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.

1. 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 14_c. 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

2. 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.