BioInteractive Regulation and Efficiency of Photosynthesis 2 — Questions and Answers
Question 1: What is the overall efficiency of photosynthesis in converting solar energy to glucose?
- Typically 1–3% under natural conditions (Correct answer)
- About 50% of all sunlight absorbed
- Exactly 10% in all plants
- 100% in C4 plants
Correct answer: Typically 1–3% under natural conditions
Plants typically convert only 1–3% of incident solar energy into chemical energy in glucose under field conditions, due to reflection, heat loss, and metabolic inefficiencies.
Question 2: How does non-photochemical quenching (NPQ) protect plants from excess light?
- It dissipates excess excitation energy as heat to prevent photo-oxidative damage (Correct answer)
- It converts excess light into extra ATP
- It speeds up the Calvin cycle
- It increases water splitting
Correct answer: It dissipates excess excitation energy as heat to prevent photo-oxidative damage
NPQ is a photoprotective mechanism where carotenoids and other components harmlessly dissipate excess absorbed light energy as heat, preventing damage to the photosystems.
Question 3: What is the role of RuBisCO activase in photosynthesis?
- It reactivates RuBisCO by removing inhibitory sugar phosphates from its active site (Correct answer)
- It synthesizes new RuBisCO protein
- It transfers electrons to RuBisCO
- It phosphorylates RuBP for use in the Calvin cycle
Correct answer: It reactivates RuBisCO by removing inhibitory sugar phosphates from its active site
RuBisCO activase uses ATP to remove inhibitors (like RuBP and other sugar phosphates) from the active site of RuBisCO, keeping it functional in the light.
Question 4: How does drought stress affect photosynthesis?
- Stomata close to conserve water, reducing CO2 entry and thus photosynthesis (Correct answer)
- Drought increases photosynthesis by concentrating leaf solutes
- Drought has no effect because water is not a reactant in the Calvin cycle
- Drought speeds up light reactions but slows the Calvin cycle
Correct answer: Stomata close to conserve water, reducing CO2 entry and thus photosynthesis
Under drought, stomata close to prevent water loss, but this simultaneously limits CO2 diffusion into the leaf, reducing Calvin cycle activity and overall photosynthesis.
Question 5: What is the significance of the RuBP regeneration step in the Calvin cycle for maintaining photosynthetic rate?
- If RuBP is not regenerated, the Calvin cycle stops and CO2 cannot be fixed (Correct answer)
- RuBP regeneration only affects the light reactions
- RuBP is stored when not needed and does not affect the rate
- RuBP is regenerated automatically without ATP
Correct answer: If RuBP is not regenerated, the Calvin cycle stops and CO2 cannot be fixed
RuBP must be continuously regenerated from 3-carbon intermediates using ATP and NADPH; without regeneration, the substrate for CO2 fixation is depleted and photosynthesis ceases.
Question 6: How does leaf age affect photosynthetic capacity?
- Young mature leaves photosynthesize most rapidly; very young and old leaves have lower rates (Correct answer)
- Old leaves always photosynthesize fastest
- Photosynthetic rate is constant throughout leaf life
- Young leaves have no chlorophyll and cannot photosynthesize
Correct answer: Young mature leaves photosynthesize most rapidly; very young and old leaves have lower rates
Photosynthetic capacity increases as leaves develop and chloroplasts mature, peaks in fully expanded leaves, then declines as leaves age and chloroplasts break down.
BioInteractive Regulation and Efficiency of Photosynthesis 2
What is the overall efficiency of photosynthesis in converting solar energy to glucose?