Mechanism of Photosynthesis

The basic equation for photosynthesis in green plant is:



If this equation is multiplied by 6 the formation of glucose, a hexose (6-carbon) sugar is indicated.
It will be seen that in the above equation water has been shown on both sides. This is necessary because the water produced in photosynthesis is new water, and not the same water which is used as raw material.

(A) Outline of Photosynthetic Reactions

There are two reactions involved in photosynthesis. The first reaction requires light and is called the light or Hill reaction. The second reaction does not require light and is called the dark or Blackman reaction. (1) The light reaction is a photochemical reaction, while the dark reaction is a thermo chemical reaction. The unit of photosynthesis is believed to consist of two types of centres, photosystem I and photosystem II. These are excited at different wavelengths of light. The two systems are linked by redox catalysts. The light reaction involves two process, photophosphorylation and photolysis of water. In photophosphorylation there is conversion of light energy into chemical energy. Photophosphorylation is of two types, cyclic photophosphorylation and noncyclic photophosphorylatiyn. (2) The dark reaction takes place through a series of steps known as the Calvin-Benson cycle. The details of different stages of photosynthesis will now be taken up.

(B) The Light or Hill Reaction

The first photosynthesis reaction takes place in the presence of light is called the light of Hill reaction. The light reaction involves two process, photophosphorylation and photolysis of water. The essential features of the light reaction are: (1) the absorption of light energy by chlorophyll, (2) the transfer of this energy and (3) the utilization of the energy in the electron transfer chain. The process by which light energy is transformed into chemical energy is called photophosphoryiation. There are two types of photophosphorylation, cyclic and non-cyclic photophophosrylation.