What metabolic functions does the liver carry out?
- Control of blood glucose levels, amino acid levels, lipid levels.
- Synthesis of bile, plasma proteins, cholesterol.
- Synthesis of red blood cells in the fetus.
- Storage of vitamins A, D and B12, iron, glycogen.
- Detoxification of alcohol, drugs
- breakdown of hormones
- destruction of red blood cells.
What does the liver store sugar as?
- the liver stores sugar in the form of glycogen.
- It is able to store approximately 100-120g of glycogen which makes up about 8% of the fresh weight of the liver.
How is the glycogen stored and used in the liver?
- The glycogen forms granules in the cytoplasm of the hepatocytes.
- This glycogen can be broken down to release glucose in the blood as required.
Which substances does the liver detoxify?
- Substances that may cause harm.
- Some compounds like hydrogen peroxide are produces in the body.
- Others, such as alcohol, may be consumed for health or recreational reasons. e.g. medicines and recreational drugs.
How does the liver detoxify toxins?
-Toxins may be rendered harmless by oxidation, reduction, methylation or by combination with another molecule.
Liver cells contain many enzymes that render toxic molecules less toxic, give two examples.
- Catalase.
- cytochrome P450
What does catalase do?
-Converts hydrogen peroxide to oxygen and water.
-Catalase has a particularly high turnover number of five million. What does this refer to?
-The number of molecules of hydrogen peroxide that that one molecule of catalase can render harmless in one second.
What is Cytochrome P450?
- A group of enzymes used to break down drugs including cocaine and various medicinal drugs.
- The cytochromes are also used in other metabolic reactions such as electron transport during respiration.
Why does the cytochrome P450’s role in metabolising drugs cause unwanted side effects of some medicinal drugs?
Their role in metabolising drugs can interfere with other metabolic roles and cause unwanted side effects of some medicinal drugs.
Alcohol or ethanol is a drug that depresses nerve activity. How can it be used in respiration?
-Alcohol contains chemical potential energy which can be used for respiration.
Describe the steps in the detoxification of alcohol.
-Alcohol is broken down by the hepatocytes by the action of the enzyme ethanol dehydrogenase.
-The resulting compound is ethanal.
-This is dehydrogenated further by the enzyme ethanal dehydrogenase.
The final compound produced is ethanoate (acetate). -This acetate combines with coenzyme A, which enters the process of aerobic respiration.
-The hydrogen atoms released from alcohol are combined with another coenzyme called NAD to form reduced NAD.
What is ‘fatty liver’ disease?
- NAD is also required to oxidise and breakdown fatty acids for use in respiration.
- If the liver has to detoxify too much alcohol, it uses up its stores of NAD and has insufficient left to deal with the fatty acids.
- These fatty acids are then converted back to lipids and stored as fat in the hepatocytes, causing the liver to become enlarged.
What can ‘fatty liver’ cause?
-Alcohol-related hepatitis or cirrhosis.
Why can we not store excess amino acids?
-Because the amino groups make them toxic. t
Why would it be wasteful to excrete the whole amino acid molecule?
The amino acid molecules contain a lot of energy.
What happens to excess amino acids?
- Excess amino acids undergo treatment in the liver to remove and excrete the amino component.
- This treatment consists of two processes: deamination followed by the ornithine cycle.
What does the process od deamination do?
-Removes the amino group and produces ammonia.
Why must ammonia not be allowed to accumulate?
-Ammonia is very soluble and highly toxic.
What is produced in deamination apart from ammonia?
-Deamination also produces an organic compound (a keto acid) which can enter respiration directly to release its energy.
Why must ammonia be converted into a less toxic form very quickly?
Because ammonia is so soluble and toxic.
Describe the ornithine cycle.
- The ammonia is combined with carbon dioxide to produce urea.
- This occurs in the ornithine cycle.
- Ammonia and carbon dioxide combine with the amino acid ornithine to produce citrulline.
- This is converted to arginine by addition to further ammonia.
- The arginine is then re-converted to ornithine by the removal of urea.
Why is ammonia converted to urea?
-Urea is both less soluble and less toxic than ammonia.
What happens to the urea next?
- It can be passed back into the blood and transported around the body to the kidneys.
- In the kidney0,s urea can be safely stored in the bladder until it is released from the body.
