Plants have a series of light-dependent and light-independent reactions to respire. In the absence of light (at night), plants must rely on their light-independent reactions to respire.
The light-independent reactions involve carbon fixation and the Calvin cycle.
The light-independent reactions take place in the stroma of the chloroplasts. The first reaction involves a five-carbon sugar called ribulose biphosphate (RuBP). RuBp is also a product of this reaction. Therefore, this reaction is a cycle: the Calvin Cycle.
Carbon dioxide enters the stroma of chloroplast by diffusion. In the stroma, it combines with RuBP in a carboxylation reaction, which is catalyzed by the enzyme ribulose biphosphate carboxylase, or rubisco. The product of the reaction is a six carbon compound which then splits to form two molecules of glycerate 3-phosphate, the first product of carbon fixation.
While one out of every six glycerate 3-phosphates are used to regenerate RuBP for the Calvin Cycle again, the remaining 5/6 glycerate 3-phosphates are reduced using energy from ATP and hydrogen ions from NADPH (which are produced by light-dependent reactions and stored). The glycerate 3-phosphates, through the reduction reaction, are reduced to a three-carbon sugar, triose phosphate, which link together to produce glucose phosphate, which forms starch (the storage form of energy as carbohydrates in plants) in the stroma by condensation.
Through these light-independent reactions, plants respire and generate energy when in the absence of light.