The Mammalian Liver: Function (OCR A Level Biology)

• Liver functions include:
  • storage of glycogen
  • formation of urea
  • detoxification

Storage of glycogen

• The liver plays a vital role in the conversion of glucose into glycogen; this process is known as glycogenesis
  • Insulin triggers this process after the pancreas detects an increase in blood glucose concentration
  • The synthesis of glycogen removes glucose molecules from the bloodstream and decreases the blood glucose concentration to within a normal range
• The glycogen produced during glycogenolysis is stored inside hepatocytes
  • Glycogen is a compact storage molecule that allows for easy release of glucose when it is needed

Formation of urea

• The protein in our diets is broken down into amino acids, which are absorbed into the blood and transported to the liver via the hepatic portal vein
• Excess amino acids are processed inside hepatocytes during a two-step process that involves:
  • deamination
  • the ornithine cycle

Deamination

• During deamination:
  
  • The amino group (NH₂) is removed from each amino acid, together with an extra hydrogen atom (H⁺)
  • NH₂ and H⁺ combine to form ammonia (NH₃)
• The part of the amino acid that remains after deamination is a keto acid which can:
  • enter the Krebs cycle to be respired
  • be converted to glucose
  • be converted to glycogen or fat for storage

Deamination of an amino acid involves removal of the amino group

The ornithine cycle

• Ammonia is a very soluble and highly toxic compound that can be very damaging if it builds up in the blood
• To avoid the toxic effects of ammonia it is converted into urea, which is less toxic than ammonia
• The series of events during which ammonia is converted into urea is known as the ornithine cycle
• During the ornithine cycle ammonia is combined with carbon dioxide to form urea
  • One molecule of urea is produced from one molecule of carbon dioxide and two amino groups
• The urea diffuses through the phospholipid bilayer of the hepatocytes and is transported to the kidneys dissolved in the blood plasma; here it is excreted

Ammonia is combined with carbon dioxide to produce urea during the ornithine cycle

Detoxification

• Detoxification is the breakdown of substances that are not needed, or are toxic
• Substances that undergo detoxification in the liver include:
  • alcohol
  • hydrogen peroxide
  • lactate
  • medicinal drugs

Detoxification of alcohol

• Once consumed alcohol, or ethanol, is absorbed in the stomach and transported in the blood until it reaches the hepatocytes
• Inside the hepatocytes the enzyme alcohol dehydrogenase converts ethanol into a molecule called ethanal, which is then converted into other molecules that enter respiration
• Continuous alcohol detoxification can cause liver problems:
  • The metabolism of ethanol generates ATP, so hepatocytes do not metabolise as much fat as usual and instead store the fat, which causes the condition known as fatty liver
  • Stored fat reduces the ability of hepatocytes to carry out other functions, and can eventually lead to severe problems such as cirrhosis, which is scarring of the liver caused by excessive alcohol consumption