2025–26 CED · AP Biology

Complete Study Guide
All 8 Units

Distilled from the official College Board Course & Exam Description. Every essential knowledge statement you need.

UNIT 1
8–11%
UNIT 2
10–13%
UNIT 3
12–16%
UNIT 4
10–15%
UNIT 5
8–11%
UNIT 6
12–16%
UNIT 7
13–20%
UNIT 8
10–15%
UNIT 01 Chemistry of Life 8–11% of exam
1.1 — Water & Hydrogen Bonding
Properties of Water
  • Water is polar — oxygen pulls electrons more strongly, creating partial charges (δ− on O, δ+ on H)
  • Polarity allows hydrogen bonding between water molecules and with other polar/charged molecules
  • High specific heat capacity — water absorbs a lot of heat before its temperature rises; stabilizes body temperature
  • High heat of vaporization — evaporation removes a lot of heat; allows evaporative cooling (sweating)
  • Cohesion — water molecules stick to each other (H-bonds); creates surface tension
  • Adhesion — water sticks to other polar surfaces (e.g., capillary action in xylem)
Know WHY each property matters biologically, not just what it is. The exam will give you a scenario and ask you to predict or explain.
1.2 — Elements of Life
Elemental Composition of Macromolecules
  • C, H, O — present in all four macromolecule classes (carbs, lipids, proteins, nucleic acids)
  • Nitrogen (N) — in nucleic acids (nitrogenous bases) and also in proteins (amine group)
  • Phosphorus (P) — in nucleic acids (phosphate backbone) and phospholipids
  • Sulfur (S) — in proteins only (some R groups; disulfide bridges)
1.3 — Building & Breaking Macromolecules
Dehydration Synthesis & Hydrolysis
  • Dehydration synthesis (condensation) — joins monomers by removing water (–OH from one + –H from other → H₂O lost, covalent bond forms). Builds polymers.
  • Hydrolysis — breaks polymers by adding water across a covalent bond. Breaks down macromolecules.
  • Both reactions apply to all four macromolecule types
1.4 — Carbohydrates
Structure & Function
  • Monomer = monosaccharide (simple sugar, e.g., glucose)
  • Polymer = polysaccharide (complex carbohydrate); can be linear or branched
  • Cellulose — structural; plant cell walls; humans can't digest it
  • Starch — energy storage in plants
  • Glycogen — energy storage in animals (liver, muscle)
Molecular structure of specific carbohydrate polymers (exact bond types, ring diagrams, etc.) — not tested.
1.5 — Lipids
Structure & Function
  • Generally nonpolar and hydrophobic — don't mix with water
  • Saturated fatty acids — all single C–C bonds; straight chain; solid at room temp (e.g., butter)
  • Unsaturated fatty acids — one or more C=C double bonds; kinked chain; liquid at room temp (e.g., olive oil)
  • More double bonds = more unsaturated = more fluid at room temp
  • Fats — energy storage, insulation
  • Phospholipids — form the lipid bilayer of cell membranes; hydrophilic head + 2 hydrophobic tails
  • Steroids (including cholesterol) — cholesterol stabilizes animal membranes; steroids function as hormones (growth, metabolism, homeostasis)
Be ready to predict what happens to membrane fluidity if you change the ratio of saturated to unsaturated fats, or change cholesterol levels.
1.6 — Nucleic Acids
DNA & RNA Structure
  • Monomer = nucleotide — made of: 5-carbon sugar + phosphate group + nitrogenous base
  • DNA sugar = deoxyribose; RNA sugar = ribose
  • DNA bases: A, T, G, C — A pairs with T (2 H-bonds), G pairs with C (3 H-bonds)
  • RNA bases: A, U, G, C — uracil replaces thymine
  • DNA = double-stranded antiparallel helix; RNA = typically single-stranded
  • Strands have directionality: 5' end (phosphate) → 3' end (hydroxyl); new nucleotides always added to the 3' end
Molecular structure of individual nucleotides — not tested in detail.
