Future Scientists

Revision note 2.12

Liver and Metabolism

The liver is a chemical-processing organ that handles absorbed nutrients, stores glycogen, forms urea and detoxifies harmful substances.

ExtendedIntegrationMetabolism

What you need to know

  • Define metabolism and identify the liver as a major metabolic organ.
  • Explain how absorbed nutrients reach and are processed by the liver.
  • Describe the storage and release of glucose as glycogen.
  • Explain deamination and the formation of urea.
  • Describe detoxification and connect the liver with digestion, circulation and excretion.

What metabolism means

Metabolism is the sum of all chemical reactions in a cell or organism. It includes reactions that build larger molecules, break molecules down, transfer energy and remove harmful products.

Liver cells contain many enzymes and receive a large blood supply, allowing them to process changing quantities of nutrients and chemicals.

Metabolic reaction

An enzyme-controlled chemical reaction taking place in a living cell.

Blood supply and absorbed nutrients

Most water-soluble products absorbed from the small intestine travel first to the liver in the hepatic portal vein. This allows the liver to adjust and process nutrient concentrations before blood reaches the general circulation.

The liver can:

  • convert nutrients into forms needed by cells or suitable for storage;
  • store some vitamins, minerals and glycogen;
  • make plasma proteins and other important molecules;
  • modify or break down hormones, medicines and toxins;
  • produce bile, which aids lipid digestion.

Lipid products absorbed into lymph reach the bloodstream by a different initial route, but the liver still plays a major part in lipid metabolism.

Glucose and glycogen

After a carbohydrate-rich meal, blood glucose rises. Under hormonal control, liver cells remove glucose and join it into insoluble glycogen for storage. When blood glucose falls, glycogen can be broken down and glucose released.

Blood glucose risesAbsorbed glucose reaches the liver.
StorageGlucose molecules are joined to form glycogen.
Blood glucose fallsStored glycogen is broken down.
ReleaseGlucose returns to the blood as needed.
Misconception AlertGlycogen and glucose are the same molecule.Select to reveal the correctionSelect to hide the correction
Correct understanding

Glucose is a small soluble sugar. Glycogen is a large storage carbohydrate made from many glucose units.

Excess amino acids and urea

The body cannot store excess amino acids in the same way that it stores glycogen or fat. In the liver, their amino group is removed by deamination. The remaining carbon-containing part can enter respiration or be converted to other molecules.

Removal of the amino group forms toxic ammonia. The liver converts ammonia into less toxic urea. Urea is carried in plasma to the kidneys, filtered from the blood and excreted in urine.

Excess amino acidCannot be stored directly.
DeaminationThe amino group is removed in the liver.
Urea formationToxic ammonia is converted to urea.
ExcretionBlood carries urea to the kidneys.

Detoxification

The liver changes harmful substances into less harmful or more easily excreted products. Enzymes break down alcohol, but the process takes time. High or repeated alcohol exposure can damage liver cells, cause fatty change, inflammation and eventually scarring that reduces liver function.

Medicines are also metabolised in the liver. This is one reason dose and timing matter: too much can overwhelm normal processing or produce harmful concentrations.

Exam tip

Detoxification does not mean the liver can instantly make any dose harmless. Enzyme pathways have limited rates, and breakdown products may also cause damage.

Quick retrieval check

1. What is metabolism?
The sum of all chemical reactions in a cell or organism.
2. Why is glucose stored as glycogen?
Glycogen is a compact, insoluble storage molecule that can be broken down when glucose is needed.
3. What happens during deamination?
The amino group is removed from an excess amino acid in the liver.
4. Trace urea from its formation to removal.
It is formed in the liver, carried dissolved in plasma to the kidneys, filtered from blood and excreted in urine.

Exam connection

Question

A person eats a meal containing more protein than is needed for growth and repair. Explain what happens to the excess amino acids.

Show the mark points
  • Excess amino acids cannot be stored directly.
  • They are deaminated in the liver.
  • The removed amino group forms toxic ammonia, which is converted to urea.
  • Urea is transported in blood to the kidneys and excreted.
  • The remaining part may be respired or converted to another storage molecule.