CLEP Biology Cellular Respiration and Photosynthesis 2 — Questions and Answers
Question 1: Which enzyme catalyzes the conversion of pyruvate to acetyl-CoA during pyruvate oxidation?
- Pyruvate kinase
- Pyruvate dehydrogenase complex (Correct answer)
- Lactate dehydrogenase
- Acetyl-CoA carboxylase
Correct answer: Pyruvate dehydrogenase complex
The pyruvate dehydrogenase complex catalyzes the oxidative decarboxylation of pyruvate to acetyl-CoA, releasing CO2 and reducing NAD+ to NADH.
Question 2: During the light-dependent reactions, what is the role of plastoquinone (PQ)?
- It accepts electrons from NADPH
- It donates electrons to P700
- It shuttles electrons from PSII to the cytochrome b6f complex (Correct answer)
- It splits water molecules
Correct answer: It shuttles electrons from PSII to the cytochrome b6f complex
Plastoquinone carries electrons from Photosystem II to the cytochrome b6f complex, also pumping H+ ions into the thylakoid lumen.
Question 3: In the citric acid cycle, which step directly produces GTP (or ATP)?
- Isocitrate → α-ketoglutarate
- Succinyl-CoA → succinate (Correct answer)
- Malate → oxaloacetate
- Citrate → isocitrate
Correct answer: Succinyl-CoA → succinate
The conversion of succinyl-CoA to succinate by succinyl-CoA synthetase is the only substrate-level phosphorylation step in the citric acid cycle.
Question 4: What is the primary function of the oxygen-evolving complex (OEC) in photosynthesis?
- Reducing NADP+ to NADPH
- Splitting water to release O2 and replenish electrons to PSII (Correct answer)
- Pumping H+ across the thylakoid membrane
- Absorbing light energy at 700 nm
Correct answer: Splitting water to release O2 and replenish electrons to PSII
The OEC (also called the water-splitting complex) oxidizes water molecules, releasing O2 gas and providing electrons to replace those lost by P680 in Photosystem II.
Question 5: How many carbon atoms enter the citric acid cycle per turn via acetyl-CoA, and how many leave as CO2?
- 2 enter, 1 leaves
- 2 enter, 2 leave (Correct answer)
- 3 enter, 3 leave
- 4 enter, 2 leave
Correct answer: 2 enter, 2 leave
Two carbons enter as acetyl-CoA and two carbons are released as CO2 (from isocitrate dehydrogenase and α-ketoglutarate dehydrogenase reactions) per turn.
Question 6: Which photosynthetic pigment has peak absorption at approximately 700 nm and is the reaction center of Photosystem I?
- P680 chlorophyll a
- Chlorophyll b
- P700 chlorophyll a (Correct answer)
- Beta-carotene
Correct answer: P700 chlorophyll a
P700 is a specialized chlorophyll a molecule that absorbs light maximally at 700 nm and serves as the reaction center of Photosystem I.
Question 7: During anaerobic fermentation in yeast, what happens to pyruvate?
- It is converted to lactate by lactate dehydrogenase
- It is decarboxylated to acetaldehyde, then reduced to ethanol (Correct answer)
- It enters the citric acid cycle as acetyl-CoA
- It is phosphorylated to form ATP
Correct answer: It is decarboxylated to acetaldehyde, then reduced to ethanol
In alcoholic fermentation, pyruvate is first decarboxylated to acetaldehyde by pyruvate decarboxylase, then reduced to ethanol by alcohol dehydrogenase using NADH.
CLEP Biology Cellular Respiration and Photosynthesis 2
Which enzyme catalyzes the conversion of pyruvate to acetyl-CoA during pyruvate oxidation?