Photosynthesis Reaction Stages 2 — Questions and Answers
Question 1: Which protein complex in the thylakoid membrane splits water molecules to release oxygen?
- Photosystem I
- Photosystem II (Correct answer)
- ATP synthase
- Ferredoxin
Correct answer: Photosystem II
Photosystem II contains the oxygen-evolving complex that oxidizes water, releasing O2, protons, and electrons.
Question 2: What is the primary electron donor that reduces NADP+ at the end of the light reactions?
- Plastoquinone
- Ferredoxin (Correct answer)
- Plastocyanin
- Cytochrome b6f
Correct answer: Ferredoxin
Ferredoxin passes electrons from Photosystem I to NADP+ reductase, which reduces NADP+ to NADPH.
Question 3: During the Calvin cycle, how many carbon dioxide molecules must be fixed to produce one molecule of G3P that exits the cycle?
- 1
- 2
- 3 (Correct answer)
- 6
Correct answer: 3
Three CO2 molecules are fixed per turn of the Calvin cycle to net one G3P molecule for export.
Question 4: Which enzyme catalyzes the regeneration of RuBP from G3P in the Calvin cycle?
- RuBisCO
- Phosphoglycerate kinase
- Phosphoribulokinase (Correct answer)
- Triose phosphate isomerase
Correct answer: Phosphoribulokinase
Phosphoribulokinase uses ATP to phosphorylate ribulose-5-phosphate, regenerating RuBP.
Question 5: In cyclic photophosphorylation, electrons cycle back to which component after leaving Photosystem I?
- Water
- Plastoquinone (Correct answer)
- NADPH
- RuBisCO
Correct answer: Plastoquinone
In cyclic flow, electrons from PSI return to plastoquinone, driving proton pumping and ATP synthesis without producing NADPH.
Question 6: What happens to most of the G3P produced in the Calvin cycle?
- It is exported immediately as glucose
- It is used to regenerate RuBP (Correct answer)
- It enters glycolysis directly
- It is stored as starch in the thylakoid
Correct answer: It is used to regenerate RuBP
Five out of every six G3P molecules produced are used to regenerate RuBP, maintaining the cycle.
Question 7: Which term describes the ATP-powered movement of protons across the thylakoid membrane that drives ATP synthesis?
- Substrate-level phosphorylation
- Photophosphorylation (Correct answer)
- Oxidative phosphorylation
- Fermentation
Correct answer: Photophosphorylation
Photophosphorylation is the light-driven synthesis of ATP via the proton gradient across the thylakoid membrane.
Photosynthesis Reaction Stages 2
Which protein complex in the thylakoid membrane splits water molecules to release oxygen?