Photosynthesis Environmental Factors 2 — Questions and Answers
Question 1: Which wavelengths of visible light are LEAST effective at driving photosynthesis?
- Red wavelengths (~680 nm)
- Blue wavelengths (~430 nm)
- Green wavelengths (~550 nm) (Correct answer)
- Violet wavelengths (~400 nm)
Correct answer: Green wavelengths (~550 nm)
Chlorophyll reflects green light rather than absorbing it, making green wavelengths the least effective for photosynthesis.
Question 2: At what approximate CO₂ concentration does photosynthesis in C3 plants typically become CO₂-saturated under normal conditions?
- 200 ppm
- 400 ppm
- 1000 ppm (Correct answer)
- 1800 ppm
Correct answer: 1000 ppm
Most C3 plants reach CO₂ saturation around 1000 ppm; above this, additional CO₂ provides little additional benefit.
Question 3: How does drought stress reduce photosynthesis even when light and CO₂ are plentiful?
- It destroys chlorophyll pigments directly
- It causes stomata to close, limiting CO₂ entry (Correct answer)
- It raises leaf temperature above the optimum
- It prevents ATP synthesis in the mitochondria
Correct answer: It causes stomata to close, limiting CO₂ entry
Under drought, plants close stomata to conserve water, which simultaneously restricts CO₂ diffusion into the leaf.
Question 4: Which environmental factor most directly determines the rate of the light-independent (Calvin cycle) reactions?
- Water availability in the mesophyll
- Temperature, by affecting enzyme activity (Correct answer)
- Light intensity reaching the chloroplast
- Soil nitrogen concentration
Correct answer: Temperature, by affecting enzyme activity
The Calvin cycle is enzyme-driven; temperature directly governs RuBisCO and other enzyme kinetics, making it the primary controlling factor.
Question 5: A plant is moved from a shaded forest floor to full sunlight. Which of the following responses is most likely in the short term?
- Immediate increase in photosynthesis limited only by CO₂
- Photoinhibition and possible bleaching of chlorophyll (Correct answer)
- Rapid synthesis of new RuBisCO enzyme
- Conversion from C3 to C4 metabolism
Correct answer: Photoinhibition and possible bleaching of chlorophyll
Sudden excess light can overwhelm the photosynthetic apparatus, leading to photoinhibition and oxidative damage to chlorophyll.
Question 6: How does increased soil salinity negatively affect photosynthesis?
- It directly denatures photosystem proteins
- It causes osmotic stress that reduces water uptake (Correct answer)
- It blocks light from reaching leaves
- It increases stomatal aperture excessively
Correct answer: It causes osmotic stress that reduces water uptake
High salinity lowers soil water potential, making it harder for roots to absorb water, which leads to water deficit and stomatal closure.
Question 7: What is the compensation point in the context of light and photosynthesis?
- The light intensity at which photosynthesis equals the maximum possible rate
- The light intensity at which photosynthesis exactly balances respiration (Correct answer)
- The CO₂ level at which carbon fixation stops
- The temperature at which all photosynthetic enzymes denature
Correct answer: The light intensity at which photosynthesis exactly balances respiration
The light compensation point is the irradiance at which the rate of photosynthesis equals the rate of respiration, resulting in zero net gas exchange.
Photosynthesis Environmental Factors 2
Which wavelengths of visible light are LEAST effective at driving photosynthesis?