Biosynthesis of Carbohydrates

BIOSYNTHESIS OF CARBOHYDRATES

Production of Monosaccharides by Photosynthesis

Carbohydrates are products of photosynthesis, a biologic process that converts electromagnetic energy into chemical energy. In the green plant, photosynthesis consists of two classes of reactions. One class comprises the so-called light reactions that actually convert electromagnetic energy into chemical potential. The other class consists of the enzymatic reactions that utilize the energy from the light reactions to fix carbon dioxide into sugar. These are referred to as the dark reactions. The results of both of these types of reactions are most simply summarized in the following equation:

Although this equation summarizes the overall relationships of the reactants and products, it gives no clue as to the nature of the chemical intermediates involved in the process. The elucidation of the reactions by which carbon dioxide is accepted into an organic compound and ultimately into sugars with regeneration of the carbon dioxide acceptor was a major achievement in biosynthetic research. The pathway of carbon in photosynthesis, as worked out primarily by Calvin and coworkers, is presented in Figure below.

                                    Carbohydrate biosynthesis

Production of sucrose

Sucrose is of considerable metabolic importance in higher plants. Studies have shown that sucrose is not only the first sugar formed in photosynthesis but also the main transport material. Newly formed sucrose is, therefore, probably the usual precursor for polysaccharide synthesis. Although an alternative pathway consisting of a reaction between glucose 1-phosphate and fructose is responsible for sucrose production in certain microorganisms, the biosynthesis of this important metabolite in higher plants apparently occurs as shown in Figure below.

Fructose 6-phosphate, derived from the photosynthetic cycle, is converted to glucose 1-phosphate, which, in turn, reacts with UTP to form UDP-glucose. UDP-glucose either reacts with fructose 6-phosphate to form first sucrose phosphate and ultimately sucrose, or with fructose to form sucrose directly. Once formed, the free sucrose may either remain in situ or may be translocated via the sieve tubes to various parts of the plants. A number of reactions, for example, hydrolysis by invertase or reversal of the synthetic sequence, convert sucrose to monosaccharides from which other oligosaccharides or polysaccharides may be derived.