In liver and muscle cells excess glucose is converted to glycogen for…

The idea

When insulin causes glucose to enter liver and muscle cells, not all of it is used immediately in respiration. Excess glucose is converted into glycogen, a large insoluble polysaccharide made from many glucose units joined together. Glycogen is stored in the liver and in muscle tissue and acts as a reserve of glucose. Storing glucose as glycogen is useful because glycogen is insoluble, so it does not affect the water potential of the cell and cannot diffuse out. When blood glucose concentration later falls, glucagon stimulates the breakdown of glycogen back into glucose, which is released into the blood. This storage and release cycle helps keep blood glucose concentration stable between meals.

What earns the marks

• Excess glucose is converted to glycogen in liver and muscle cells.
• Glycogen is a storage polysaccharide made from glucose units.
• Glycogen is stored in the liver and in muscle tissue.
• Glycogen is insoluble and does not diffuse out of cells.
• When blood glucose concentration falls, glycogen can be broken down to release glucose.
• The storage and release of glycogen helps keep blood glucose concentration stable.

Common mistakes and how to put them right

• Confusing glycogen with glucagon.

  Put it right: Glycogen is the storage carbohydrate while glucagon is the hormone that stimulates its breakdown.

• Stating that glycogen is stored in the blood.

  Put it right: Glycogen is stored inside liver and muscle cells.

• Saying excess glucose is converted to starch in humans.

  Put it right: Starch is a plant storage carbohydrate, whereas humans store glycogen.

Exam tips

• Spell glycogen and glucagon correctly and use each term only for its correct meaning.
• Name both storage sites, the liver and muscle cells, when describing glycogen storage.
• Link glycogen storage to the later release of glucose when blood glucose concentration falls.