Sucrose is synthesized in green leaves during the active period of photosynthesis and sucrose is converted into starch. Large excess of sucrose is transported to some tissues, such as grains, where it is converted into starch again and stored to fulfill future needs. On germination, the stored starch in the seeds is re-converted into sucrose to provide energy for the growth of juvenile seedling. In the developing seeds, sucrose first breakdown by the enzyme sucrose synthesis to UDP-glucose and fructose. This is reversible reaction.
In the following step UDP-glucose change to glucose-I phosphate than ADP-glucose. Glucose form ADP glucose is then in corporated into primer molecule (dextrin) by starch synthesis while several types of hydrolase enzymes and starch phosphorylase can hydrolyse starch to glucose. Glucose vs. phosphorylated in glucose 6 phosphate and than isomerised in glucose-I phosphate and converted into UDP glucose. And at last UDP-glucose and fructose-6 phosphate join to form sucrose. We can show the inter conversion of starch-sucrose by a flow chart:
- (a) Conversion of sucrose into starch in developing seeds.
- (b) Conversion of starch into sucrose on germination.