Photosynthesis Carbon Fixation 3 — Questions and Answers
Question 1: During the regeneration phase of the Calvin cycle, how many ATP molecules are consumed to regenerate three molecules of RuBP from five molecules of G3P?
- 3
- 5 (Correct answer)
- 6
- 9
Correct answer: 5
Three ATP are used by phosphoribulokinase to phosphorylate three molecules of ribulose-5-phosphate, regenerating three RuBP molecules.
Question 2: What is photorespiration and which enzyme is responsible for initiating it?
- CO₂ release driven by ATP synthase
- O₂ addition to RuBP catalyzed by RuBisCO (Correct answer)
- Glucose breakdown in the light by pyruvate kinase
- Water splitting catalyzed by the oxygen-evolving complex
Correct answer: O₂ addition to RuBP catalyzed by RuBisCO
Photorespiration occurs when RuBisCO adds O₂ (instead of CO₂) to RuBP, producing 2-phosphoglycolate, which must be recycled in a costly process that releases CO₂.
Question 3: In which cellular compartment of the chloroplast does carbon fixation (the Calvin cycle) take place?
- Thylakoid lumen
- Outer membrane space
- Stroma (Correct answer)
- Intermembrane space
Correct answer: Stroma
The Calvin cycle, including carbon fixation by RuBisCO, occurs in the stroma — the fluid-filled space surrounding the thylakoid membranes.
Question 4: For every three CO₂ molecules fixed by the Calvin cycle, how many molecules of G3P (glyceraldehyde-3-phosphate) are produced in net?
- 1 (Correct answer)
- 2
- 3
- 6
Correct answer: 1
Fixing 3 CO₂ produces 6 G3P molecules total, but 5 are used to regenerate RuBP, leaving a net gain of 1 G3P per turn of the cycle.
Question 5: Which intermediate in the Calvin cycle is directly phosphorylated using ATP to produce 1,3-bisphosphoglycerate?
- G3P
- RuBP
- 3-phosphoglycerate (Correct answer)
- Ribulose-5-phosphate
Correct answer: 3-phosphoglycerate
Phosphoglycerate kinase phosphorylates 3-PGA using ATP to form 1,3-bisphosphoglycerate, which is then reduced by NADPH to produce G3P.
Question 6: In C4 plants, where does the Calvin cycle (secondary carbon fixation) actually take place?
- Mesophyll cell chloroplasts
- Bundle sheath cell chloroplasts (Correct answer)
- Guard cell cytoplasm
- Epidermal cell vacuoles
Correct answer: Bundle sheath cell chloroplasts
In C4 plants, CO₂ is released from 4-carbon acids in the bundle sheath cells, where RuBisCO and the Calvin cycle operate in a high-CO₂ environment.
Question 7: What molecule serves as the carbon dioxide acceptor in the CAM pathway during nighttime fixation?
- RuBP
- Phosphoenolpyruvate (PEP) (Correct answer)
- Oxaloacetate
- 3-phosphoglycerate
Correct answer: Phosphoenolpyruvate (PEP)
PEP carboxylase uses phosphoenolpyruvate (PEP) as the CO₂ acceptor at night, producing oxaloacetate which is rapidly converted to malate for storage.
Photosynthesis Carbon Fixation 3
During the regeneration phase of the Calvin cycle, how many ATP molecules are consumed to regenerate three molecules of RuBP from five molecules of G3P?