AP Biology Unit 3: Cellular Energetics
Enzymes, photosynthesis and cellular respiration.
Unit overview
Cells capture, store and use energy through coupled reactions. Enzymes speed reactions by lowering activation energy; their activity depends on shape, so temperature, pH, and competitive or noncompetitive inhibitors all change reaction rates.
Photosynthesis captures light energy in the thylakoid membranes, where an electron transport chain pumps protons and ATP synthase makes ATP; the Calvin cycle then uses ATP and NADPH to fix carbon dioxide into sugar. Cellular respiration releases energy from glucose through glycolysis, the Krebs cycle and oxidative phosphorylation, where a proton gradient across the inner mitochondrial membrane drives ATP synthesis. Fermentation regenerates NAD+ when oxygen is absent.
Key topics
- Enzymes
- Active sites, activation energy, denaturation and inhibition.
- Photosynthesis
- Light-dependent reactions and the Calvin cycle.
- Cellular respiration
- Glycolysis, Krebs cycle, electron transport chain and chemiosmosis.
- Fermentation
- Regenerating NAD+ so glycolysis can continue without oxygen.
Common mistakes
- Saying enzymes are used up or that they add energy to reactions.
- Saying plants photosynthesise instead of respiring; plant cells do both.
- Forgetting that oxygen's role is to be the final electron acceptor in the electron transport chain.
Exam tips
- Graph questions often ask you to explain a change in rate; link it to enzyme shape and active-site interactions.
- Follow the protons: most energy questions come down to how a gradient is built and used.
More practice on this site
See the AP Biology exam format and every unit on the course page.