Photosynthesis Light Reactions 3 — Questions and Answers
Question 1: Which component of the thylakoid membrane directly uses the proton gradient to synthesize ATP?
- Photosystem I
- Cytochrome b6f complex
- ATP synthase (CF1-CF0) (Correct answer)
- Ferredoxin-NADP+ reductase
Correct answer: ATP synthase (CF1-CF0)
The chloroplast ATP synthase (CF1-CF0 complex) uses the proton-motive force created by the proton gradient across the thylakoid membrane to drive ATP synthesis.
Question 2: What is the significance of the high proton concentration in the thylakoid lumen?
- It directly reduces NADP+ to NADPH
- It creates a proton-motive force that drives ATP synthesis (Correct answer)
- It activates the oxygen-evolving complex
- It absorbs light energy for photosystems
Correct answer: It creates a proton-motive force that drives ATP synthesis
The accumulation of protons in the thylakoid lumen creates a electrochemical gradient (proton-motive force) that powers ATP synthase to produce ATP.
Question 3: Which electron carrier in the light reactions is embedded in the thylakoid membrane and carries both electrons and protons?
- Plastocyanin
- Ferredoxin
- Plastoquinone (Correct answer)
- Cytochrome f
Correct answer: Plastoquinone
Plastoquinone (PQ) is a lipid-soluble molecule embedded in the thylakoid membrane that picks up electrons and protons from PSII and delivers them to the cytochrome b6f complex.
Question 4: At which absorption wavelength does Photosystem I show maximum efficiency?
- 680 nm
- 700 nm (Correct answer)
- 720 nm
- 660 nm
Correct answer: 700 nm
Photosystem I has its reaction center chlorophyll a (P700) with an absorption maximum at 700 nm, hence its designation.
Question 5: What happens to the electrons that are used to reduce NADP+ in the light reactions?
- They return to Photosystem II via the electron transport chain
- They are used to split water molecules
- They enter the Calvin cycle to fix CO2
- They are permanently incorporated into NADPH (Correct answer)
Correct answer: They are permanently incorporated into NADPH
The electrons transferred to NADP+ remain in NADPH, which then carries them to the Calvin cycle where they are used in the reduction of 3-PGA.
Question 6: Why does the red drop phenomenon occur in photosynthesis?
- Chlorophyll absorbs too much red light and gets damaged
- Light above 680 nm cannot excite Photosystem II, drastically reducing efficiency (Correct answer)
- Red light inhibits the oxygen-evolving complex
- Red wavelengths cause stomatal closure
Correct answer: Light above 680 nm cannot excite Photosystem II, drastically reducing efficiency
The red drop shows that light wavelengths above 680 nm (which can only excite PSI) cannot efficiently drive noncyclic electron flow without PSII being activated.
Question 7: Which of the following correctly describes the Emerson enhancement effect?
- Red light alone is more effective than any two wavelengths combined
- Two wavelengths of light together produce more photosynthesis than either alone (Correct answer)
- Photosystem I enhances the activity of Photosystem II by donating electrons
- Longer wavelengths always produce more photosynthesis than shorter wavelengths
Correct answer: Two wavelengths of light together produce more photosynthesis than either alone
The Emerson enhancement effect shows that illuminating with two wavelengths simultaneously (e.g., 680 nm + 700 nm) produces more photosynthesis than the sum of each alone, evidence for two cooperating photosystems.
Photosynthesis Light Reactions 3
Which component of the thylakoid membrane directly uses the proton gradient to synthesize ATP?