 are lighter. Spend your study time accordingly — and don’t shortchange natural selection, which tends to carry the largest single share of points.

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Unit 1: Chemistry of Life — 8–11%

1. Polarity & Hydrogen Bonding. Water has polar covalent bonds between H and O, which gives it a partial positive and partial negative end. Adjacent water molecules attract each other through hydrogen bonds.
2. Cohesion, Adhesion & Surface Tension. Water sticks to itself (cohesion), sticks to other polar surfaces (adhesion), and forms a taut film at its surface (surface tension) — all consequences of hydrogen bonding.
3. Specific Heat & Heat of Vaporization. Two distinct thermal properties of water. Its high specific heat means water resists changes in temperature, helping organisms maintain a stable internal temperature. Its high heat of vaporization means water absorbs a great deal of energy before it evaporates, which is what makes evaporative cooling (sweating in animals, transpiration in plants) so effective.
4. Macromolecules. The four major classes of biological macromolecules: carbohydrates, lipids, nucleic acids, and proteins. Built primarily from carbon, hydrogen, and oxygen (plus nitrogen, phosphorus, and sulfur). Proteins, nucleic acids, and polysaccharides are true polymers of repeating monomers; lipids are grouped in because of their biological importance but are not polymers in the same sense.
5. Dehydration Synthesis & Hydrolysis. Opposite reactions. Dehydration synthesis joins two monomers and releases water. Hydrolysis uses water to split polymers back into monomers.
6. Carbohydrates. Monosaccharides (like glucose) linked by covalent bonds form polysaccharides. Key examples: cellulose (plant cell walls), starch (plant energy storage), and glycogen (animal energy storage).
7. Lipids. Nonpolar, hydrophobic molecules including fats (energy storage), steroids like cholesterol (membrane stability, hormones), and phospholipids (membrane structure). Saturated fatty acids have only single C–C bonds; unsaturated fatty acids have double bonds that kink the chain.
8. Phospholipid Bilayer. Two layers of phospholipids with hydrophilic heads facing the aqueous environment and hydrophobic tails tucked inward — the fundamental architecture of all cell membranes.
9. Nucleic Acids (DNA & RNA). Polymers of nucleotides, each with a five-carbon sugar (deoxyribose or ribose), a phosphate, and a nitrogenous base (A, T, G, C, or U). They store and transmit genetic information.
10. Antiparallel Double Helix & Base Pairing. Two DNA strands run in opposite 5′–3′ directions, held together by hydrogen bonds. A pairs with T (or U in RNA); C pairs with G.
11. Amino Acids & Peptide Bonds. The monomers of proteins. Each amino acid has a central carbon, an amino group, a carboxyl group, and a variable R group that determines whether it is polar, nonpolar, or ionic. Peptide bonds link amino acids into polypeptides.
12. Protein Structure (Primary, Secondary, Tertiary, Quaternary). Primary: amino acid sequence. Secondary: local folding into alpha-helices and beta-pleated sheets, held by hydrogen bonds. Tertiary: overall 3D shape from R-group interactions and disulfide bridges. Quaternary: assembly of multiple polypeptide subunits.
13. Denaturation. Loss of protein shape (and therefore function) due to changes in temperature, pH, or chemical environment that disrupt the bonds holding a protein in its folded form.

