Carbohydrate Synthesis

The dark reactions use ATP and NADPH to convert CO 2 into carbohydrate. The first step is fixing CO 2 into organic carbon. The basic reaction is addition of CO 2 to a phosphorylated acceptor. This step requires no direct input of energy. Two types of plants exist, which use different acceptor molecules. In so‐called C‐3 plants, the acceptor is a 5‐carbon, doubly phosphorylated acceptor, and two 3‐carbon phosphorylated compounds are formed. In C‐4 plants, the acceptor is phosphoenolpyruvate, and the carboxylation makes the 4‐carbon acid oxaloacetatic acid and releases inorganic phosphate. The ATP and NADPH from the light reactions are used for making the acceptors and converting the first products into glucose.