AP Biology Unit 3 — Cellular Energetics — is, per released FRQs, the most commonly tested unit on the exam. Roughly 1 of every 3 FRQs touches it directly, and it also feeds into Units 4 and 8 indirectly through experimental design. If you're tight on review time, this is the unit to lock down.
The four concepts the exam tests
- Enzymes — how they lower activation energy, how factors (pH, temperature, inhibitors) alter their function.
- Photosynthesis — light-dependent reactions, Calvin cycle, the inputs and outputs of each.
- Cellular respiration — glycolysis, Krebs, electron transport chain; aerobic vs. anaerobic yields.
- Fitness in environmental contexts — how energetic processes affect organism survival.
Enzymes: the FRQ you'll definitely see
Classic FRQ setup: a data table shows enzyme activity (rate of product formed per minute) at different pH values. You'll be asked to: graph it, identify the optimal pH, predict the effect of a competitive inhibitor, and design an experiment to distinguish between competitive and non-competitive inhibition.
Photosynthesis: the input/output table to memorize
| Stage | Inputs | Outputs | Location |
|---|---|---|---|
| Light-dependent reactions | H₂O, light, NADP⁺, ADP + Pᵢ | O₂, NADPH, ATP | Thylakoid membrane |
| Calvin cycle (light-independent) | CO₂, NADPH, ATP | G3P → glucose, NADP⁺, ADP | Stroma |
The Calvin cycle does not directly use light. It uses the products of the light-dependent reactions. Students lose points every year for writing "sunlight" as an input to the Calvin cycle. Don't.
Cellular respiration: the yields are the answer
| Stage | Inputs | Products | ATP yield |
|---|---|---|---|
| Glycolysis | Glucose, 2 ATP, 2 NAD⁺ | 2 Pyruvate, 2 NADH, 4 ATP | +2 net |
| Pyruvate oxidation | 2 Pyruvate, 2 NAD⁺ | 2 Acetyl-CoA, 2 NADH, 2 CO₂ | 0 |
| Krebs cycle | 2 Acetyl-CoA, 6 NAD⁺, 2 FAD, 2 ADP | 6 NADH, 2 FADH₂, 2 ATP, 4 CO₂ | +2 |
| Electron transport chain | 10 NADH, 2 FADH₂, O₂ | H₂O, ~34 ATP | ~+34 |
Total per glucose: about 38 ATP in ideal conditions, often quoted as 32–36 in modern textbooks because of inefficiencies in the shuttles. Either range gets the mark on the FRQ.
The experimental-design question
Unit 3 FRQs love to ask "design an experiment to test whether X affects Y." The rubric wants: a hypothesis stated as a cause-effect claim, an independent variable with specified levels, a dependent variable with a measurable unit, a control group, and at least one confounding variable you're holding constant. Miss any of those and you lose that point.
Template: "Hypothesis: [X] affects [Y] by [mechanism]. IV: [X], at levels [A, B, C]. DV: [Y], measured in [units]. Control: [setup without X]. Constants: [temperature, pH, time, volume]."
