AP Biology Cheat Sheet 2026 — Processes, Genetics, Ecology & Evolution

🔬 Cell Biology — Membranes & Transport

Type	What moves	Energy	Examples
Simple diffusion	Nonpolar/small molecules	Passive (no ATP)	O₂, CO₂, lipids move down concentration gradient
Facilitated diffusion	Polar/large molecules via protein channels	Passive (no ATP)	Glucose (GLUT), ions through channel proteins
Osmosis	Water via aquaporins	Passive (no ATP)	Water moves toward lower water potential (higher solute)
Active transport	Molecules against gradient	ATP required	Na⁺/K⁺ pump (3 Na⁺ out, 2 K⁺ in)
Endocytosis / exocytosis	Large particles via vesicles	ATP required	Phagocytosis (in), secretion of hormones (out)

Water potential shortcut: Ψ = Ψs + Ψp. Pure water Ψ = 0. Adding solute makes Ψs negative → water moves in. Adding pressure makes Ψp positive → water moves out. Water flows from high Ψ to low Ψ.

⚡ Cellular Respiration

Stage	Location	Inputs	Outputs	ATP yield
Glycolysis	Cytoplasm	Glucose (C₆)	2 pyruvate, 2 NADH, 2 ATP (net)	2 ATP (net)
Pyruvate oxidation	Mitochondrial matrix	2 pyruvate	2 Acetyl-CoA, 2 CO₂, 2 NADH	0 ATP
Krebs cycle	Mitochondrial matrix	2 Acetyl-CoA	4 CO₂, 6 NADH, 2 FADH₂, 2 ATP	2 ATP
ETC / Oxidative phosphorylation	Inner mitochondrial membrane	10 NADH, 2 FADH₂, O₂	H₂O, ~32–34 ATP	~32–34 ATP

Key shortcut: NADH → ~2.5 ATP; FADH₂ → ~1.5 ATP via ETC. The actual AP Bio yield to know is ~30–32 ATP total per glucose (some textbooks say 36–38; the actual biological yield is lower).

🌿 Photosynthesis

Stage	Location	Inputs	Outputs
Light-dependent reactions	Thylakoid membranes	H₂O, light energy, ADP, NADP⁺	O₂, ATP, NADPH
Calvin cycle (light-independent)	Stroma	CO₂, ATP, NADPH	G3P (→ glucose), ADP, NADP⁺

Electron flow shortcut: PSII → (ETC/ATP synthase) → PSI → NADP⁺ reductase → NADPH. Water is split at PSII (O₂ released). Cyclic electron flow: PSI only → ATP but no NADPH and no O₂.

C3 vs C4 vs CAM: C3 (most plants) — CO₂ directly enters Calvin cycle, stomata open day; C4 (corn, sugarcane) — CO₂ fixed in mesophyll first (PEP carboxylase), reduces photorespiration; CAM (cacti) — stomata open at night, store malic acid.

🧬 Genetics & Heredity Shortcuts

Concept	Rule / Shortcut
Monohybrid cross (Aa × Aa)	Offspring ratio: 1 AA : 2 Aa : 1 aa → phenotype 3:1 dominant:recessive
Dihybrid cross (AaBb × AaBb)	Phenotype ratio: 9:3:3:1 (A_B_ : A_bb : aaB_ : aabb)
Incomplete dominance	Aa = blended phenotype (red × white = pink); ratio 1:2:1 phenotype
Codominance	Both alleles expressed simultaneously (AB blood type)
X-linked recessive	Carrier female (X^A X^a) × normal male → 50% of sons affected; females need 2 copies to show
Chi-square test	χ² = Σ(O−E)²/E. Compare to critical value at df = (categories − 1). p > 0.05 → fail to reject H₀
Hardy-Weinberg	p² + 2pq + q² = 1; p + q = 1. No evolution if: large pop, random mating, no mutation/selection/migration

🦠 Gene Expression & Regulation

Step	Where	Key enzyme	Product
DNA replication	Nucleus	DNA polymerase (reads 3'→5', builds 5'→3')	2 identical DNA strands
Transcription	Nucleus	RNA polymerase	pre-mRNA → mRNA (after splicing introns)
Translation	Ribosome (cytoplasm or RER)	Ribosome + tRNA	Polypeptide (protein)

Lac operon shortcut (E. coli): Default = repressor blocks transcription. Lactose present → allolactose binds repressor → operator free → genes transcribed. Glucose also present → CAP site inactive → low transcription even if lactose present. Genes OFF when: no lactose (repressor ON) or glucose present (CAP inactive).

🌍 Evolution & Natural Selection

Mechanism	How it changes allele frequencies	Direction
Natural selection	Differential reproductive success based on phenotype	Directional / stabilizing / disruptive
Genetic drift	Random chance — especially in small populations	Unpredictable; reduces variation
Bottleneck effect	Sudden population reduction → loss of alleles	Reduces variation
Founder effect	Small group starts new population	Reduces variation; different from source pop
Gene flow	Immigration/emigration brings in or removes alleles	Usually reduces differences between populations
Mutation	Ultimate source of all new alleles	Random; raw material for evolution
Sexual selection	Mate choice or competition drives traits	Intersexual (mate choice) or intrasexual (competition)

Speciation types: Allopatric (geographic barrier → reproductive isolation) vs. Sympatric (same area — polyploidy in plants, habitat differentiation). Prezygotic barriers: temporal, behavioral, mechanical, gametic isolation. Postzygotic: hybrid inviability/sterility.

🌲 Ecology — Populations, Communities & Ecosystems

Concept	Quick Rule
Logistic growth	dN/dt = rN × (K−N)/K. At K (carrying capacity): growth = 0. S-curve. J-curve = exponential (no limits).
r-selected vs K-selected	r: small, many offspring, short lifespan (bacteria, insects). K: large, few offspring, long lifespan, parental care (elephants, humans).
10% rule (energy transfer)	Only ~10% of energy passes to next trophic level. Autotrophs → herbivores → carnivores → top carnivores: each step loses ~90%
Primary productivity	GPP = total photosynthesis. NPP = GPP − respiration. NPP is what's available to consumers.
Nitrogen cycle	N₂ → NH₄⁺ (nitrogen fixation by bacteria) → NO₂⁻ → NO₃⁻ (nitrification) → N₂ (denitrification). Plants use NH₄⁺ or NO₃⁻.
Keystone species	Disproportionate effect on ecosystem relative to biomass (sea otters → sea urchins → kelp)
Ecological succession	Primary: bare rock → pioneer species (lichens) → shrubs → climax community. Secondary: starts from soil disturbance.

🧪 AP Bio Lab Essentials

Lab	What you measure	Key trick
Osmosis (potato/dialysis tube)	% mass change of tissue	% change = (final − initial) / initial × 100. Tissue gains mass in hypotonic solution.
Photosynthesis (floating leaf disks)	Rate of O₂ production (disks rise)	More light or CO₂ → faster rise. Boiling water kills leaves (control).
Cellular respiration (respirometer)	O₂ consumed via KOH absorbing CO₂	KOH absorbs CO₂ so only O₂ consumption measured. Germinating seeds vs. dry seeds.
Mitosis/meiosis (onion root)	Frequency of cells in each phase	Most cells in interphase (G1+S+G2 take longest). Frequency ∝ time spent in phase.
Gel electrophoresis	Fragment size separation	Smaller fragments move FARTHER. DNA is negatively charged → migrates toward (+) end.
Chi-square (genetics crosses)	Observed vs. expected ratios	If χ² < critical value → ratios consistent with hypothesis (fail to reject H₀)