Photosynthesis is an Oxidation and Reduction process. Water is oxidised to release O2 and CO2 is reduced to form sugars. The first phase requires light and is called light reaction or Hill’s reaction.

1. Light reaction: It is a photochemical reaction whereas dark reaction is a thermochemical reaction.

Solar energy is trapped by chlorophyll and stored in the form of chemical energy (assimilatory power)as ATP and reducing power NADPH 1 H1. NADPH 1 H1 alone are known as reducing powers. This reaction takes place in thylakoid membrane of the chloroplast. Oxygen is evolved as a result of splitting of water molecules by light.

Light reaction is discussed in two phases: i. Photo-oxidation Phase:

• Absorption of light energy.
• Transfer of energy from accessory pigments to reaction centre.
• Activation of Chlorophyll ‘a’ molecule.

ii. Photo Chemical Phase:.

• Photolysis of water and oxygen evolutio.
• Electron transport and synthesis of assimilatory power.

2. Dark reaction (Biosynthetic phase): Fixation and reduction of CO2 into carbohydrates with the help of assimilatory power produced during light reaction. This reaction does not require light and is not directly light driven. Hence, it is called as Dark reaction or Calvin- Benson cycle (Figure 13.9).