(cane sugar, beet sugar, saccharose) A nonreducing disaccharide of glucose and fructose linked through a high-energy bond between carbon two of fructose and carbon one of glucose. Sucrose is the major transport sugar in higher plants. It is formed in the chloro-plasts and transported through the phloem to other organs and tissues, where it is either metabolized for energy or utilized in the synthesis of reserve and structural polysaccharides. Present evidence indicates that in vivo synthesis of sucrose is a two-step reaction involving fructose 6-phosphate and glucose. These react to form sucrose 6-phosphate, which is then dephosphorylated by the enzyme sucrose phosphatase. The enzyme sucrose synthase can catalyse the direct manufacture of sucrose from UDP-glucose and fructose, but in the cell sucrose synthase is thought to be the major route of sucrose breakdown, allowing direct formation of * nucleoside diphosphate sugars from sucrose. The enzyme invertase also splits sucrose, to D-glucose and D-fructose, but its physiological significance is not clear. The mixture of D-glucose and D-fructose formed on hydrolysis by invertase is often termed invert sugar since the optical activity is inverted from dextrorotatory (sucrose solution) to laevorotatory (D-glucose and D-fructose mixture). Invert sugar is found in many fruits. Table sugar is composed of sucrose crystals. Production is mostly from sugar cane (Saccharum officinarum), grown in the tropics, and sugar beet (Beta vulgaris), grown in temperate zones. Small amounts are obtained from other sources, e.g. sugar maple (Acer saccharum) and sweet sorghum (Sorghum bicolor). |