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Unit 2: Cells — 10–13%

14. Prokaryote vs. Eukaryote. The two fundamental cell categories. Prokaryotes (bacteria and archaea) are typically single-celled and lack a nucleus or membrane-bound organelles; their DNA is concentrated in a nucleoid region, usually as a single circular chromosome, and many also carry smaller circular plasmids. Eukaryotes (animals, plants, fungi, and protists) have a true nucleus, multiple linear chromosomes wrapped around histones, and membrane-bound organelles. Mitochondria and chloroplasts are thought to have arisen through endosymbiosis.
15. Ribosomes. Non-membrane-bound structures made of rRNA and protein. They synthesize proteins by reading mRNA. Found in all domains of life — evidence of common ancestry.
16. Endomembrane System. Interconnected network of membranes including the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and plasma membrane — working together to modify, package, and transport proteins and lipids.
17. Endoplasmic Reticulum (Rough & Smooth). Rough ER is studded with ribosomes and synthesizes membrane-bound and secreted proteins. Smooth ER lacks ribosomes and handles lipid synthesis and detoxification.
18. Golgi Apparatus. Stack of flattened membrane sacs that modifies, sorts, and packages proteins and lipids (for example, through glycosylation) for transport to their final destinations.
19. Mitochondria. Double-membrane organelles where aerobic cellular respiration occurs. The inner membrane is folded into cristae, increasing the surface area available for the electron transport chain and ATP synthesis.
20. Lysosomes. Membrane-enclosed sacs containing hydrolytic enzymes that digest macromolecules and worn-out organelles. Also play a role in apoptosis.
21. Chloroplasts. Double-membrane organelles in plants and photosynthetic algae that carry out photosynthesis. Contain stroma, thylakoids, and stacks of thylakoids called grana.
22. Surface Area-to-Volume Ratio. As cells get larger, volume increases faster than surface area, limiting the rate at which materials can be exchanged with the environment. This constrains cell size and drives structures like microvilli, root hairs, and highly folded membranes.
23. Fluid Mosaic Model. Model of the plasma membrane: a phospholipid bilayer with embedded proteins, cholesterol, glycoproteins, and glycolipids that can move laterally — hence “fluid” and “mosaic.”
24. Selective Permeability. The plasma membrane’s property of allowing some substances to cross freely while restricting others. Small nonpolar molecules cross easily; ions and large polar molecules need transport proteins.
25. Passive vs. Active Transport. Passive transport (diffusion, osmosis, facilitated diffusion) moves substances down their concentration gradient with no ATP required. Active transport moves substances against their gradient and requires energy (for example, the Na⁺/K⁺ pump).
26. Facilitated Diffusion & Aquaporins. Passive movement of polar or charged particles through specific channel or carrier proteins. Aquaporins are channel proteins that allow rapid water movement across membranes.
27. Osmosis & Water Potential. Osmosis is the diffusion of water across a selectively permeable membrane, from regions of high water potential (low solute) to low water potential (high solute).
28. Tonicity (Hypotonic, Hypertonic, Isotonic). Describes the relative solute concentration of two solutions. Cells shrink in hypertonic solutions (water out), swell or burst in hypotonic solutions (water in), and stay the same size in isotonic solutions.
29. Endocytosis & Exocytosis. Bulk transport processes that move large molecules or particles into the cell (endocytosis) or out of the cell (exocytosis) via vesicles, requiring energy.
30. Endosymbiotic Theory. Mitochondria and chloroplasts originated when a primitive eukaryotic cell engulfed free-living prokaryotes. Evidence: these organelles have their own circular DNA, their own ribosomes (similar to bacterial ribosomes), and double membranes.

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Unit 3: Cellular Energetics — 12–16%

