The role of the liver in glycogenesis, glycogenolysis and gluconeogenesis.

The idea

The liver acts as a glucose buffer. Glycogenesis is the conversion of glucose into glycogen for storage, occurring in liver and muscle cells when blood glucose is high and insulin is present. Glycogenolysis is the breakdown of stored glycogen into glucose, which leaves liver cells and enters the blood; it is stimulated by glucagon and adrenaline. AQA mark schemes accept describing glycogenolysis as the hydrolysis of glycogen. Gluconeogenesis is the production of glucose from non-carbohydrate sources, mainly glycerol and amino acids; it is stimulated by glucagon and increases as glycogen stores fall during fasting. Muscle glycogen cannot raise blood glucose because muscle cells lack glucose-6-phosphatase, so glucose-6-phosphate is used in respiration rather than released.

What earns the marks

• Glycogenesis described as the conversion of glucose into glycogen for storage, lowering blood glucose concentration.
• Glycogenolysis described as the breakdown of glycogen into glucose, raising blood glucose concentration; AQA mark schemes accept the hydrolysis of glycogen.
• Gluconeogenesis described as the production of glucose from non-carbohydrate sources such as glycerol and amino acids.
• Glucose released by the liver enters the blood and becomes available for respiration by body cells.
• Each process linked to its hormonal control: insulin promotes glycogenesis; glucagon and adrenaline promote glycogenolysis, while glucagon also stimulates gluconeogenesis.
• Liver cells (hepatocytes) identified as the site of these processes; naming hepatocytes is creditworthy but not essential if liver cells are clearly indicated.

Common mistakes and how to put them right

• Mixing up the three terms, most often writing glycogenolysis when gluconeogenesis is meant.

  Put it right: Genesis makes it, lysis splits it, gluconeogenesis makes new glucose from non-carbohydrate sources.

• Claiming muscle glycogen can be released into the blood as glucose.

  Put it right: Muscle cells lack glucose-6-phosphatase, so muscle glycogen is used locally in respiration and cannot raise blood glucose.

• Stating that glucagon only stimulates gluconeogenesis.

  Put it right: Glucagon stimulates both glycogenolysis and gluconeogenesis.

Exam tips

• Learn the three words by their endings: genesis makes it, lysis splits it, and gluconeogenesis makes new glucose from something that was not carbohydrate.
• Always state the direction each process moves blood glucose concentration, and name the hormone that controls it, remembering glucagon stimulates both glycogenolysis and gluconeogenesis.
• When explaining why liver glycogen but not muscle glycogen can raise blood glucose, refer to glucose-6-phosphatase in liver cells.