Ruben, Kamen and Hassid (1941) proposed that O2 evolved during photosynthesis comes from water. The O2 is actually released during photolysis of water in PS-II of light reaction as: 4H2O⇌Mn+Cl−4OH−+4H+4OH−⇌2H2O+O2+4e− Calvin (1954-55) traced the pathway of carbon fixation by using (14CO2) and gave the C3 cycle. During dark reaction of photosynthesis assimilation of CO2 takes place to form carbohydrate. In the process of glycolysis (common path of aerobic and anaerobic respiration) the glucose molecule breaks down into two molecules of pyruvic acid.