31. Enzymes & Active Site. Biological catalysts (usually proteins) that lower activation energy and speed up reactions. Substrates bind the active site, which is shaped to fit specific molecules.
32. Competitive vs. Noncompetitive Inhibition. Competitive inhibitors bind the active site and block the substrate. Noncompetitive inhibitors bind an allosteric site, changing the enzyme’s shape and reducing its activity.
33. ATP (Adenosine Triphosphate). The cell’s main energy currency. Hydrolysis of ATP to ADP and inorganic phosphate (P_i) is exergonic because the products are more stable than ATP. The released free energy is then coupled to cellular work like active transport, synthesis, and movement. (Avoid the common shorthand that “energy is stored in the bond” — breaking any bond requires energy; the net energy release comes from forming the more stable products.)
34. Metabolic Pathway. A sequence of enzyme-catalyzed reactions in which the product of each step becomes the reactant for the next. Examples: glycolysis, the Krebs cycle, the Calvin cycle.
35. Photosynthesis. The process that uses light energy, CO₂, and H₂O to produce carbohydrates and O₂. The overall equation: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂.
36. Chloroplast Structure (Stroma, Thylakoid, Grana). Stroma: fluid interior where the Calvin cycle occurs. Thylakoids: flattened membrane sacs where the light reactions occur. Grana: stacks of thylakoids.
37. Light Reactions & Photosystems I and II. Light-dependent reactions in the thylakoid membrane. Light excites electrons in chlorophyll in photosystems I and II, generating ATP and NADPH while splitting water and releasing O₂.
38. Calvin Cycle. Light-independent reactions in the stroma that fix CO₂ using the enzyme rubisco, powered by ATP and NADPH from the light reactions. The immediate product is G3P (glyceraldehyde-3-phosphate), a three-carbon sugar that the plant then uses to build glucose and other carbohydrates.
39. Electron Transport Chain (ETC). A series of membrane-bound proteins that pass electrons in redox reactions, pumping protons across a membrane to build an electrochemical gradient. Found in the thylakoid membrane (photosynthesis) and the inner mitochondrial membrane (respiration).
40. Chemiosmosis & ATP Synthase. Protons flow back down their gradient through ATP synthase, and this proton flow drives the synthesis of ATP from ADP and inorganic phosphate. Called photophosphorylation in photosynthesis and oxidative phosphorylation in respiration.
41. Cellular Respiration. The process that uses biological macromolecules to produce ATP. Aerobic respiration uses oxygen as the terminal electron acceptor and occurs in three stages: glycolysis, the Krebs cycle, and oxidative phosphorylation.
42. Glycolysis. Glucose is split into two pyruvate molecules in the cytosol, producing a small net yield of ATP and NADH. Does not require oxygen.
43. Krebs (Citric Acid) Cycle. Before the cycle, each pyruvate from glycolysis is first oxidized to acetyl-CoA (the link reaction) in the mitochondrial matrix, releasing one CO₂ and one NADH. Acetyl-CoA then enters the cycle by combining with oxaloacetate. Over one turn, the acetyl group is fully oxidized and released as 2 CO₂. The cycle also reduces NAD⁺ to NADH and FAD to FADH₂, and produces a small amount of ATP (or GTP) by substrate-level phosphorylation.
44. Oxidative Phosphorylation. The production of ATP via the electron transport chain and chemiosmosis in the inner mitochondrial membrane. Produces most of the ATP from aerobic cellular respiration.
45. NADH & FADH₂. Electron carriers (coenzymes) produced during glycolysis and the Krebs cycle that deliver high-energy electrons to the electron transport chain.
46. Fermentation. Anaerobic pathway that allows glycolysis to continue when oxygen is unavailable. Regenerates NAD⁺ by producing lactic acid (in muscle cells) or alcohol and CO₂ (in yeast).
47. Common Ancestry of Metabolism. Core metabolic pathways like glycolysis and oxidative phosphorylation are conserved across bacteria, archaea, and eukaryotes — evidence that all life descends from a common ancestor.

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Unit 4: Cell Communication and Cell Cycle — 10–15%

48. Cell Signaling. Cells communicate through direct contact or by releasing chemical signals that bind receptors on nearby or distant cells. Local regulators act on neighboring cells; hormones act over long distances in the bloodstream.
49. Signal Transduction Pathway. A series of molecular events that converts an external signal into a cellular response. Typically involves reception, transduction (often via phosphorylation cascades), and response.
50. Ligand & Receptor. A ligand is a signaling molecule that binds a specific receptor protein on or inside a target cell. Binding changes the shape of the receptor and initiates the pathway.
51. G Protein-Coupled Receptors. A major family of membrane receptors in eukaryotes that activate intracellular signaling through associated G proteins. A key example of a receptor that triggers a signaling cascade.
52. Second Messenger (cAMP). Small intracellular molecules like cyclic AMP that relay and amplify signals from membrane receptors to intracellular targets.
53. Phosphorylation Cascade. A sequence of protein activations in which each enzyme adds a phosphate to the next, amplifying the signal and producing the final cellular response.
54. Apoptosis. Programmed cell death. A normal, regulated process that removes damaged or unneeded cells during development and tissue maintenance.
55. Negative Feedback. A response that reduces the initial stimulus and returns the system to its set point — the most common way organisms maintain homeostasis (e.g., blood sugar regulation by insulin and glucagon).
56. Positive Feedback. A response that amplifies the initial stimulus, driving the system further from its set point. Examples include the onset of labor in childbirth and blood clotting.
57. Homeostasis. The maintenance of stable internal conditions (temperature, pH, blood sugar, water balance) through regulatory mechanisms, usually negative feedback.
58. Cell Cycle & Interphase. The regulated sequence of events that produces two daughter cells. Interphase includes G1 (growth), S (DNA synthesis), and G2 (final preparation). Then comes mitosis and cytokinesis. G0 is a non-dividing phase in which cells may be metabolically active but have exited the cell cycle.
59. Mitosis. Division of the nucleus into two genetically identical daughter nuclei. Proceeds through prophase, metaphase, anaphase, and telophase. Plays a role in growth, tissue repair, and asexual reproduction.
60. Cytokinesis. Division of the cytoplasm, producing two separate daughter cells. In animal cells, a cleavage furrow pinches the cell in two; in plant cells, a cell plate forms.
61. Cyclins & Cyclin-Dependent Kinases (CDKs). Regulatory proteins whose interactions control progression through cell cycle checkpoints. CDK activity rises and falls with the cycle as cyclins are produced and degraded.
62. Checkpoints & Cancer. Internal control points (at G1/S, G2/M, and during mitosis) that halt the cell cycle if conditions are not right. Disruptions to checkpoint regulation can lead to uncontrolled division and cancer.

