What Happens to Each Nutrient
| Nutrient absorbed | How it is assimilated |
|---|---|
| Glucose | Used in aerobic respiration to release energy for cell activities. Excess glucose is converted to glycogen and stored in the liver and muscles. If glycogen stores are full, glucose is converted to fat. |
| Amino acids | Used to synthesise new proteins for growth, repair, enzymes, antibodies and hormones. Excess amino acids cannot be stored: they are deaminated in the liver. |
| Fatty acids and glycerol | Reassembled into lipids and stored under the skin (subcutaneous fat) and around organs. Also used to make cell membranes (phospholipids). |
| Vitamins and minerals | Used for specific metabolic functions: e.g. Iron → haemoglobin; calcium → bones and teeth; vitamin D → calcium absorption. |
The Role of the Liver
The liver receives blood directly from the small intestine via the hepatic portal vein. It processes absorbed nutrients:
- Glucose regulation:
- After a meal (blood glucose high): insulin stimulates the liver to convert glucose → glycogen (glycogenesis).
- Between meals (blood glucose low): glucagon stimulates the liver to convert glycogen → glucose (glycogenolysis).
- Deamination of excess amino acids:
- The amino group (−NH₂) is removed from the amino acid.
- The amino group is converted to urea (CO(NH₂)₂), which is transported in the blood to the kidneys and excreted in urine.
- The remaining carbon-containing part (keto acid) is used for respiration or converted to fat/carbohydrate.
- Bile production: The liver produces bile, stored in the gall bladder.
- Detoxification: The liver breaks down alcohol and other toxins.
Deamination Explained (Supplement)
Why can't amino acids be stored? Unlike glucose (stored as glycogen) and fats (stored in adipose tissue), there is no storage molecule for amino acids. The body cannot build up a reserve of amino acids.
Deamination process:
- Excess amino acids are transported to the liver.
- The amino group (−NH₂) is removed: this is deamination.
- The amino group is combined with CO₂ to form urea: 2NH₃ + CO₂ → CO(NH₂)₂ + H₂O.
- Urea dissolves in blood plasma, is filtered by the kidneys, and excreted in urine.
- The remaining part of the amino acid (the keto acid) enters respiration pathways or is converted to fat/glycogen.
Exam link: Urea is a waste product of metabolism: so its removal by the kidneys is excretion, not egestion.