Photosynthesis Environmental Factors 3 — Questions and Answers
Question 1: Which of the following best describes how C4 plants adapt to hot, sunny environments compared to C3 plants?
- C4 plants have more chlorophyll per leaf area
- C4 plants concentrate CO₂ around RuBisCO, reducing photorespiration (Correct answer)
- C4 plants open stomata only at night to avoid heat
- C4 plants use a different form of ATP synthase that tolerates high temperatures
Correct answer: C4 plants concentrate CO₂ around RuBisCO, reducing photorespiration
C4 plants pre-fix CO₂ in mesophyll cells and shuttle it to bundle sheath cells where RuBisCO operates in a high-CO₂ environment, suppressing the wasteful photorespiration pathway.
Question 2: A farmer notices that crop yield drops significantly when nighttime temperatures remain unusually high. Which physiological process best explains this?
- Accelerated respiration consuming carbon fixed during the day (Correct answer)
- Reduced dew formation limiting leaf water content
- Inhibition of stomatal opening the following morning
- Destruction of phytochrome pigments that control flowering
Correct answer: Accelerated respiration consuming carbon fixed during the day
High nighttime temperatures increase dark respiration, causing plants to consume more of the sugars produced by daytime photosynthesis.
Question 3: What happens to the quantum yield of photosynthesis (moles of CO₂ fixed per mole of photons absorbed) as temperature increases above the optimum?
- It increases because enzymes are more active
- It stays constant because quantum yield depends only on light
- It decreases because photorespiration and protein denaturation accelerate (Correct answer)
- It first increases then stays constant
Correct answer: It decreases because photorespiration and protein denaturation accelerate
Above the temperature optimum, photorespiration rises and enzymes begin to denature, reducing the efficiency of carbon fixation per photon absorbed.
Question 4: Which gas, when elevated in the atmosphere, directly supplies the substrate for carbon fixation in photosynthesis?
- Oxygen (O₂)
- Carbon dioxide (CO₂) (Correct answer)
- Nitrogen (N₂)
- Water vapor (H₂O)
Correct answer: Carbon dioxide (CO₂)
CO₂ is the inorganic carbon substrate that RuBisCO fixes into organic molecules during the Calvin cycle.
Question 5: Under which condition would a C3 plant most likely suffer the greatest reduction in net photosynthesis due to photorespiration?
- Cool temperatures and high CO₂
- Hot temperatures and low CO₂ (Correct answer)
- Moderate temperatures and moderate humidity
- Cool temperatures and low light
Correct answer: Hot temperatures and low CO₂
Photorespiration is favored by high temperature (which increases O₂ solubility relative to CO₂) and low CO₂ (which shifts RuBisCO toward oxygenase activity).
Question 6: How does elevated atmospheric CO₂ typically affect stomatal conductance in most plant species?
- Stomata open wider to allow more CO₂ entry
- Stomata close partially, reducing water loss (Correct answer)
- Stomata convert from passive to active transport
- Stomata become insensitive to abscisic acid signals
Correct answer: Stomata close partially, reducing water loss
Under elevated CO₂, many plants partially close their stomata because internal CO₂ demand is met at lower conductance, improving water-use efficiency.
Question 7: Which UV radiation wavelength range can directly damage the photosynthetic machinery of plants not adapted to high UV exposure?
- UV-A (315–400 nm)
- UV-B (280–315 nm) (Correct answer)
- UV-C (100–280 nm)
- Near-infrared (700–1000 nm)
Correct answer: UV-B (280–315 nm)
UV-B radiation can damage the D1 protein of Photosystem II and nucleic acids, impairing the photosynthetic electron transport chain.
Photosynthesis Environmental Factors 3
Which of the following best describes how C4 plants adapt to hot, sunny environments compared to C3 plants?