Question 1

Where does the Calvin cycle take place?

Accepted Answer

The Calvin cycle takes place in the stroma of the chloroplast. It is also called the C3 or reductive pentose phosphate pathway.

Question 2

What happens during carboxylation?

Accepted Answer

RuBisCO catalyses the addition of CO2 to the five-carbon RuBP molecule. An unstable six-carbon intermediate immediately splits into two molecules of 3-phosphoglyceric acid, also called 3-PGA.

Question 3

How are ATP and NADPH used in the Calvin cycle?

Accepted Answer

ATP provides phosphorylation energy during reduction and regeneration, while NADPH provides reducing power during the conversion of phosphorylated 3-PGA into G3P. ATP and NADPH are formed during the light reaction and are consumed in the stroma, producing ADP and NADP+.

Question 4

Why are three turns needed to obtain one net G3P?

Accepted Answer

Three turns fix three CO2 molecules and form six G3P molecules. Five of those G3P molecules regenerate three RuBP molecules, so only one G3P remains as net output. This uses nine ATP and six NADPH.

Question 5

Does glucose leave the Calvin cycle directly?

Accepted Answer

No. After six turns, the direct net product is two G3P molecules. These G3P molecules are used in later reactions to produce one glucose-equivalent.

Calvin Cycle in 3D: Fixing CO2 in the Stroma

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Practice