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Unit 5: Heredity — 8–11%

63. Meiosis. Two-stage cell division that produces four haploid gametes from one diploid cell. Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids — essential for sexual reproduction.
64. Homologous Chromosomes. Paired chromosomes (one maternal, one paternal) that carry the same genes in the same order but may have different alleles.
65. Crossing Over (Recombination). In prophase I of meiosis, non-sister chromatids of homologous chromosomes exchange segments of DNA at chiasmata. Increases genetic variation in gametes.
66. Independent Assortment. During metaphase I of meiosis, homologous pairs line up randomly at the metaphase plate, so maternal and paternal chromosomes are distributed independently to daughter cells. A major source of genetic variation.
67. Nondisjunction. Failure of chromosomes or sister chromatids to separate properly during meiosis. Produces gametes with the wrong chromosome number, leading to aneuploidy in offspring (e.g., trisomy).
68. Haploid & Diploid. Haploid (n) cells have one set of chromosomes (gametes); diploid (2n) cells have two sets (most body cells). Fertilization restores the diploid number.
69. Mendel’s Laws (Segregation & Independent Assortment). Law of segregation: the two alleles for a gene separate during gamete formation. Law of independent assortment: alleles for different genes on different chromosomes are inherited independently.
70. Genotype vs. Phenotype. Genotype is the genetic makeup (the set of alleles an organism carries). Phenotype is the observable expression of those alleles, shaped by the environment.
71. Homozygous, Heterozygous, Dominant, Recessive. Homozygous means both alleles for a gene are the same; heterozygous means they are different. A dominant allele masks a recessive allele in the heterozygote.
72. Punnett Square & Test Cross. A Punnett square predicts the probability of genotypes and phenotypes in offspring. A test cross (breeding an unknown genotype with a homozygous recessive individual) reveals whether the unknown is homozygous or heterozygous.
73. Codominance & Incomplete Dominance. In codominance, both alleles are fully expressed in the heterozygote (for example, the I^AI^B genotype in the ABO blood group produces blood type AB, with both A and B antigens on the red blood cell surface). In incomplete dominance, the heterozygote shows an intermediate phenotype (e.g., pink flowers from a red/white cross). Note that ABO is also a classic example of multiple alleles (I^A, I^B, and i) at a single gene locus.
74. Sex-Linked Traits, Pleiotropy & Phenotypic Plasticity. Sex-linked traits are carried on sex chromosomes (often X) and show distinctive inheritance patterns across sexes. Pleiotropy is when a single gene affects multiple traits. Phenotypic plasticity is when the same genotype produces different phenotypes under different environmental conditions.

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Unit 6: Gene Expression and Regulation — 12–16%

