liver function

• metabolic functions of liver :

  • control of blood sugar levels, amino acid levels and lipid levels
  • synthesis of bile, plasma proteins, cholesterol
  • synthesis of red blood cells in the foetus
  • storage of vitamin A, D and B12, iron and glycogen
  • detoxification of alcohol, drugs
  • breakdown of hormones
  • destruction of red blood cells
• Storage of glycogen

  liver stores sugar in form of glycogen

  • able to store approximately 100-120 g of glycogen, making up about 8% of fresh weight of liver
  • the glycogen forms granules in the cytoplasm of the hepatocytes

  → this glycogen can be broken down to release glucose into blood as required

  glycogen is compact, it does not affect water potential and has many terminals to release sugars quickly
• Detoxification

  one important role of liver is to detoxify substances that may cause harm

  • some of these substances such as hydrogen peroxide are produced in body whereas others such as alcohol are consumed
  • toxins can be rendered harmless by oxidation, reduction, methylation or combination with another molecule
  • liver cells contain many enzymes such as
• Detoxification 2

  catalase

  • converts hydrogen peroxide to oxygen and water , has high turnover number
• Detoxification 3

  cytochrome P450 , which is a group of enzymes used to breakdown drugs including cocaine + various medicinal drugs

  • also sued in other metabolic reactions such as electron transport during respiration

  →their role in metabolising drugs can interfere with other metabolic roles + cause unwanted side effects of some medicinal drugs

  • P450 enzymes show a lot of variation between individuals + as they breakdown medicinal drugs, different end products may be produced

  →reason why people suffer from side effects that others do not
• Detoxification of alcohol

  alcohol is a drug that depresses nerve activity , but also contains chemical potential energy, which can be used for respiration

  • ethanol dehydrogenase breaks down alcohol in the hepatocytes

  → ethanal is formed

  • ethanal dehydrogenated further by enzyme ethanal dehydrogenase
  • the final compound is ethanoate (acetate)
  • this acetate is combined with coenzymes A to form acetyl coenzymes A, which enters the process of aerobic respiration
• Detoxification of alcohol 2

  • the hydrogen atoms released from alcohol are combined with another coenzymes

  →called NAD, to form to form reduced NAD

  • NAD is also required to oxidise + breakdown fatty acids for use in respiration

  these fatty acids are then converted back to lipids + stored as fat in hepatocytes, causing the liver to become enlarged

  → this is a condition known as ‘fatty liver’, which can lead to alcohol-related hepatitis or to cirrhosis
• Figure 1

  
• Formation of urea

  every day we need 40-60 g of protein

  • excess amino acids cannot be stored, because the amino groups make them toxic

  → however amino acid molecules contain a lot of energy, so it would be wasteful to excrete the whole molecule

  therefore excess amino acids undergo treatment in the liver to remove + excrete the amino component

  → this treatment consists of 2 processes : Deamination followed by the Ornithine cycle
• Deamination

  removes amino group and produces ammonia

  • ammonia is very soluble and highly toxic

  → therefore ammonia must not be allowed to accumulate

  deamination also produces an organic compound, a keto acid, which can enter respiration directly to release its energy
• The Ornithine Cycle

  because ammonia is highly soluble and toxic, it must be converted to a less toxic form very quickly

  • the ammonia is combined with carbon dioxide to produce urea
  • ammonia and carbon dioxide combine with the amino acid ornithine to produce citrulline

  → converted to arginine by addition of further ammonia; arginine is then re-converted to ornithine by removal of urea
• The Ornithine Cycle 2

  Urea is both less soluble and toxic than ammonia and so it can be passed back into blood + transported around the body to kidneys

  • in the kidneys the urea is filtered out of the blood + concentration in urine
  • urine can be safely stored in bladder until released from body