1.7 — Proteins
Amino Acids & Protein Structure
  • Monomer = amino acid — central carbon + H + carboxyl group (–COOH) + amine group (–NH₂) + variable R group
  • R group determines properties: hydrophobic/nonpolar, hydrophilic/polar, or ionic
  • Amino acids joined by peptide bonds (covalent; formed by dehydration synthesis between carboxyl and amine groups)
  • Primary structure — specific sequence of amino acids; determines everything else
  • Secondary structure — local folding via H-bonds along the backbone; produces α-helices and β-pleated sheets
  • Tertiary structure — overall 3D shape from R-group interactions (H-bonds, hydrophobic interactions, ionic bonds, disulfide bridges)
  • Quaternary structure — multiple polypeptide chains interacting (e.g., hemoglobin)
  • All four levels together determine function — if shape changes, function changes (denaturation)
The exam loves asking what happens when a protein is denatured (heat, pH change, etc.) — always connect it to loss of shape → loss of function.
Molecular structure of specific amino acids — not tested.
UNIT 02 Cells 10–13% of exam
2.1 — Cell Structure & Function
Organelles
  • Ribosomes — made of rRNA + protein; synthesize proteins from mRNA; found in all cells (prokaryotes and eukaryotes); evidence of common ancestry
  • Endomembrane system — ER, Golgi, lysosomes, vacuoles, vesicles, nuclear envelope, plasma membrane — all work together to modify, package, and transport lipids, polysaccharides, and proteins
  • Rough ER — has ribosomes; protein synthesis and initial processing
  • Smooth ER — no ribosomes; lipid synthesis, detoxification
  • Golgi complex — flattened membrane sacs; folds, modifies, and packages proteins; sorts them for trafficking
  • Mitochondria — double membrane; site of aerobic cellular respiration; inner membrane highly folded (cristae) to maximize ATP production surface
  • Chloroplasts — double membrane; site of photosynthesis; plants and photosynthetic algae only
  • Lysosomes — contain hydrolytic enzymes; digest macromolecules and old organelles; also involved in apoptosis (programmed cell death)
  • Vacuoles — in plants: large central vacuole maintains turgor pressure, stores water/nutrients; in animals: smaller, more numerous
Don't just memorize what each organelle does — be ready to explain how structure enables function (e.g., WHY does the inner mitochondrial membrane fold?).
2.2 — Cell Size
Surface Area-to-Volume Ratio
  • SA:V ratio limits cell size — as cells grow, volume increases faster than surface area
  • Smaller cells have higher SA:V → more efficient exchange of materials with environment
  • Larger cells have lower SA:V → harder to get nutrients in / waste out
  • Solutions: membrane folds (cristae, microvilli), cell division, elongated shapes
  • Same principle applies to organisms: smaller organisms have higher metabolic rate per unit body mass because they exchange heat faster
Volume of sphere: V = (4/3)πr³  |  SA of sphere: SA = 4πr²
Volume of cube: V = s³  |  SA of cube: SA = 6s²
2.3–2.6 — Membranes & Transport
Fluid Mosaic Model & Membrane Permeability
  • Fluid mosaic model — phospholipid bilayer with embedded proteins, cholesterol, glycoproteins, and glycolipids; all components can move laterally
  • Phospholipids: hydrophilic phosphate heads face outward (toward water); hydrophobic fatty acid tails face inward
  • Selective permeability: small nonpolar molecules (O₂, CO₂, N₂) pass freely; large polar molecules and ions need transport proteins
  • Small polar uncharged molecules (H₂O, NH₃) pass slowly in small amounts
  • Passive transport — net movement from high → low concentration; no energy required (simple diffusion, facilitated diffusion)
  • Facilitated diffusion — uses channel or carrier proteins; still down concentration gradient; no ATP; ions (Na⁺, K⁺) need channel proteins; large polar molecules need carrier proteins; aquaporins move water
  • Active transport — moves molecules against concentration gradient (low → high); requires ATP; uses transport proteins (e.g., Na⁺/K⁺ pump)
  • Endocytosis — cell engulfs material by membrane folding inward; requires energy
  • Exocytosis — vesicle fuses with plasma membrane; releases contents outside; requires energy
2.7 — Tonicity & Osmoregulation
Osmosis & Water Potential
  • Osmosis — diffusion of water across a selectively permeable membrane from low solute → high solute (equivalently: high water potential → low water potential)
  • Hypotonic solution — lower solute than cell; water enters cell (cell swells/lyses in animal; turgor pressure in plant)
  • Hypertonic solution — higher solute than cell; water leaves cell (cell shrinks/crenates in animal; plasmolysis in plant)
  • Isotonic solution — equal solute concentration; no net water movement
  • Water potential (ψ) = pressure potential (ψₚ) + solute potential (ψₛ)
  • Solute potential: ψₛ = −iCRT (i = ionization constant, C = molarity, R = 0.0831 L·bar/mol·K, T = temp in Kelvin)
ψ = ψₚ + ψₛ  |  ψₛ = −iCRT
Water potential math shows up on FRQs. Water always moves from higher ψ to lower ψ. Pure water = ψ of 0 (highest possible).
