Photosynthesis Cellular Respiration 3 — Questions and Answers
Question 1: Which carbon compound enters the Krebs cycle to begin each turn?
- Acetyl-CoA (Correct answer)
- Pyruvate
- Oxaloacetate
- Citrate
Correct answer: Acetyl-CoA
Acetyl-CoA (a 2-carbon compound) combines with oxaloacetate at the start of each Krebs cycle turn, forming citrate.
Question 2: How many turns of the Krebs cycle are required to fully oxidize one glucose molecule?
- 2 turns (Correct answer)
- 1 turn
- 4 turns
- 6 turns
Correct answer: 2 turns
One glucose yields two pyruvates, each producing one acetyl-CoA, so the Krebs cycle runs twice per glucose.
Question 3: What is the role of coenzyme A (CoA) in cellular respiration?
- It carries acetyl groups into the Krebs cycle (Correct answer)
- It carries electrons to the electron transport chain
- It acts as a proton pump in the inner mitochondrial membrane
- It phosphorylates ADP to form ATP
Correct answer: It carries acetyl groups into the Krebs cycle
CoA forms acetyl-CoA by attaching to the 2-carbon acetyl group derived from pyruvate, delivering it to the Krebs cycle.
Question 4: Where in the cell does the Krebs cycle take place?
- Mitochondrial matrix (Correct answer)
- Inner mitochondrial membrane
- Cytoplasm
- Intermembrane space
Correct answer: Mitochondrial matrix
The Krebs cycle enzymes are dissolved in the mitochondrial matrix, where the cycle reactions occur.
Question 5: Which Krebs cycle intermediate directly donates electrons to FAD to produce FADH2?
- Succinate (Correct answer)
- Malate
- Isocitrate
- Alpha-ketoglutarate
Correct answer: Succinate
Succinate is oxidized to fumarate by succinate dehydrogenase, which transfers electrons to FAD, producing FADH2.
Question 6: What net ATP yield does the Krebs cycle produce directly per glucose (via substrate-level phosphorylation)?
- 2 ATP (as GTP) (Correct answer)
- 4 ATP
- 6 ATP
- 0 ATP
Correct answer: 2 ATP (as GTP)
The Krebs cycle produces 1 GTP (equivalent to ATP) per turn, so 2 GTP total for one glucose molecule.
Question 7: A student inhibits alpha-ketoglutarate dehydrogenase in a cell. Which Krebs cycle intermediate would accumulate?
- Alpha-ketoglutarate (Correct answer)
- Citrate
- Succinate
- Oxaloacetate
Correct answer: Alpha-ketoglutarate
Alpha-ketoglutarate dehydrogenase converts alpha-ketoglutarate to succinyl-CoA; inhibiting it causes alpha-ketoglutarate to build up.
Photosynthesis Cellular Respiration 3
Which carbon compound enters the Krebs cycle to begin each turn?