Where are the ATP and NADPH Used?

We have learnt that the products of the light reaction are ATP, NADPH and O2. Of these, O2 diffuses out of the chloroplast, while ATP and NADPH are used to drive the processes leading to the synthesis of food — more accurately, of sugars. This is the biosynthetic phase of photosynthesis.

• This phase does not directly depend on the presence of light, but it is dependent on the products of the light reaction — ATP and NADPH — besides CO2 and H2O.
• This can be verified simply: immediately after light becomes unavailable, the biosynthetic process continues for some time and then stops. If light is then made available, the synthesis starts again.

Because the biosynthetic phase continues briefly even after light is removed, and yet cannot go on without the light reaction's products, calling it the 'dark reaction' is really a misnomer.

Scientists were keen to find out what the first product was when CO2 is fixed. Just after World War II, among several efforts to put radioisotopes to beneficial use, the work of Melvin Calvin stands out. His use of radioactive 14C in algal photosynthesis studies led to the discovery that the first CO2 fixation product was a 3-carbon organic acid. He also worked out the complete biosynthetic pathway; hence it was named the Calvin cycle after him. The first product identified was 3-phosphoglyceric acid, in short PGA, which contains three carbon atoms. …