Photosynthesis Determinants of Photosynthetic Rate 2 — Questions and Answers
Question 1: Which gas must be available in sufficient concentration for the Calvin cycle to proceed at maximum rate?
- Oxygen
- Nitrogen
- Carbon dioxide (Correct answer)
- Hydrogen
Correct answer: Carbon dioxide
CO₂ is the carbon source fixed by RuBisCO in the Calvin cycle, so its concentration directly limits the rate of carbon fixation.
Question 2: How does increasing temperature from 10°C to 30°C generally affect photosynthetic rate in a C3 plant?
- Rate decreases steadily
- Rate increases up to the enzyme optimum then may plateau or decline (Correct answer)
- Rate is unaffected by temperature
- Rate decreases then increases
Correct answer: Rate increases up to the enzyme optimum then may plateau or decline
Enzymatic reactions including RuBisCO speed up with temperature until the enzyme's optimum, beyond which denaturation reduces the rate.
Question 3: A plant is moved from 400 ppm CO₂ to 800 ppm CO₂ under constant light and temperature. What is the most likely immediate effect?
- Stomata close, reducing photosynthesis
- Photorespiration increases significantly
- Carbon fixation rate increases (Correct answer)
- Chlorophyll degrades faster
Correct answer: Carbon fixation rate increases
Higher CO₂ concentration increases substrate availability for RuBisCO, directly boosting carbon fixation.
Question 4: Which wavelengths of light are LEAST effective at driving photosynthesis?
- Red (~680 nm)
- Blue (~450 nm)
- Green (~550 nm) (Correct answer)
- Far-red (~700 nm)
Correct answer: Green (~550 nm)
Chlorophyll reflects most green light rather than absorbing it, making green wavelengths the least effective for photosynthesis.
Question 5: At very high light intensities, photosynthesis in a C3 plant often levels off. Which factor is most likely limiting at that point?
- Light absorption capacity
- CO₂ concentration or enzyme capacity (Correct answer)
- Water in the vascular tissue
- Leaf surface area
Correct answer: CO₂ concentration or enzyme capacity
Once light is saturating, CO₂ availability or the enzymatic capacity of the Calvin cycle becomes the rate-limiting factor.
Question 6: How does water stress affect photosynthetic rate?
- It increases the rate by concentrating CO₂ inside leaves
- It has no effect because water is not a direct reactant in light reactions
- It decreases the rate primarily by causing stomatal closure (Correct answer)
- It increases the rate by stimulating chlorophyll synthesis
Correct answer: It decreases the rate primarily by causing stomatal closure
Under water stress, plants close stomata to conserve water, which restricts CO₂ entry and reduces photosynthesis.
Question 7: A scientist doubles the leaf area of an experimental plant while keeping all other variables constant. What is the most accurate prediction?
- Total photosynthesis doubles because more chloroplasts are available to capture light (Correct answer)
- Photosynthetic rate per unit leaf area doubles
- Total photosynthesis is unaffected because the plant's enzyme pool is fixed
- Photosynthesis decreases due to self-shading
Correct answer: Total photosynthesis doubles because more chloroplasts are available to capture light
Doubling leaf area doubles the number of chloroplasts exposed to light, so total photosynthetic output approximately doubles, assuming light and CO₂ are not limiting.
Photosynthesis Determinants of Photosynthetic Rate 2
Which gas must be available in sufficient concentration for the Calvin cycle to proceed at maximum rate?