Photosynthesis Energy Conversion 3 — Questions and Answers
Question 1: What is the quantum yield of photosynthesis and why is it important?
- The number of CO2 molecules fixed per day
- The ratio of photons absorbed to O2 released, measuring efficiency (Correct answer)
- The rate of ATP production per minute
- The amount of glucose stored per leaf
Correct answer: The ratio of photons absorbed to O2 released, measuring efficiency
Quantum yield (Φ) measures photosynthetic efficiency as the number of O2 molecules released per photon absorbed; the theoretical maximum is ~0.125 (8 photons per O2).
Question 2: In the Z-scheme, electrons ultimately flow from which molecule to which molecule?
- NADPH → H2O
- H2O → NADP+ (Correct answer)
- CO2 → glucose
- ATP → ADP
Correct answer: H2O → NADP+
The Z-scheme traces electron flow from the oxidation of water (splitting O–H bonds) through PSII and PSI to the reduction of NADP+ into NADPH.
Question 3: Why does the Calvin cycle require more ATP than NADPH on a per-CO2 basis?
- Because CO2 fixation itself consumes ATP
- Because the regeneration of RuBP requires extra ATP beyond what's needed for G3P reduction (Correct answer)
- Because NADPH is recycled but ATP is not
- Because photorespiration destroys extra NADPH
Correct answer: Because the regeneration of RuBP requires extra ATP beyond what's needed for G3P reduction
For every 3 CO2 fixed, 9 ATP and 6 NADPH are needed; the extra 3 ATP (vs. 6 NADPH) are consumed specifically in the phosphorylation step that regenerates RuBP.
Question 4: Which pigment extends light absorption into wavelengths that chlorophyll a cannot efficiently capture?
- Phytochrome
- Carotenoids (β-carotene and xanthophylls) (Correct answer)
- Cryptochrome
- Phycoerythrin (in land plants)
Correct answer: Carotenoids (β-carotene and xanthophylls)
Carotenoids absorb blue-green and blue light (400–500 nm) and transfer that energy to chlorophyll a, broadening the usable spectrum of sunlight.
Question 5: What happens to the energy of a photon that is absorbed by an antenna pigment but NOT transferred to the reaction center?
- It is stored as ATP
- It is re-emitted as fluorescence or dissipated as heat (Correct answer)
- It splits a water molecule directly
- It converts CO2 to glucose
Correct answer: It is re-emitted as fluorescence or dissipated as heat
Excess excitation energy not funneled to a reaction center is released as fluorescence (longer-wavelength light) or thermal energy, a process exploited in chlorophyll fluorescence assays.
Question 6: What is the significance of P700 absorbing longer-wavelength light than P680?
- P700 generates more ATP per photon
- P700 electrons are at a lower redox potential and can reduce ferredoxin (Correct answer)
- P700 directly oxidizes water
- P700 converts light energy into heat
Correct answer: P700 electrons are at a lower redox potential and can reduce ferredoxin
Although P700 absorbs lower-energy photons, the energy boost is sufficient to excite electrons to a very negative redox potential, enabling reduction of ferredoxin and ultimately NADP+.
Question 7: How do inhibitors like DCMU (diuron) reveal the electron transport pathway in thylakoids?
- By blocking rubisco and stopping CO2 fixation
- By blocking plastoquinone binding in PSII, halting electron flow from PSII to PSI (Correct answer)
- By preventing ADP phosphorylation in ATP synthase
- By inhibiting ferredoxin-NADP+ reductase
Correct answer: By blocking plastoquinone binding in PSII, halting electron flow from PSII to PSI
DCMU competes with plastoquinone (PQ) at the QB site of PSII, blocking electron transfer to the PQ pool and thus to PSI, which allows researchers to isolate PSII activity.
Photosynthesis Energy Conversion 3
What is the quantum yield of photosynthesis and why is it important?