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AP Biology Unit 3: Cellular Energetics — Worked Examples

Light Reactions and Electron Transport in Photosynthesis

Hard

A researcher treats isolated chloroplasts with DCMU, a herbicide that blocks electron flow from photosystem II (PSII) to the plastoquinone pool. The chloroplasts are then illuminated. Which of the following correctly predicts the effect on photosynthesis?

  1. O2 production stops and NADPH production stops; ATP production via cyclic electron flow may continue ✓ Correct
  2. O2 production continues but NADPH production stops because PSI is unaffected
  3. Both O2 and NADPH production stop completely, and no ATP is produced by any mechanism
  4. O2 production stops, NADPH production stops, and ATP production stops because all require linear electron flow
Solution

DCMU blocks electron transfer from PSII to plastoquinone. This halts linear (noncyclic) electron flow entirely: (1) O2 production stops because PSII no longer needs to replace electrons from water splitting; (2) NADPH production stops because no electrons reach NADP+ reductase via the linear chain. However, PSI can still operate in cyclic electron flow, recycling electrons through cytochrome b6f to generate a proton gradient and produce some ATP. Choice B incorrectly assumes O2 comes from a process independent of PSII electron flow. Choice C and D both incorrectly claim ATP production is impossible — cyclic electron flow around PSI can still generate ATP.

ATP Yield Under Anaerobic Conditions

Hard

Yeast cells are grown in a sealed container with glucose as the sole carbon source. Initially the container has oxygen, but as the yeast consume it, conditions become anaerobic. A student measures ATP production rate per glucose molecule under both conditions. Which statement best explains the change in ATP yield per glucose when conditions shift from aerobic to anaerobic?

  1. Glycolysis stops in the absence of oxygen, eliminating all ATP production
  2. Without oxygen as the final electron acceptor, the electron transport chain cannot function, so only substrate-level phosphorylation in glycolysis produces ATP ✓ Correct
  3. Fermentation produces fewer ATP than glycolysis because it consumes ATP to regenerate NAD+
  4. The Krebs cycle continues but produces less ATP without oxygen to drive the proton gradient
Solution

In the absence of oxygen, there is no final electron acceptor for the electron transport chain (ETC), so oxidative phosphorylation cannot occur. The ETC accounts for ~34 of the ~36-38 ATP from aerobic respiration. Without it, only the 2 ATP from glycolysis (via substrate-level phosphorylation) are produced. Fermentation (producing ethanol + CO2 in yeast) serves solely to regenerate NAD+ so glycolysis can continue. Choice A is wrong because glycolysis does not require oxygen. Choice C is wrong because fermentation does not consume ATP — it regenerates NAD+. Choice D is wrong because the Krebs cycle effectively stops without NAD+ and FAD being recycled by the ETC.