How many times must the calvin-benson-bassham cycle take up co2 in order for subsequent synthesis to produce sufficient pool of triose phosphate so that it is available for export?

a. 5
b. 3
c. 6
d. 1

B (3 times)

Explanation:

Calvin cycle is the light independent phase of photosynthetis, which occurs after the completion of the light dependent phase. The Calvin cycle can be categorised into three stages viz: Fixation, Reduction, and Regeneration.

Reduction: In the stroma (matrix of the chloroplast), Co2, which enters via the stomata is fixed. A plant enzyme known as Ribulosephosphate carboxylase (RUBISCO) catalyzes the reaction between 3 molecules of CO2 and 3 molecules of Ribulosebiphosphate (RUBP), a Co2 acceptor. RUBP is a 5-carbon molecule.

N.B: per cycle, only one CO2 and RUBP molecule is involved, and for each CO2 that reacts with one RUBP, 2 molecules of 3-phospho glycerate (PGA) is produced. PGA is a 3-carbon molecule. In total for the 3 molecules of CO2 and RUBP, 6molecules of PGA is formed.

Reduction: NADH from the light stage is used to reduce 6 molecules of PGA to 6 molecules of Glyceraldehyde-3-phosphate (G-3-P) with ATP as the energy source.

Regeneration: Only one of the G-3-P molecules leaves the Calvin cycle to contribute to the formation of other compounds needed by the plant. Because G3P has 3-carbon atoms, it takes 3-times to fix 3molecules of CO2 in order to be able to export one G3P since one CO2 and RUBP molecule is used per cycle.

But each cycle forms 2molecules of G-3-P, therefore, three cycles will form 6molecules of G-3-P. One is exported out of the Calvin cycle and the remaining 5 G-3-P molecules are used to regenerate RUBP, which enables the cycle to continue by accepting C02 again.

Please note that G-3-P (triose phosphate) is only ready for export when the net carbon atoms are sufficient for RUBP regeneration.