Photosynthesis Calvin Cycle 2

Question 1

Which enzyme catalyzes the carboxylation of RuBP in the Calvin cycle?

Accepted Answer

RuBisCO

Answer

RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the fixation of CO₂ onto RuBP.

Question 2

How many turns of the Calvin cycle are needed to produce one molecule of glucose?

Accepted Answer

6

Answer

Six turns fix 6 CO₂ molecules, providing enough carbon to synthesize one glucose (C₆H₁₂O₆).

Question 3

What is the oxidation state of carbon in CO₂ compared to the carbon in G3P?

Accepted Answer

CO₂ is more oxidized than G3P

Answer

Carbon in CO₂ is fully oxidized (+4), while carbon in G3P is partially reduced, reflecting the energy invested in the Calvin cycle.

Question 4

Which phase of the Calvin cycle directly consumes ATP without consuming NADPH?

Accepted Answer

Regeneration of RuBP

Answer

The regeneration phase uses 3 ATP (per 3 CO₂ fixed) exclusively to phosphorylate ribulose-5-phosphate back to RuBP.

Question 5

What would happen to the Calvin cycle if NADPH were suddenly unavailable?

Accepted Answer

3-PGA would accumulate and G3P production would halt

Answer

Without NADPH, the reduction of 3-PGA to G3P cannot occur, so 3-PGA accumulates while G3P output drops to zero.

Photosynthesis Practice Test