Biosynthetic phase catalyses assimilation of CO₂ to carbohydrates. The reactions are known as carbon reactions. They occur in the stroma of chloroplasts. The reactions do not require light. Instead assimilatory power (ATP and NADPH) produced during photochemical phase is used in fixation and reduction of CO₂. All the enzymes required for the process are present in the matrix or stroma of the chloroplast.