W HERE ARE THE ATP AND NADPH U SED ? We learnt that the products of light reaction are ATP, NADPH and O . Of these O diffuses out of the chloroplast while ATP and NADPH are used to drive the processes leading to the synthesis of food, more accurately, sugars. This is the biosynthetic phase of photosynthesis.
This process does not directly depend on the presence of light but is dependent on the products of the light reaction, i.e., ATP and NADPH, besides CO and H O. You may wonder how this could be verified; it is simple: immediately after light becomes unavailable, the biosynthetic process continues for some time, and then stops. If then, light is made available, the synthesis starts again. Can we, hence, say that calling the biosynthetic phase as the dark dark dark dark dark reaction reaction reaction reaction reaction is a misnomer?
Discuss this amongst yourselves. Let us now see how the ATP and NADPH are used in the biosynthetic phase. We saw earlier that CO is combined with H O to produce (CH O) n or sugars. It was of interest to scientists to find out how this reaction proceeded, or rather what was the first product formed when CO is taken into a reaction or fixed.
Just after world war II, among the several efforts to put radioisotopes to beneficial use, the work of Melvin Calvin is exemplary. The use of radioactive C by him in algal photosynthesis studies led to the discovery that the first CO fixation product was a -carbon organic acid. He also contributed to working out the complete biosynthetic pathway; hence it was called Calvin cycle after him. The first product identified was -phosphoglyceric acid or in short PGA .
How many carbon atoms does it have? Scientists also tried to know whether all plants have PGA as the first product of CO fixation, or whether any other product was formed in other plants. Experiments conducted over a wide range of plants led to the discovery