Starch is the energy-storage polysaccharide of plants. It is stored as starch grains in chloroplasts and in amyloplasts of storage organs such as potato tubers and seeds. Starch is a mixture of two polymers of α-glucose.Amylose (about 20 to 30% of starch) is an unbranched chain of α-glucose joined by 1,4-glycosidic bonds. Because of the angle of these bonds, the chain coils into a helix, held by hydrogen bonds. The helix is compact, so a lot of glucose fits into a small space.Amylopectin (about 70 to 80%) also has 1,4-glycosidic bonds, but it has side branches joined by 1,6-glycosidic bonds roughly every 20 to 30 glucose units. Enzymes remove glucose from the ends of chains, and the branches give many ends, so glucose can be released more rapidly.
• insoluble, so it does not lower the water potential of the cell or draw in water by osmosis
• large, so it cannot diffuse out of the cell
• compact, because of coiling and branching
• easily hydrolysed to α-glucose, which is the respiratory substrateStarch is digested by amylase, which hydrolyses 1,4-glycosidic bonds to give maltose; maltase then hydrolyses maltose to glucose.Worked example: an amylopectin molecule of 6000 glucose units with a branch every 25 units has about 6000 ÷ 25 = 240 branch points, each with a free end for enzymes to act on.
• insoluble, so it does not lower the water potential of the cell or draw in water by osmosis
• large, so it cannot diffuse out of the cell
• compact, because of coiling and branching
• easily hydrolysed to α-glucose, which is the respiratory substrateStarch is digested by amylase, which hydrolyses 1,4-glycosidic bonds to give maltose; maltase then hydrolyses maltose to glucose.Worked example: an amylopectin molecule of 6000 glucose units with a branch every 25 units has about 6000 ÷ 25 = 240 branch points, each with a free end for enzymes to act on.