Photosynthesis Determinants of Photosynthetic Rate 2

Question 1

Which gas must be available in sufficient concentration for the Calvin cycle to proceed at maximum rate?

Accepted Answer

Carbon dioxide

Answer

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?

Accepted Answer

Rate increases up to the enzyme optimum then may plateau or decline

Answer

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?

Accepted Answer

Carbon fixation rate increases

Answer

Higher CO₂ concentration increases substrate availability for RuBisCO, directly boosting carbon fixation.

Question 4

Which wavelengths of light are LEAST effective at driving photosynthesis?

Accepted Answer

Green (~550 nm)

Answer

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?

Accepted Answer

CO₂ concentration or enzyme capacity

Answer

Once light is saturating, CO₂ availability or the enzymatic capacity of the Calvin cycle becomes the rate-limiting factor.

Photosynthesis Practice Test