75. DNA Replication (Semiconservative). Each original DNA strand serves as a template for a new complementary strand, so each daughter molecule contains one old and one new strand. Synthesis proceeds 5′ to 3′.
76. Key Replication Enzymes. Helicase unwinds the double helix. Topoisomerase relaxes supercoiling. DNA polymerase synthesizes the new strand (only 5′ to 3′, and it requires an RNA primer). Ligase seals fragments on the lagging strand.
77. Leading vs. Lagging Strand. DNA polymerase builds the leading strand continuously in the direction of the replication fork and builds the lagging strand discontinuously in short Okazaki fragments that ligase later joins.
78. Transcription. RNA polymerase reads one strand of DNA (3′ to 5′) and synthesizes a complementary mRNA molecule (5′ to 3′). The starting point for gene expression.
79. mRNA, tRNA & rRNA. mRNA carries the genetic message from DNA to the ribosome. tRNA brings the correct amino acid to match each codon (via its anticodon). rRNA is a structural and catalytic component of the ribosome itself.
80. Introns & Exons (RNA Splicing). In eukaryotes, pre-mRNA contains non-coding introns and coding exons. Splicing removes introns and joins exons. Alternative splicing produces different mature mRNAs — and therefore different proteins — from a single gene.
81. 5′ Cap & Poly-A Tail. Modifications added to eukaryotic mRNA. The 5′ cap is a modified guanine nucleotide (7-methylguanosine, or m⁷G) that helps ribosomes recognize the mRNA and protects it from degradation; the poly-A tail (a string of adenine nucleotides at the 3′ end) increases mRNA stability and helps it exit the nucleus.
82. Translation. Ribosomes read mRNA in three-nucleotide codons and assemble the corresponding amino acids into a polypeptide. Proceeds in three stages: initiation (at the AUG start codon), elongation, and termination (at a stop codon).
83. Codon & Anticodon. A codon is a three-nucleotide sequence on mRNA that codes for one amino acid. An anticodon is the complementary three-nucleotide sequence on tRNA. The genetic code is nearly universal, which is further evidence of common ancestry.
84. Promoter, Enhancer & Transcription Factors. Promoters are DNA sequences just upstream of a gene where RNA polymerase and general transcription factors assemble to begin transcription. Enhancers are DNA sequences (often far from the gene) where specific transcription factors bind to boost transcription, typically by looping the DNA to contact the promoter. Regulating these interactions is a main way eukaryotes control gene expression.
85. Operons (lac Operon). Groups of genes in prokaryotes that are transcribed together and controlled by a single promoter. The lac operon in E. coli is the classic inducible example. Lactose (via allolactose) removes the LacI repressor, but the operon is most strongly transcribed only when glucose is also low. Low glucose raises cAMP, which binds CAP and actively recruits RNA polymerase.
86. Epigenetics. Reversible chemical modifications to DNA or histones (like methylation or acetylation) that change gene expression without changing the underlying DNA sequence.
87. Point Mutations. Single-nucleotide substitutions. A silent mutation does not change the amino acid; a missense mutation changes one amino acid; a nonsense mutation creates a premature stop codon.
88. Frameshift Mutations. Insertions or deletions of one or more nucleotides (in numbers not divisible by three) that shift the reading frame. Usually catastrophic because nearly every codon downstream changes.
89. Retrovirus & Reverse Transcriptase. A retrovirus (like HIV) has an RNA genome. Reverse transcriptase copies the viral RNA into DNA, which integrates into the host genome — an exception to the usual DNA → RNA → protein flow.
90. Gel Electrophoresis. Technique that separates DNA fragments by size. Smaller fragments travel faster through the gel toward the positive electrode. Used in DNA fingerprinting, forensic analysis, and PCR verification.
91. PCR & Bacterial Transformation. Polymerase chain reaction (PCR) amplifies specific DNA fragments through cycles of denaturing, annealing primers, and extending new strands. Bacterial transformation introduces foreign DNA (often plasmids) into bacterial cells — a core technique in genetic engineering.

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Unit 7: Natural Selection — 13–20%

