Photosynthesis Determinants of Photosynthetic Rate 3 — Questions and Answers
Question 1: Which mineral nutrient deficiency most directly reduces the rate of photosynthesis by limiting chlorophyll production?
- Phosphorus
- Magnesium (Correct answer)
- Potassium
- Calcium
Correct answer: Magnesium
Magnesium is the central atom in the chlorophyll molecule, so its deficiency leads to chlorosis and reduced photosynthetic capacity.
Question 2: In a compensation point experiment, net photosynthesis equals zero. What does this mean?
- The plant has stopped all metabolic activity
- Gross photosynthesis exactly equals the rate of respiration (Correct answer)
- CO₂ concentration has dropped to zero
- All chlorophyll has been bleached
Correct answer: Gross photosynthesis exactly equals the rate of respiration
At the compensation point, carbon fixed by photosynthesis equals carbon released by respiration, so there is no net gas exchange.
Question 3: How does increasing nitrogen supply typically affect photosynthetic rate?
- Decreases it by acidifying the chloroplast
- Has no effect because nitrogen is not used in photosynthesis
- Increases it by enabling more RuBisCO and chlorophyll synthesis (Correct answer)
- Decreases it by closing stomata
Correct answer: Increases it by enabling more RuBisCO and chlorophyll synthesis
Nitrogen is essential for synthesizing proteins such as RuBisCO and for the porphyrin ring of chlorophyll, so more nitrogen supports greater photosynthetic capacity.
Question 4: Why does photorespiration reduce net photosynthesis in C3 plants at high temperatures?
- RuBisCO oxygenates O₂ instead of CO₂, releasing CO₂ without fixing carbon (Correct answer)
- High temperature destroys chlorophyll directly
- ATP synthase stops functioning above 30°C
- Stomata open wider, releasing fixed carbon
Correct answer: RuBisCO oxygenates O₂ instead of CO₂, releasing CO₂ without fixing carbon
At high temperatures, O₂ solubility increases relative to CO₂ and RuBisCO's oxygenase activity rises, consuming energy and releasing CO₂ through photorespiration.
Question 5: Which adaptation allows C4 plants to maintain higher photosynthetic rates in hot, dry conditions compared to C3 plants?
- They lack stomata, eliminating water loss
- They concentrate CO₂ around RuBisCO in bundle sheath cells, suppressing photorespiration (Correct answer)
- They use a different ATP-generating pathway that does not require light
- They store water in vacuoles that release CO₂ directly to chloroplasts
Correct answer: They concentrate CO₂ around RuBisCO in bundle sheath cells, suppressing photorespiration
C4 plants use PEP carboxylase in mesophyll cells to fix CO₂ into 4-carbon acids, then concentrate CO₂ in bundle sheath cells where RuBisCO operates, minimizing photorespiration.
Question 6: A greenhouse manager wants to maximize crop yield. Which combination of environmental changes would most increase photosynthetic rate?
- Reduce light intensity and increase CO₂
- Increase light intensity, raise CO₂ to ~1000 ppm, and maintain optimal temperature (Correct answer)
- Reduce temperature to 5°C and flood the soil
- Increase UV radiation and reduce watering
Correct answer: Increase light intensity, raise CO₂ to ~1000 ppm, and maintain optimal temperature
Simultaneously increasing light, CO₂, and maintaining optimal temperature removes multiple limiting factors at once, producing the greatest increase in photosynthetic rate.
Question 7: How does partial stomatal closure affect the relationship between CO₂ concentration inside the leaf and photosynthetic rate?
- Intracellular CO₂ rises, boosting photosynthesis
- Intracellular CO₂ falls because diffusion into the leaf is restricted, lowering photosynthesis (Correct answer)
- It has no effect on internal CO₂ levels
- CO₂ is produced internally at a higher rate to compensate
Correct answer: Intracellular CO₂ falls because diffusion into the leaf is restricted, lowering photosynthesis
Partial stomatal closure increases diffusion resistance, reducing CO₂ influx and lowering the sub-stomatal CO₂ concentration available to RuBisCO.
Photosynthesis Determinants of Photosynthetic Rate 3
Which mineral nutrient deficiency most directly reduces the rate of photosynthesis by limiting chlorophyll production?