Glycogen is the energy-storage polysaccharide of animals and fungi. In humans it is stored as small granules, mainly in liver and muscle cells.Like starch, glycogen is a polymer of α-glucose, with 1,4-glycosidic bonds in the chains and 1,6-glycosidic bonds at the branch points. It is similar to amylopectin, but more highly branched: a branch occurs about every 8 to 12 glucose units, compared with every 20 to 30 in amylopectin. The chains between branches are also shorter.
How the structure suits its function:
• Very branched, so there are many free ends. Enzymes remove glucose from these ends, so glucose can be released very rapidly. This suits animals, which are active and have high metabolic rates.
• Compact, so a lot of glucose is stored in a small volume.
• Insoluble, so it does not lower the water potential of the cell.
• Large, so it cannot diffuse out of the cell.Liver glycogen is broken down to glucose that is released into the blood to keep blood glucose concentration steady. Muscle glycogen is used by the muscle cells themselves during exercise.Comparing the stores: amylose is unbranched and coiled; amylopectin is branched; glycogen is the most branched and the most compact.Worked example: a glycogen molecule of 30 000 glucose units with a branch every 10 units has about 30 000 ÷ 10 = 3000 branch points. An amylopectin molecule of the same size branching every 25 units has only about 1200.
How the structure suits its function:• Very branched, so there are many free ends. Enzymes remove glucose from these ends, so glucose can be released very rapidly. This suits animals, which are active and have high metabolic rates.
• Compact, so a lot of glucose is stored in a small volume.
• Insoluble, so it does not lower the water potential of the cell.
• Large, so it cannot diffuse out of the cell.Liver glycogen is broken down to glucose that is released into the blood to keep blood glucose concentration steady. Muscle glycogen is used by the muscle cells themselves during exercise.Comparing the stores: amylose is unbranched and coiled; amylopectin is branched; glycogen is the most branched and the most compact.Worked example: a glycogen molecule of 30 000 glucose units with a branch every 10 units has about 30 000 ÷ 10 = 3000 branch points. An amylopectin molecule of the same size branching every 25 units has only about 1200.