Biochemistry II: Carbohydrate Synthesis

Carbohydrate Synthesis

The dark reactions use ATP and NADPH to convert CO₂ into carbohydrate. The first step is fixing CO₂ into organic carbon. The basic reaction is addition of CO₂ 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.