Photosynthesis Cellular Respiration 2 — Questions and Answers
Question 1: Which molecule serves as the primary electron carrier in the electron transport chain during cellular respiration?
- NADH (Correct answer)
- ATP
- Pyruvate
- Acetyl-CoA
Correct answer: NADH
NADH donates electrons to Complex I of the electron transport chain, driving the proton gradient used for ATP synthesis.
Question 2: At which pH range does the intermembrane space of the mitochondria sit during active cellular respiration?
- Around pH 4–5 (Correct answer)
- Around pH 7–8
- Around pH 9–10
- Around pH 6–6.5
Correct answer: Around pH 4–5
Protons pumped into the intermembrane space by the ETC complexes make it more acidic, roughly pH 4–5.
Question 3: How many ATP molecules are produced per FADH2 via oxidative phosphorylation?
- ~1.5 ATP (Correct answer)
- ~2.5 ATP
- ~3 ATP
- ~4 ATP
Correct answer: ~1.5 ATP
FADH2 enters the ETC at Complex II, bypassing Complex I, so it yields approximately 1.5 ATP compared to NADH's ~2.5 ATP.
Question 4: What happens to pyruvate when oxygen is unavailable in human muscle cells?
- It is converted to lactate (Correct answer)
- It enters the Krebs cycle directly
- It is converted to ethanol
- It is broken down into CO2 and water
Correct answer: It is converted to lactate
In oxygen-deprived human muscle cells, pyruvate accepts electrons from NADH and is reduced to lactate, regenerating NAD+.
Question 5: Which enzyme catalyzes the conversion of pyruvate to acetyl-CoA?
- Pyruvate dehydrogenase complex (Correct answer)
- Pyruvate kinase
- Citrate synthase
- Isocitrate dehydrogenase
Correct answer: Pyruvate dehydrogenase complex
The pyruvate dehydrogenase complex oxidatively decarboxylates pyruvate, producing acetyl-CoA, CO2, and NADH.
Question 6: Rotenone is a poison that blocks Complex I of the electron transport chain. What would be the immediate effect on ATP production?
- ATP production drops sharply because NADH cannot donate electrons (Correct answer)
- ATP production increases because protons accumulate
- ATP production is unaffected because Complex II compensates fully
- ATP production shifts entirely to the Krebs cycle
Correct answer: ATP production drops sharply because NADH cannot donate electrons
Blocking Complex I prevents NADH oxidation, collapsing the proton gradient and drastically reducing ATP synthesis.
Question 7: Which of the following correctly describes substrate-level phosphorylation?
- ATP is formed by direct transfer of a phosphate group from a substrate to ADP (Correct answer)
- ATP is formed using the proton gradient across the inner mitochondrial membrane
- ATP is formed by photons exciting chlorophyll electrons
- ATP is formed by the hydrolysis of GTP
Correct answer: ATP is formed by direct transfer of a phosphate group from a substrate to ADP
Substrate-level phosphorylation directly transfers a high-energy phosphate from a metabolic intermediate to ADP, as in glycolysis and the Krebs cycle.
Photosynthesis Cellular Respiration 2
Which molecule serves as the primary electron carrier in the electron transport chain during cellular respiration?