Calvin and colleagues determined the pathway of carbohydratesynthesis in plants

Question

Calvin and colleagues determined the pathway of carbohydratesynthesis in plants by studying the incorporation of radioactivecarbon dioxide into biological compounds. Suppose thatphotosynthesis is proceeding at a steady pace in a typicalexperiment with the lights on, and carbon dioxide is being combinedwith ribulose-bisphosphate (RuBP) to produce 3-phosphoglycerate(3PG). Then suddenly the source of carbon dioxide is eliminated.What changes in the concentrations of 3PG and RuBP would occurimmediately? What will happen over time?

Explanation / Answer

      Calvin cycle is also knownas the C3 cycle which is used to fix carbon-dioxide inthe dark phase of phototynthesis.       The first formed stableproduct in this cycle is a 3 carbon compound called3-phosphoglycerate.       Hence this pathway is alsoknown as C3 pathway.       The primary acceptor ofcarbondioxide in this cycle is Ribulose 1, 5- bisphosphate which isconverted to 2 molecules       of 3-phosphoglycerate forman un-stable 6 carbon compound.       When the source ofcarbondioxide is suddenly eliminated, the concentration ofRibulose-5-phosphate will remain the       same while theconcentration of 3-phosphoglycerate will decrease.       This is because, Ribulose1,5-bis phosphate (RuBP) condenses with Carbondioxide and isconverted to 2 molecules of       3-phosphoglycerate; whenthe corbondioxide source is suddenly eliminated; there is nocarbondioxide for condensation       with RuBP.       So, the originalconcentration of RuBP remains the same while the concentration of3-phosphoglycerate decreases as it       is not formed due to lackof condensation.       Over time theconcentration of RuBP increases and the concentration of3-phosphoglycerate further decreases.       This is because, in calvincycle there is a phase called regeneration where RuBP isregenerated in the last step.       Due to this regeneration,the concentration of RuBP increases and as there is no supply ofcarbondioxide the       concentration of3-phosphoglycerate decreases.

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