92. Natural Selection. Darwin’s mechanism of evolution: individuals with phenotypes better suited to their environment survive and reproduce more, passing favorable traits to offspring. Acts on individuals, but populations evolve.
93. Adaptation. A heritable trait favored by natural selection because it increases an organism’s fitness in a particular environment. Adaptations are the product of evolution, not the result of organisms actively trying to change in response to their environment.
94. Phenotypic Variation. The raw material of natural selection. Without variation in heritable traits, selection has nothing to act on. Sources include mutation, recombination, and sexual reproduction.
95. Evolutionary Fitness. Not about strength or speed, but about reproductive success — the number of viable, fertile offspring an organism produces that survive to reproduce themselves.
96. Artificial Selection. Humans deliberately breeding organisms for desired traits. Dramatic changes in dogs, crops, and livestock in relatively short time spans support the power of selection as an evolutionary force.
97. Genetic Drift. Random changes in allele frequencies, most pronounced in small populations. Unlike natural selection, drift is non-adaptive — favorable and unfavorable alleles can be lost by chance.
98. Bottleneck Effect. A form of genetic drift caused by a drastic reduction in population size (often from a disaster). The surviving population has reduced genetic diversity, which can persist even as the population recovers.
99. Founder Effect. A form of genetic drift that occurs when a small group colonizes a new area. The new population’s gene pool reflects the founders’ alleles, which may differ substantially from the original population’s.
100. Gene Flow & Migration. Movement of alleles between populations as individuals migrate and interbreed. Tends to reduce differences between populations and can prevent them from diverging into separate species.
101. Gene Pool. The total collection of all alleles for all genes in a population. Evolution, at the population level, is a change in allele frequencies in the gene pool across generations.
102. Hardy–Weinberg Equilibrium. A mathematical model for a non-evolving population, described by p² + 2pq + q² = 1. Allele frequencies stay constant from generation to generation under five conditions: large population, no migration, no mutation, random mating, and no natural selection.
103. Using Hardy–Weinberg. Since these conditions are rarely all met, Hardy–Weinberg is most useful as a null model. If observed genotype frequencies deviate significantly from predicted values, one or more assumptions have been violated — usually indicating that selection, drift, migration, mutation, or nonrandom mating is acting on the population (i.e., evolution is occurring).
104. Homologous & Vestigial Structures. Homologous structures (like the limb bones of mammals) have a common evolutionary origin, even if they now serve different functions. Vestigial structures (like the human tailbone) are reduced remnants of features that were functional in ancestors — both provide evidence of common ancestry.
105. Molecular Evidence for Evolution. Comparing DNA and protein sequences across species reveals evolutionary relationships. More similar sequences generally indicate more recent common ancestry.
106. Phylogenetic Trees & Cladograms. Branching diagrams that represent hypothetical evolutionary relationships. Nodes represent the most recent common ancestor of the groups that branch from them. Depending on how the diagram is built, branch lengths may or may not carry meaning — some trees scale branches to time or to amount of molecular change, while others (often called cladograms) show only the branching pattern.
107. Speciation. The formation of new species when populations become reproductively isolated. The biological species concept defines a species as a group that can interbreed to produce viable, fertile offspring.
108. Reproductive Isolation. Mechanisms that prevent interbreeding. Pre-zygotic barriers (habitat, temporal, behavioral, mechanical, gametic) prevent fertilization. Post-zygotic barriers (reduced hybrid viability, reduced hybrid fertility) act after a zygote forms.
109. Allopatric vs. Sympatric Speciation. Allopatric speciation occurs when populations are geographically separated (e.g., by a new mountain range). Sympatric speciation happens within the same geographic area, often through polyploidy in plants or niche differentiation.
110. Divergent Evolution & Adaptive Radiation. Divergent evolution occurs when a population splits and adapts to different environments, producing phenotypic diversity. Adaptive radiation is a rapid burst of speciation when new habitats or resources become available — Darwin’s Galápagos finches are the classic example.
111. Convergent Evolution. Similar selective pressures produce similar adaptations in unrelated lineages. Dolphins and sharks share a streamlined body shape despite being a mammal and a fish.
112. Punctuated Equilibrium vs. Gradualism. Two models for the tempo of evolution. Gradualism: slow, steady change over long periods. Punctuated equilibrium: long periods of stasis punctuated by rapid bursts of change, often during speciation events.
113. RNA World Hypothesis. Proposes that the earliest self-replicating molecules were RNA. RNA can both store information (like DNA) and catalyze reactions (like proteins), which makes it a plausible candidate for the first step in the origin of life. Geological evidence places that origin between about 3.9 and 3.5 billion years ago.