2.8–2.10 — Transport Mechanisms & Origins
Active Transport & Endosymbiosis
  • Na⁺/K⁺ pump uses ATP to move 3 Na⁺ out and 2 K⁺ in → creates electrochemical gradient → maintains membrane potential
  • Endosymbiosis theory — mitochondria and chloroplasts were once free-living prokaryotes engulfed by a host cell; evidence: double membranes, own DNA, ribosomes similar to bacteria, reproduce by binary fission
  • Prokaryotes lack membrane-bound organelles but have internal specialized regions
  • Eukaryotes have internal membranes that compartmentalize reactions — reduces competing interactions, increases surface area
  • Cell walls (bacteria, archaea, fungi, plants) provide structural boundary and protect from osmotic lysis
UNIT 03 Cellular Energetics 12–16% of exam
3.1–3.2 — Enzymes
Enzyme Structure, Function & Environmental Factors
  • Enzymes are biological catalysts — lower activation energy; speed up reactions without being consumed
  • Enzymes are specific — shape of active site fits a specific substrate (induced fit model)
  • If enzyme shape changes → active site changes → enzyme stops working (denaturation)
  • Factors affecting enzyme activity:
    • Temperature — optimal range; too hot = denaturation; too cold = slower (less kinetic energy)
    • pH — each enzyme has an optimal pH; change in pH alters R-group charges → changes shape
    • Substrate concentration — more substrate → faster reaction, up to a saturation point
    • Enzyme concentration — more enzyme → faster reaction (if substrate not limiting)
    • Inhibitors — competitive (block active site); noncompetitive (bind elsewhere, change shape)
Enzyme experiments are a favorite FRQ topic. Know how to set up controls, identify independent/dependent variables, and predict results of changing one factor.
3.3 — Cellular Energy
ATP & Energy Transfer
  • ATP (adenosine triphosphate) is the primary energy currency of cells
  • Energy is released when the terminal phosphate bond is hydrolyzed: ATP → ADP + Pᵢ + energy
  • Cells use free energy from exergonic reactions (like cellular respiration) to drive endergonic reactions (like biosynthesis) through ATP coupling
  • Redox reactions — electrons transferred between molecules; oxidation = loss of electrons; reduction = gain of electrons (OIL RIG)
  • Electron carriers: NAD⁺/NADH and FAD/FADH₂ — carry high-energy electrons from glucose breakdown to the electron transport chain
3.4 — Photosynthesis
Light Reactions & Calvin Cycle
  • Overall: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
  • Occurs in chloroplasts: thylakoid membrane (light reactions) + stroma (Calvin cycle)
  • Light-dependent reactions (in thylakoids):
    • Chlorophyll absorbs light → excites electrons
    • Water is split (photolysis) → releases O₂ as byproduct
    • Electrons move through ETC → produce ATP (via ATP synthase) and NADPH
  • Calvin cycle / light-independent reactions (in stroma):
    • Uses ATP + NADPH from light reactions
    • CO₂ is fixed (added to RuBP by RuBisCO) → eventually produces G3P → used to build glucose
  • Factors that affect photosynthesis rate: light intensity, CO₂ concentration, water availability, temperature
3.5 — Cellular Respiration
Glycolysis, Krebs Cycle & Oxidative Phosphorylation
  • Overall: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP
  • Glycolysis (cytoplasm) — glucose (6C) → 2 pyruvate (3C); net 2 ATP + 2 NADH; no O₂ needed
  • Pyruvate oxidation (mitochondrial matrix) — pyruvate → acetyl-CoA + CO₂; produces NADH
  • Krebs cycle / citric acid cycle (matrix) — acetyl-CoA feeds in; releases CO₂; produces NADH, FADH₂, and 2 ATP per glucose
  • Oxidative phosphorylation / ETC (inner mitochondrial membrane) — NADH and FADH₂ donate electrons → electrons move down ETC → energy pumps H⁺ ions → H⁺ flows back through ATP synthase (chemiosmosis) → ~34 ATP produced
  • O₂ is the final electron acceptor → forms water
  • Fermentation (anaerobic) — when O₂ unavailable; regenerates NAD⁺ so glycolysis can continue; produces lactate (animals) or ethanol + CO₂ (yeast); only 2 ATP net
You don't need to memorize exact ATP counts — but know which stages produce ATP, NADH, FADH₂, and CO₂, and where each stage occurs.
