We learnt that the products of 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, 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 CO2 and H2O. 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 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 CO2 is combined with H2O to produce (CH2O)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 CO2 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 14C by him in algal photosynthesis studies led to the discovery that the first CO2 fixation product was a 3-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 3-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 CO2 fixation, or whether any other product was formed in other plants.
Experiments conducted over a wide range of plants led to the discovery of another group of plants, where the first stable product of CO2 fixation was again an organic acid, but one which had 4 carbon atoms in it.
This acid was identified to be oxaloacetic acid or OAA. Since then CO2 assimilation during photosynthesis was said to be of two main types: those plants in which the first product of CO2 fixation is a C3 acid (PGA), i.e., the C3 pathway, and those in which the first product was a C4 acid (OAA), i.e., the C4 pathway.
These two groups of plants showed other associated characteristics that we will discuss later.