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Unit 8: Ecology — 10–15%

114. Ecological Levels of Organization. Life is organized in a nested hierarchy: individual → population (members of one species in an area) → community (interacting populations of different species) → ecosystem (community plus its abiotic surroundings) → biome (regional groupings of ecosystems with similar climate and vegetation).
115. Biotic vs. Abiotic Factors. Biotic factors are the living components of an ecosystem — predators, prey, competitors, parasites, mutualists, decomposers. Abiotic factors are the nonliving components — temperature, water, sunlight, pH, soil chemistry, salinity. Both shape where species can live and how populations grow.
116. Endotherms vs. Ectotherms. Endotherms (like mammals and birds) generate heat metabolically and maintain a stable body temperature. Ectotherms (like reptiles and fish) rely on the external environment and regulate temperature behaviorally.
117. Food Chains, Food Webs & Trophic Levels. Food chains trace the linear flow of energy from one organism to the next. Food webs show the more realistic network of feeding relationships. Trophic levels include producers, primary consumers, secondary consumers, tertiary consumers, and decomposers. Roughly 10% of energy passes from one level to the next.
118. Autotrophs vs. Heterotrophs. Autotrophs make their own food — photoautotrophs use sunlight (plants, algae), while chemoautotrophs use inorganic chemicals (some bacteria and archaea). Heterotrophs (carnivores, herbivores, omnivores, decomposers, scavengers) consume other organisms for energy.
119. Biogeochemical Cycles (Water, Carbon, Nitrogen, Phosphorus). The global recycling of matter between biotic and abiotic reservoirs. The water cycle moves H₂O through evaporation, condensation, and precipitation. The carbon cycle runs on photosynthesis, respiration, decomposition, and combustion. The nitrogen cycle depends on microbial fixation, nitrification, and denitrification. The phosphorus cycle has no atmospheric phase and moves slowly through rock weathering.
120. Exponential Growth. Population growth with no limits. Modeled by dN/dt = r_max N. Produces a characteristic J-shaped curve. Rare in nature except briefly, such as when a species colonizes a new environment.
121. Logistic Growth & Carrying Capacity. Population growth that slows as resources become limited, modeled by dN/dt = r_max N ((K−N)/K). Produces an S-shaped curve. Carrying capacity (K) is the maximum population size the environment can sustain.
122. Density-Dependent vs. Density-Independent Factors. Density-dependent factors (disease, predation, competition for resources) intensify as populations grow. Density-independent factors (weather, natural disasters) affect populations regardless of size.
123. Simpson’s Diversity Index. A measure of community diversity that accounts for both species richness (number of species) and relative abundance. The AP Biology formula sheet uses D = 1 − Σ(n/N)², in which higher values (closer to 1) indicate greater diversity. Be aware that some other texts use D = Σ(n/N)², where the interpretation is reversed — higher values mean less diversity.
124. Symbiosis: Mutualism, Commensalism, Parasitism. Long-term interactions between different species. Mutualism benefits both (+/+). Commensalism benefits one while the other is unaffected (+/0). Parasitism benefits one at the other’s expense (+/−).
125. Competition & Niche. Competition for shared resources can be intraspecific (within a species) or interspecific (between species). A niche is the role and resource use of a species — no two species can occupy exactly the same niche indefinitely (competitive exclusion).
126. Predator-Prey Interactions. Drive population dynamics, natural selection, and community structure. Can produce oscillating population cycles, where predator numbers lag slightly behind prey numbers.
127. Keystone Species. A species whose effect on its community is disproportionately large relative to its abundance. Removing a keystone species — like sea otters in a kelp forest — can cause the whole community to collapse.
128. Invasive Species. Non-native species that outcompete native species for resources, often because they escape their usual predators or competitors. Examples: kudzu and zebra mussels.
129. Biomagnification & Eutrophication. Biomagnification: toxins (like DDT or mercury) accumulate at increasing concentrations up the food chain. Eutrophication: nutrient runoff (especially nitrogen and phosphorus) causes algal blooms that deplete oxygen in aquatic ecosystems.

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How to Use This List

Don’t try to memorize all of these terms in one sitting, and don’t just memorize definitions. For each term, ask yourself three questions: What structure or process is this? What does it do? How does it connect to the Big Ideas? The exam rewards students who can place a term in context, connect it to the four Big Ideas (Evolution, Energetics, Information Storage and Transmission, Systems Interactions), and link it to related concepts across other units — for example, seeing how the structure of a phospholipid (Unit 1) explains the selective permeability of a membrane (Unit 2), or how a mutation in a signal transduction receptor (Units 4 and 6) can lead to uncontrolled cell division and cancer.

A good pace is one unit per day, returning to earlier units as you add new ones. After you have worked through a unit’s terms, run the corresponding stimulus-based drills on the AP Biology practice page — that is where you will see these terms in the form the exam actually tests them, embedded in experimental data, graphs, models, and diagrams.

Good luck — and remember that on the AP Biology exam, knowing the terms is just the starting line. Points come from using them as evidence in your reasoning.

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