Photosynthesis Calvin Cycle 4

Question 1

Which carbon-containing molecule exits the chloroplast stroma to be used for sucrose synthesis?

Accepted Answer

G3P

Answer

3-PGA

Answer

RuBP

Answer

Oxaloacetate

Question 2

How many molecules of CO₂ must be fixed to produce enough G3P to synthesize one molecule of glyceraldehyde-3-phosphate (net)?

Accepted Answer

3

Answer

1

Answer

2

Answer

6

Question 3

What happens to the Calvin cycle when light intensity drops to zero in a leaf?

Accepted Answer

It slows and stops as ATP and NADPH from the light reactions are depleted

Answer

It continues indefinitely using stored ATP and NADPH

Answer

It reverses, releasing CO₂ back into the atmosphere

Answer

It switches to using oxygen as an electron donor

Question 4

Which enzyme phosphorylates 3-PGA to form 1,3-bisphosphoglycerate in the Calvin cycle?

Accepted Answer

Phosphoglycerate kinase

Answer

RuBisCO

Answer

G3P dehydrogenase

Answer

Phosphoribulokinase

Question 5

In terms of carbon bookkeeping, what happens to the 18 carbon atoms from 6 RuBP when 6 CO₂ are fixed?

Accepted Answer

All 24 carbons become 12 G3P molecules, of which 10 regenerate RuBP and 2 are exported

Answer

All 18 carbons form glucose directly

Answer

12 carbons go to G3P and 6 carbons are released as CO₂

Answer

6 carbons form G3P and 12 carbons regenerate RuBP

Question 6

Why is G3P considered a key branch-point metabolite in plant metabolism?

Accepted Answer

It can enter glycolysis, be used for starch synthesis, or be exported for sucrose production

Answer

It is only used for glucose synthesis in the chloroplast

Answer

It directly powers ATP synthesis in the stroma

Answer

It cannot leave the chloroplast and must be converted to starch

Question 7

Which regulatory mechanism increases RuBisCO activity in the light?

Accepted Answer

RuBisCO is activated by rubisco activase, which uses ATP and is powered by light

Answer

RuBisCO is inhibited by NADPH in darkness

Answer

Light directly phosphorylates RuBisCO to activate it

Answer

High CO₂ concentration from respiration activates RuBisCO at night