UNIT 04 Cell Communication & Cell Cycle 10–15% of exam
4.1–4.3 — Signal Transduction
Cell Communication: Reception → Transduction → Response
  • Cells communicate via chemical signals (ligands); signals bind to specific receptors (lock-and-key specificity)
  • Three-stage model:
    • Reception — signal (ligand) binds to receptor (on surface or inside cell)
    • Transduction — signal is converted and amplified inside the cell through a cascade of molecular changes; often involves second messengers (e.g., cAMP) and phosphorylation
    • Response — cell carries out a specific action (gene expression, enzyme activation, movement, etc.)
  • Signal molecules include: hormones, neurotransmitters, growth factors
  • Nonpolar signals (like steroid hormones) can cross the membrane and bind receptors inside the cell
  • Polar/large signals bind surface receptors (like G-protein coupled receptors or receptor tyrosine kinases)
4.4 — Feedback Mechanisms
Positive & Negative Feedback
  • Negative feedback — response counteracts the stimulus; maintains homeostasis (e.g., thermoregulation, blood glucose regulation by insulin/glucagon)
  • Positive feedback — response amplifies the stimulus; used to drive processes to completion (e.g., blood clotting, childbirth contractions, action potentials)
  • Most homeostatic mechanisms use negative feedback
Be able to identify whether a diagram shows positive or negative feedback and predict what happens if a component is disrupted.
4.5–4.6 — Cell Cycle & Its Regulation
Mitosis & Checkpoints
  • Cell cycle: G₁ (growth) → S (DNA replication) → G₂ (prep for division) → M (mitosis) → cytokinesis
  • Mitosis phases: Prophase → Metaphase → Anaphase → Telophase (PMAT)
  • Results in 2 genetically identical daughter cells (same chromosome number as parent)
  • Checkpoints regulate the cycle:
    • G₁ checkpoint — checks: is cell big enough? Is DNA undamaged? Are conditions favorable?
    • G₂ checkpoint — checks: was DNA replicated correctly?
    • M checkpoint (spindle checkpoint) — checks: are all chromosomes attached to spindle fibers?
