Photosynthesis Calvin Cycle 2 — Questions and Answers
Question 1: Which enzyme catalyzes the carboxylation of RuBP in the Calvin cycle?
- RuBisCO (Correct answer)
- Phosphoglycerate kinase
- Glyceraldehyde-3-phosphate dehydrogenase
- Rubisco activase
Correct answer: RuBisCO
RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the fixation of CO₂ onto RuBP.
Question 2: How many turns of the Calvin cycle are needed to produce one molecule of glucose?
- 3
- 6 (Correct answer)
- 12
- 18
Correct answer: 6
Six turns fix 6 CO₂ molecules, providing enough carbon to synthesize one glucose (C₆H₁₂O₆).
Question 3: What is the oxidation state of carbon in CO₂ compared to the carbon in G3P?
- CO₂ is more oxidized than G3P (Correct answer)
- CO₂ is more reduced than G3P
- Both have the same oxidation state
- G3P is more oxidized than CO₂
Correct answer: CO₂ is more oxidized than G3P
Carbon in CO₂ is fully oxidized (+4), while carbon in G3P is partially reduced, reflecting the energy invested in the Calvin cycle.
Question 4: Which phase of the Calvin cycle directly consumes ATP without consuming NADPH?
- Carbon fixation
- Reduction
- Regeneration of RuBP (Correct answer)
- All phases use ATP equally
Correct answer: Regeneration of RuBP
The regeneration phase uses 3 ATP (per 3 CO₂ fixed) exclusively to phosphorylate ribulose-5-phosphate back to RuBP.
Question 5: What would happen to the Calvin cycle if NADPH were suddenly unavailable?
- Carbon fixation would stop immediately
- 3-PGA would accumulate and G3P production would halt (Correct answer)
- RuBP would accumulate
- ATP synthesis would increase to compensate
Correct answer: 3-PGA would accumulate and G3P production would halt
Without NADPH, the reduction of 3-PGA to G3P cannot occur, so 3-PGA accumulates while G3P output drops to zero.
Question 6: Which metabolite is considered both a product and a reactant within the Calvin cycle?
- G3P
- 3-PGA
- RuBP (Correct answer)
- CO₂
Correct answer: RuBP
RuBP is consumed when it accepts CO₂ and regenerated at the end of the cycle, making it both a reactant and a product.
Question 7: In the Calvin cycle, what is the direct product formed when RuBP reacts with CO₂?
- Two molecules of 3-PGA (Correct answer)
- One molecule of 6-carbon sugar
- One molecule of G3P and one CO₂
- Two molecules of RuBP
Correct answer: Two molecules of 3-PGA
The unstable 6-carbon intermediate immediately splits into two 3-carbon molecules of 3-phosphoglycerate (3-PGA).
Photosynthesis Calvin Cycle 2
Which enzyme catalyzes the carboxylation of RuBP in the Calvin cycle?