The light reactions of photosynthesis transform the energy light into high-energy compound ATP and NADPH. The dark reaction reduced CO2 to form carbohydrates. These reactions are called dak reactions because these reactions do not directly depend on photons of light. The process of dark reaction is completed into the following steps.
- Calvin cycle or C3 cycle: This was invented by Melvin Calvin and his colleagues. They were investigating CO2 fixation in C3 plant (chlorella green algae) by using a radioactive isotope of carbon 14c. During Calvin cycle atmospheric CO2 fixed and formed 3 carbon compounds called phosphoglyceric acid (PGA) in the presence of RUBP. The released PGA then changed into glucose. The overall processes of the Calvin cycle can be represented by the following equation
6CO2+ 6RUBP+ 12NADPH+ 12H+ + 18ATP + 12H2O → C6 H12 O6 + 12NADP + 18ADP + 18iP + 6RUBP + 12H2O
- Hatch and Slack cycle/ C4 cycle: In several plants like Zea mays, Euphorbia, Amaranthus, and Chenopodium first stable production for initial CO2 fixation, in dark reactions are oxaloacetic acid, aspartic acid, and malic acid, they are also known as C4 plants because all are four carbon compounds. During the dark phase, CO2 combines with phosphoenol pyruvic acid to form oxaloacetic acid in nesophy11 chloroplasts. It is further reduced to malic acid is transferred to chloroplasts in bundle sheath. Where liberated CO2 comes in contact with RUBP to undergo the Calvin cycle.