  • Cyclins and CDKs — proteins that regulate progression through checkpoints
  • Proto-oncogenes — normal genes that promote cell division; if mutated → oncogenes → uncontrolled division → cancer
  • Tumor suppressor genes (e.g., p53) — normally stop the cycle or trigger apoptosis when DNA is damaged; if mutated/lost → cancer
  • Apoptosis — programmed cell death; essential for development and removing damaged cells
UNIT 05 Heredity 8–11% of exam
5.1–5.2 — Meiosis & Genetic Diversity
Meiosis
  • Produces 4 genetically unique haploid (n) gametes from one diploid (2n) cell
  • Meiosis I — homologous chromosomes separate (reduction division); crossing over occurs in prophase I
  • Meiosis II — sister chromatids separate (like mitosis)
  • Crossing over (recombination) — exchange of segments between homologous chromosomes during prophase I → creates new allele combinations (genetic recombination)
  • Independent assortment — homologous pairs orient randomly at metaphase I → each gamete gets a random mix of maternal/paternal chromosomes
  • Both crossing over and independent assortment increase genetic diversity
5.3 — Mendelian Genetics
Inheritance Patterns
  • Dominant allele masks recessive allele in heterozygotes
  • Law of Segregation — two alleles separate during gamete formation; each gamete gets one allele
  • Law of Independent Assortment — alleles for different genes assort independently (true for genes on different chromosomes)
  • Use Punnett squares to predict offspring genotype and phenotype ratios
  • Monohybrid cross (Aa × Aa) → 3:1 phenotype ratio, 1:2:1 genotype ratio
  • Dihybrid cross (AaBb × AaBb) → 9:3:3:1 phenotype ratio
  • Testcross — cross unknown genotype with homozygous recessive to determine genotype
5.4 — Non-Mendelian Genetics
Extensions Beyond Simple Dominance
  • Incomplete dominance — heterozygote shows intermediate phenotype (e.g., red + white = pink)
  • Codominance — both alleles fully expressed in heterozygote (e.g., AB blood type)
  • Multiple alleles — more than 2 alleles exist in population (e.g., ABO blood types: Iᴬ, Iᴮ, i)
  • Polygenic inheritance — multiple genes affect one trait → continuous variation (e.g., skin color, height)
  • Epistasis — one gene masks or modifies expression of another gene
  • Sex-linked traits — genes on sex chromosomes (usually X); males (XY) are hemizygous for X-linked traits; more common in males for recessive X-linked traits
  • Linked genes — genes on the same chromosome tend to be inherited together; crossing over can separate them
5.5 — Environmental Effects on Phenotype
Gene–Environment Interactions
  • Phenotype is not determined by genotype alone — environment can modify expression
  • Examples: Himalayan rabbits (fur color depends on temperature), plants growing in different soils, human height (nutrition affects final height)
  • Same genotype can produce different phenotypes in different environments
UNIT 06 Gene Expression & Regulation 12–16% of exam
6.1–6.4 — Central Dogma
DNA Replication → Transcription → Translation
  • DNA replication — semi-conservative (each new molecule has one old strand + one new); occurs at replication forks; DNA polymerase adds nucleotides 5'→3'; requires a primer
  • Transcription — DNA → mRNA; RNA polymerase reads template strand 3'→5' and builds mRNA 5'→3'; occurs in nucleus (eukaryotes)
  • RNA processing (eukaryotes only) — introns removed, exons spliced together; 5' cap and poly-A tail added; allows mRNA to leave nucleus
  • Translation — mRNA → protein; occurs at ribosomes; tRNA brings amino acids matching codons; starts at AUG (Met); ends at stop codon (UAA, UAG, UGA)
  • Codon = 3-base sequence on mRNA; anticodon = complementary sequence on tRNA
  • Genetic code is universal (same in all organisms → evidence of common ancestry) and degenerate (multiple codons can code for same amino acid)
6.5–6.6 — Gene Regulation & Cell Specialization
How Gene Expression Is Controlled
  • Not all genes are expressed in every cell at every time — gene regulation controls which proteins are made
  • Prokaryote regulation — operons (e.g., lac operon): repressor proteins block transcription; when inducer is present, repressor is inactivated and genes are expressed
  • Eukaryote regulation — transcription factors bind to promoters/enhancers to activate or repress transcription; chromatin remodeling (histone modification, DNA methylation) affects accessibility
  • All cells in an organism have the same DNA — differentiation results from differential gene expression
  • Stem cells can differentiate into specialized cell types based on which genes are turned on/off
  • Epigenetics — heritable changes in gene expression without changes to DNA sequence (e.g., methylation, acetylation)
6.7 — Mutations
Types & Effects of Mutations
  • Point mutations — change in a single nucleotide:
    • Silent — same amino acid (due to degeneracy of genetic code); no effect
    • Missense — different amino acid; may or may not affect function
    • Nonsense — creates premature stop codon → shortened, usually nonfunctional protein
  • Frameshift mutations — insertion or deletion of nucleotides (not in multiples of 3) → shifts reading frame → almost always disrupts protein function drastically
