Photosynthesis Carbon Fixation 3

Question 1

During the regeneration phase of the Calvin cycle, how many ATP molecules are consumed to regenerate three molecules of RuBP from five molecules of G3P?

Accepted Answer

5

Answer

Three ATP are used by phosphoribulokinase to phosphorylate three molecules of ribulose-5-phosphate, regenerating three RuBP molecules.

Question 2

What is photorespiration and which enzyme is responsible for initiating it?

Accepted Answer

O₂ addition to RuBP catalyzed by RuBisCO

Answer

Photorespiration occurs when RuBisCO adds O₂ (instead of CO₂) to RuBP, producing 2-phosphoglycolate, which must be recycled in a costly process that releases CO₂.

Question 3

In which cellular compartment of the chloroplast does carbon fixation (the Calvin cycle) take place?

Accepted Answer

Stroma

Answer

The Calvin cycle, including carbon fixation by RuBisCO, occurs in the stroma — the fluid-filled space surrounding the thylakoid membranes.

Question 4

For every three CO₂ molecules fixed by the Calvin cycle, how many molecules of G3P (glyceraldehyde-3-phosphate) are produced in net?

Accepted Answer

1

Answer

Fixing 3 CO₂ produces 6 G3P molecules total, but 5 are used to regenerate RuBP, leaving a net gain of 1 G3P per turn of the cycle.

Question 5

Which intermediate in the Calvin cycle is directly phosphorylated using ATP to produce 1,3-bisphosphoglycerate?

Accepted Answer

3-phosphoglycerate

Answer

Phosphoglycerate kinase phosphorylates 3-PGA using ATP to form 1,3-bisphosphoglycerate, which is then reduced by NADPH to produce G3P.

Photosynthesis Practice Test

Photosynthesis Practice Test

Photosynthesis Carbon Fixation 3