Dark reaction is also known as light-independent phase. Unlike, light reaction, it does not require light as an essential factor. Thus, can takes place both in the presence or absence of light. The term dark reaction does not mean that it takes place only in dark period or at night. CO2 fixation occurs in both C3 and C4 cycle. In C3 cycle, CO2 is added by the enzyme, RuBisco to a 5 carbon compound RuBP that is converted to 2 molecules of 3-carbon PGA. In C4 cycle the first product of CO2 fixation(takes place in mesophyll) is a 4-carbon compound, oxaloacetic acid. It is seen in some tropical plants.