liver and skin

• Proteins make up the structural materials of cells, excess protein cannot be stored so must be removed
• Some proteins are broken down (although incomplete) at all times :

  1. Worn out cells (e.g. red blood cells) are source of protein and broken down into amino acids
  2. Most amino acids are used to make new protein
  3. Small amount is lost via urine, skin, hair, fingernails
  4. Protein built up from amino acids become primary constituents of cell structures, enzymes, antibodies
• If energy supply has been used up, protein can be metabolised for energy :

  1. Amino group (NH2) is removed from amino acids (deamination) in liver aided by enzymes
  2. Converted into ammonia (NH3) by liver cells ; ammonia is toxic to cells
  3. Converted into urea which is eliminated from the body in urine
  4. Remaining parts (mainly carbon and hydrogen) are turned into carbohydrate
• Detoxifies alcohol and other drugs (e.g. antibiotics)
• Deactivates many hormones and converts them into a form that can be excreted by kidneys 
• Breaks down haemoglobin from dead red blood cells to produce bile pigments, passed out in faces
• Main function of skin is to provide protective covering over surface of body and regulate body temperature
• the skins role in excretion is to secrete water through sweat glands and secrete some drugs like salicylic acid
• sweat contains sodium chloride, lactic acid, urea dissolved in the water
• Sweat glands are located in lower layers of skin, a duct carries sweat to hair follicle or to the skins surface where it opens at a pore. the cells surrounding sweat glands can contract and squeeze sweat to skin surface
• amino acid + oxygen -- enzyme --> carbohydrate + ammonia
• energy + carbon dioxide + ammonia --> urea + water