CLEP Biology Cellular Respiration and Photosynthesis 4 — Questions and Answers
Question 1: What is the role of ubiquinone (Coenzyme Q) in the electron transport chain?
- It pumps protons directly across the inner mitochondrial membrane
- It accepts electrons from Complexes I and II and shuttles them to Complex III (Correct answer)
- It serves as the terminal electron acceptor, combining electrons with oxygen
- It regenerates NAD+ from NADH in the matrix
Correct answer: It accepts electrons from Complexes I and II and shuttles them to Complex III
Ubiquinone is a mobile, lipid-soluble electron carrier that collects electrons from both Complex I and Complex II and delivers them to Complex III.
Question 2: How many ATP molecules are produced per NADH oxidized by the mitochondrial electron transport chain (using the P/O ratio of ~2.5)?
- 1.0 ATP
- 1.5 ATP
- 2.5 ATP (Correct answer)
- 3.0 ATP
Correct answer: 2.5 ATP
Modern measurements of the P/O ratio indicate that approximately 2.5 ATP molecules are synthesized per NADH oxidized, replacing the older estimate of 3 ATP.
Question 3: Photorespiration in C3 plants occurs when Rubisco fixes oxygen instead of CO2. What is the net effect on the plant?
- It generates extra ATP and NADPH for the Calvin cycle
- It consumes ATP and releases CO2, reducing photosynthetic efficiency (Correct answer)
- It produces glucose as a byproduct of O2 fixation
- It increases the plant's water-use efficiency
Correct answer: It consumes ATP and releases CO2, reducing photosynthetic efficiency
When Rubisco reacts with O2 (oxygenase activity), the products enter the photorespiratory pathway, which consumes ATP and NADPH and releases CO2 without producing sugar.
Question 4: In non-cyclic electron flow (the Z-scheme), what is the ultimate electron donor and acceptor?
- NADPH is the donor; O2 is the acceptor
- H2O is the donor; NADP+ is the acceptor (Correct answer)
- PSII is the donor; PSI is the acceptor
- Ferredoxin is the donor; plastocyanin is the acceptor
Correct answer: H2O is the donor; NADP+ is the acceptor
Water (H2O) donates electrons (via the OEC to PSII), and NADP+ is the final electron acceptor, being reduced to NADPH by ferredoxin-NADP+ reductase at PSI.
Question 5: Which of the following best explains why the inner mitochondrial membrane must be impermeable to protons (H+)?
- To prevent ATP from leaking out of the mitochondrial matrix
- To maintain the proton gradient necessary for ATP synthesis by ATP synthase (Correct answer)
- To keep NADH and FADH2 separated from the ETC complexes
- To prevent oxygen from entering and inhibiting the citric acid cycle
Correct answer: To maintain the proton gradient necessary for ATP synthesis by ATP synthase
The impermeability of the inner membrane to H+ is essential because the proton gradient (proton-motive force) is the driving force for ATP synthase; any leak would dissipate the gradient and uncouple oxidative phosphorylation.
Question 6: During cyclic electron flow in photosynthesis, which of the following is produced?
- ATP and NADPH
- ATP only (Correct answer)
- NADPH only
- Both O2 and NADPH
Correct answer: ATP only
In cyclic electron flow, electrons cycle from PSI through ferredoxin and the cytochrome b6f complex back to plastocyanin and PSI, generating only ATP (via proton pumping) without producing NADPH or O2.
Question 7: Which metabolic pathway is shared by both aerobic respiration and fermentation?
- Oxidative phosphorylation
- The citric acid cycle
- Glycolysis (Correct answer)
- Pyruvate oxidation
Correct answer: Glycolysis
Glycolysis occurs in the cytoplasm of all cells and is the common first stage of both aerobic respiration and fermentation, converting glucose to pyruvate.
CLEP Biology Cellular Respiration and Photosynthesis 4
What is the role of ubiquinone (Coenzyme Q) in the electron transport chain?