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. UDPglucose
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.