Evolution of Photosynthesis and Aerobic Respiration
It is assumed that the earlier form of the bacteria utilised H2S for the preparation of food. The following reaction shows this process:
CO2 + H2S Light energy Sugar = (CH2O)n + Sulphur (S)
Since this reaction takes place in the presence of sunlight, it is called as photosynthesis. In this type of photosynthesis H,S is used.
Later, a more efficient photosynthetic system evolved in which H,O instead of H, S was used as an electron donor. This led to the release of free oxygen (0,). This is the usual form of photosynthesis with which you are familiar.
CO2 + H2O + Solar energy Sugar = (CH2O)n + 02
The photosynthesis which takes place by utilising water, helped in the evolution of autotrophic mode of nutrition and release of free oxygen in the atmosphere. The release of free oxygen made possible a far more efficient method of getting energy from glucose by respiration. The free oxygen got converted into ozone (03) in the presence of ultraviolet radiation and formed an ozone layer in the upper atmosphere. The formation of this layer prevented the incoming ultraviole rays which otherwise could have broken down the proteins, nucleic acids etc. Thus, the formation of ozone layer made existence of various
forms of life on earth possible.
The free 02 generated by photosynthesis was perhaps more like a poison to anaerobic bacteria initially. But some bacteria soon developed a metabolic system in which oxygen was used for the oxidation of food (respiration). This was a much more efficient system for the release of energy from organic molecules, compared with the earlier system of respiration, that is glycolysis. Some of these aerobic bacteria lost the ability of H2S photosynthesis and became perhaps the ancestor of another class cf organisms, that used the mitochondria for respiration. In mitochondria, the energy released by respiration is stored in
the form of ATP molecules for further use.