Photosynthesis is an oxidation-reduction process completed into two phases:
(A) Light reaction or Hill reaction.
(B) Dark reaction or Blackman reaction.
(A) Light reaction: This process takes place in the presence of light. This reaction occurs in light, therefore technically called photochemical phase. The light reaction takes place in chlorophyll, containing thylakoids of chloroplasts.
The light reaction can be divided into several steps
1. Some light of definite wavelengths is absorbed by chlorophyll.
2. During the light reaction, photolysis of water takes place.
3. O2 is released during photosynthesis, comes from H2O, absorbed from the soil.
4. Synthesis of ATP and NADPH2 takes place. Synthesis of ATP in the presence of light is called photophosphoiylation. H ion produced in photolysis is picked up by a special compound NADP, activated by an electron. This is the end of the phase of light reaction and products of the light reaction, which is ATP and NADPH2, are next used in the dark reaction.
(B) Dark reaction or Blackmann reaction-
It is a light-independent process, in which reduction of CO, takes place. In reality, it is a biochemical process. Though it does not depend on light, energy and reducing agent synthesised in the last step are used in it. So, it is called dark reaction or biochemical reaction. The reactions of this step take place in the cavity of chloroplast called stroma. This step is called C3 or Calvin cycle because the first product contains 3 carbon atoms.
Assimilation of carbon takes place through the Calvin cycle. In this cycle, assimilation of carbon takes place in the form of 3 carbon atom compound viz. Phosphoglyceric acid (PGA). So, the cycle is called C3 and the plants in which this cycle is found are called C3 plants.
But in some plants, first fixed product in the cycle is made up of four carbon atoms called oxaloacetate. This cycle of reduction of carbon is called C4 cycle and the plants in which this cycle is found are called C4 plants.
