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Volume I · MMXXVI AP Biology
Library› AP Biology› Unit 3: Cellular Energetics
⁂   AP Biology · Unit 3

3. Cellular Energetics

12–16% of the AP exam. 8 key topics. 4 common errors analyzed.

12–16% exam weight 16 instructional periods

Unit 3: Cellular Energetics

Unit 3 covers cellular energetics — a core component of the AP Biology curriculum. Master these concepts to build the mechanistic vocabulary the FRQ rubrics demand.

Key topics in this unit

  1. Photosynthesis: light-dependent reactions — photosystems I and II, electron transport chain, ATP synthase, NADPH production
  2. Photosynthesis: Calvin cycle (light-independent reactions) — carbon fixation, G3P production, RuBisCO
  3. Cellular respiration: glycolysis — location (cytoplasm), inputs/outputs, net ATP yield
  4. Cellular respiration: pyruvate oxidation and Krebs cycle — acetyl-CoA entry, CO2 release, NADH/FADH2 production
  5. Cellular respiration: oxidative phosphorylation — electron transport chain, chemiosmosis, ATP synthase
  6. ATP yield comparison across aerobic respiration stages
  7. Anaerobic pathways: lactic acid fermentation, alcoholic fermentation
  8. Factors affecting rates of photosynthesis and cellular respiration

Common errors — what the rubric penalizes

These are the most frequently penalized mistakes on AP exam FRQs and MCQs. Review each before your next practice session.

Common error

Placing glycolysis in the mitochondria rather than the cytoplasm — a frequent and heavily penalized error on FRQs

Common error

Confusing NADPH (produced in photosynthesis, used in Calvin cycle) with NADH (produced in respiration), and mixing up FAD/FADH2 with NAD+/NADH

Common error

Reversing the direction of the proton gradient — protons accumulate in the intermembrane space (mitochondria) or thylakoid lumen (chloroplasts), not the matrix/stroma

Common error

Stating that fermentation produces ATP directly, when fermentation's role is to regenerate NAD+ so glycolysis can continue

Study checklist

  • Can you list all key topics from memory without this page?
  • Have you worked through at least one FRQ-style question per topic?
  • Can you explain each common error above — and why the correct answer differs?
  • Have you reviewed the relevant College Board CED section and formula sheet entries?
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