  • Mutations are the original source of all genetic variation; can be beneficial, neutral, or harmful
6.8 — Biotechnology
Key Tools & Applications
  • Gel electrophoresis — separates DNA/protein fragments by size using electric current; smaller fragments travel farther
  • PCR (polymerase chain reaction) — amplifies specific DNA sequences; requires primers, DNA polymerase (Taq), and thermal cycling
  • Restriction enzymes — cut DNA at specific sequences; used in cloning and gel electrophoresis
  • Recombinant DNA — DNA from two different sources combined (e.g., inserting human insulin gene into bacteria)
  • CRISPR-Cas9 — precise gene editing tool; guide RNA directs Cas9 enzyme to cut specific DNA sequence
  • Transformation — bacteria take up foreign DNA; used to produce recombinant proteins
UNIT 07 Natural Selection 13–20% of exam — HIGHEST WEIGHT
7.1–7.2 — Natural Selection
Mechanism of Natural Selection
  • Four conditions required for natural selection:
    • Variation — individuals in a population vary in traits
    • Heritability — variation is heritable (passed to offspring)
    • Differential survival/reproduction — some variants survive and reproduce better
    • Selection pressure — environment favors certain traits
  • Acts on phenotype; changes in allele frequencies in the population over time
  • Types: directional, stabilizing, disruptive selection
7.3 — Artificial Selection
Human-Directed Selection
  • Humans select organisms with desired traits to breed → demonstrates that heritable variation exists and selection changes populations
  • Evidence that natural selection can work (same mechanism, human as selector)
  • Examples: dog breeds, crop plants, livestock
7.4–7.5 — Population Genetics & Hardy-Weinberg
Hardy-Weinberg Equilibrium
  • H-W equilibrium — allele frequencies don't change if: large population, random mating, no mutation, no gene flow, no natural selection
  • If any condition is violated → evolution is occurring
  • H-W equations:
p + q = 1  (allele frequencies)
p² + 2pq + q² = 1  (genotype frequencies)
  • p = frequency of dominant allele; q = frequency of recessive allele
  • p² = homozygous dominant; 2pq = heterozygous; q² = homozygous recessive
  • If you know the frequency of homozygous recessives (q²), you can solve for everything else
H-W problems are common on the exam. Practice solving for p, q, and all three genotype frequencies from a given q² value.
7.6–7.8 — Evidence of Evolution & Continuing Change
Evidence & Mechanisms of Evolution
  • Fossil record — shows changes in organisms over time; transitional forms
  • Comparative anatomy — homologous structures (same origin, different function) and analogous structures (different origin, similar function; convergent evolution)
  • Molecular evidence — DNA/protein sequence similarities; more similar sequences = more closely related
  • Biogeography — distribution of species reflects evolutionary history
  • Other evolutionary mechanisms:
    • Genetic drift — random changes in allele frequency; stronger in small populations; bottleneck effect; founder effect
    • Gene flow — movement of alleles between populations via migration; can introduce new alleles
    • Mutation — ultimate source of new alleles
    • Sexual selection — selection based on mate choice
7.9–7.10 — Phylogeny & Speciation
Phylogenetic Trees & How New Species Form
  • Phylogenetic trees — show evolutionary relationships; nodes = common ancestors; branches = lineages; outgroups provide reference
  • Read trees by finding the most recent common ancestor of two groups
  • Speciation — formation of new species; requires reproductive isolation
  • Allopatric speciation — geographic barrier separates populations → diverge independently → eventually reproductively isolated
  • Sympatric speciation — new species form in same geographic area (e.g., polyploidy in plants)
  • Reproductive isolating mechanisms: prezygotic (prevent mating or fertilization) and postzygotic (hybrids are infertile or don't survive)
7.11–7.12 — Variation & Origins of Life
Sources of Variation & Abiogenesis
  • Sources of genetic variation: mutation, sexual reproduction (crossing over + independent assortment + random fertilization)
  • Variation within a population is essential for long-term survival — provides raw material for selection
  • Origin of life — early Earth had reducing atmosphere; organic monomers could form abiotically (Miller-Urey experiment); RNA world hypothesis (RNA was first self-replicating molecule); membranes formed from lipids
UNIT 08 Ecology 10–15% of exam
8.1 — Responses to the Environment
Behavioral & Physiological Responses
  • Organisms respond to environmental changes through behavioral (movement, activity patterns) and physiological (internal chemical/physical changes) mechanisms
  • Organisms exchange information with each other in response to environmental cues — can change behavior (e.g., predator warnings, fight-or-flight)
  • Communication can involve chemical signals, visual displays, sounds, or touch
Specific mechanisms of behavior (detailed neuroscience, specific hormones involved) — not tested at this level.
