Photosynthesis Calvin Cycle 5

Question 1

What would be the immediate effect on the Calvin cycle if the stroma suddenly became more acidic (lower pH)?

Accepted Answer

Calvin cycle enzymes would denature or lose optimal activity

Answer

Calvin cycle enzymes, like most enzymes, have an optimal pH near neutral; acidification of the stroma would reduce their catalytic efficiency.

Question 2

Which of the following correctly describes the stoichiometry of the full Calvin cycle for producing one G3P net?

Accepted Answer

3 CO₂ + 9 ATP + 6 NADPH → 1 G3P + 3 H₂O

Answer

To net one G3P, the Calvin cycle consumes 3 CO₂, 9 ATP, and 6 NADPH, releasing 3 H₂O as a byproduct of hydrolysis steps.

Question 3

Thioredoxin activates several Calvin cycle enzymes in the light by:

Accepted Answer

Donating electrons to reduce disulfide bonds in the enzymes

Answer

Reduced thioredoxin (supplied by the ferredoxin-thioredoxin system in the light) reduces regulatory disulfide bonds on enzymes like fructose-1,6-bisphosphatase, activating them.

Question 4

If a mutant plant lacked phosphoribulokinase, which outcome would result?

Accepted Answer

RuBP could not be regenerated and the cycle would stop after one turn

Answer

Phosphoribulokinase converts ribulose-5-phosphate to RuBP; without it, the cycle cannot regenerate its CO₂ acceptor and would cease after existing RuBP is consumed.

Question 5

How does an increase in the CO₂:O₂ ratio in the chloroplast affect carbon fixation?

Accepted Answer

It favors RuBisCO's carboxylase activity and increases Calvin cycle output

Answer

RuBisCO's carboxylase activity is competitively favored when CO₂ concentration is high relative to O₂, increasing carbon fixation and reducing photorespiration.

Photosynthesis Practice Test

Photosynthesis Practice Test

Photosynthesis Calvin Cycle 5