Photosynthesis Calvin Cycle 3 — Questions and Answers
Question 1: How does photorespiration differ from the Calvin cycle in terms of RuBisCO activity?
- RuBisCO fixes O₂ instead of CO₂, releasing CO₂ and wasting energy (Correct answer)
- RuBisCO is inactive during photorespiration
- RuBisCO produces extra G3P during photorespiration
- Photorespiration regenerates RuBP faster
Correct answer: RuBisCO fixes O₂ instead of CO₂, releasing CO₂ and wasting energy
During photorespiration, RuBisCO adds O₂ to RuBP instead of CO₂, producing a 2-carbon compound that is metabolized to release CO₂ without making sugar.
Question 2: C4 plants minimize photorespiration primarily by doing what?
- Running the Calvin cycle in mesophyll cells only
- Concentrating CO₂ around RuBisCO in bundle sheath cells (Correct answer)
- Eliminating RuBisCO entirely
- Using a different carboxylase that ignores O₂
Correct answer: Concentrating CO₂ around RuBisCO in bundle sheath cells
C4 plants initially fix CO₂ via PEP carboxylase in mesophyll cells, then release concentrated CO₂ near RuBisCO in bundle sheath cells, suppressing O₂ competition.
Question 3: What is the net output of the Calvin cycle per 3 CO₂ fixed?
- One G3P, 3 ATP consumed, 3 NADPH consumed
- One G3P, 9 ATP consumed, 6 NADPH consumed (Correct answer)
- Two G3P, 6 ATP consumed, 6 NADPH consumed
- One glucose, 6 ATP consumed, 6 NADPH consumed
Correct answer: One G3P, 9 ATP consumed, 6 NADPH consumed
Per 3 CO₂ fixed, the cycle produces one net G3P while consuming 9 ATP and 6 NADPH.
Question 4: Which of the following best describes the role of ATP in the regeneration phase of the Calvin cycle?
- It reduces 1,3-bisphosphoglycerate to G3P
- It phosphorylates ribulose-5-phosphate to form RuBP (Correct answer)
- It carboxylates RuBP
- It oxidizes NADPH to NADP⁺
Correct answer: It phosphorylates ribulose-5-phosphate to form RuBP
ATP donates a phosphate group to ribulose-5-phosphate, converting it to RuBP so the cycle can continue.
Question 5: If a plant is exposed to radioactive ¹⁴CO₂ and illuminated for only 2 seconds, which compound would first become labeled?
- G3P
- RuBP
- 3-PGA (Correct answer)
- Glucose
Correct answer: 3-PGA
The first stable product of CO₂ fixation is 3-PGA; Calvin's experiments confirmed this through short-pulse ¹⁴C labeling.
Question 6: CAM plants open their stomata at night to take in CO₂ primarily to:
- Run the Calvin cycle in darkness
- Store CO₂ as malate and release it for the Calvin cycle during the day (Correct answer)
- Avoid photorespiration by using a different pathway at night
- Increase oxygen uptake for mitochondrial respiration
Correct answer: Store CO₂ as malate and release it for the Calvin cycle during the day
CAM plants fix CO₂ into malate at night and decarboxylate it during the day, supplying CO₂ to RuBisCO while stomata remain closed to conserve water.
Question 7: What is the correct ratio of ATP to NADPH required per CO₂ fixed in the Calvin cycle?
- 1:1
- 2:1
- 3:2 (Correct answer)
- 2:3
Correct answer: 3:2
The Calvin cycle requires 3 ATP and 2 NADPH for each CO₂ fixed, giving a 3:2 ratio.
Photosynthesis Calvin Cycle 3
How does photorespiration differ from the Calvin cycle in terms of RuBisCO activity?