8.2 — Energy Flow Through Ecosystems
Trophic Levels, Energy Transfer & Biogeochemical Cycles
  • Producers (autotrophs) fix energy via photosynthesis or chemosynthesis; form the base of all food webs
  • ~10% rule — only about 10% of energy transfers between trophic levels; the rest is lost as heat
  • Food chains and food webs show who eats whom; energy flows in one direction (unidirectional)
  • Biogeochemical cycles (new to 2025–26 CED) — matter is recycled; energy is not:
    • Carbon cycle — photosynthesis fixes CO₂; respiration releases it; decomposers break down organic matter; fossil fuel burning releases stored carbon
    • Nitrogen cycle — N₂ fixed by bacteria → NH₃ → NO₃⁻ (nitrification) → absorbed by plants → decomposed → denitrification releases N₂ back to atmosphere
    • Phosphorus cycle — no atmospheric component; weathering of rocks → soil → plants → consumers → decomposers
    • Water cycle — evaporation, condensation, precipitation, transpiration, runoff
Biogeochemical cycles were newly added to this unit for 2025–26. Make sure you study these — they're almost certainly going to be tested.
8.3–8.4 — Population Ecology
Population Growth & Limiting Factors
  • Exponential growth — occurs when resources are unlimited; J-shaped curve; dN/dt = rN
  • Logistic growth — growth slows as population approaches carrying capacity (K); S-shaped (sigmoidal) curve; dN/dt = rN(K–N)/K
  • Density-dependent factors — effect intensifies as population grows (competition, predation, disease, starvation)
  • Density-independent factors — affect population regardless of density (natural disasters, temperature extremes)
  • r-selected species — many offspring, little parental care, short lifespan; good at colonizing new environments
  • K-selected species — few offspring, high parental care, long lifespan; competitive in stable environments
Exponential: dN/dt = rN
Logistic: dN/dt = rN[(K−N)/K]
8.5 — Community Ecology
Species Interactions & Succession
  • Competition (–/–) — both species harmed; leads to competitive exclusion or resource partitioning
  • Predation (+/–) — predator benefits, prey harmed; drives coevolution
  • Mutualism (+/+) — both species benefit (e.g., mycorrhizae, nitrogen-fixing bacteria in legumes)
  • Commensalism (+/0) — one benefits, other unaffected
  • Parasitism (+/–) — parasite benefits, host harmed
  • Keystone species — has disproportionately large effect on community relative to its abundance; removal collapses community structure
  • Primary succession — begins on bare rock/no soil; pioneer species (lichens) → soil builds → more complex communities
  • Secondary succession — begins after disturbance in area with existing soil; faster than primary
8.6 — Biodiversity
Importance & Threats to Biodiversity
  • Biodiversity includes genetic diversity, species diversity, and ecosystem diversity
  • High biodiversity → greater ecosystem stability and resilience
  • Major threats: habitat destruction, invasive species, pollution, overexploitation, climate change
  • Island biogeography — species richness determined by island size and distance from mainland (immigration vs. extinction rates)
8.7 — Disruptions in Ecosystems
Human Impact & Ecosystem Disruptions
  • Disruptions (natural or human-caused) can alter energy flow, species interactions, and nutrient cycles
  • Eutrophication — excess nutrients (N, P) → algal bloom → O₂ depletion → dead zone
  • Biological magnification — toxins accumulate at higher trophic levels (e.g., DDT, mercury in fish)
  • Climate change affects biodiversity, phenology, species ranges, and cycle timing
  • Introduced/invasive species can disrupt food webs, outcompete native species
Ecology FRQs often involve analyzing a graph of population growth or energy flow and making predictions — practice reading and interpreting these.