The biosynthesis and catabolism of sucrose is an important part of carbohydrate metabolism.
Sucrose is the major form of transport sugar and is translocated through the phloem tissues to other parts of the plant. It is conceivable that photosynthetically fixed carbon from leaf tissues may be transported to the fruits as sucrose during fruit development. Sucrose is biosynthesized from glucose-1-phosphate by three major steps (Fig. 3.3).
The first reaction involves the conversion of glucose-1-phosphate to UDP-glucose, by UDP-glucose pyrophosphorylase in the presence of UTP (uridine triphosphate). UDP-glucose is also an important substrate for the biosynthesis of cell wall components such as cellulose. UDP-glucose is converted to sucrose-6-phosphate by the enzyme sucrose phosphate synthase, which utilizes fructose-6-phosphate during this reaction. Finally, sucrose is formed from sucrose-6-phosphate by the action of phosphatase with the liberation of the inorganic phosphate.