1. Excretion
The removal of metabolic waste from the body. e.g. carbon dioxide and urea Where do these come from?

2. Urea Protein can’t be stored but does contain energy.
In the liver, excess amino acids are broken down.
Deamination is the removal of an amino group to make ammonia and a keto acid.
Ammonia is toxic so it is then converted into urea.
The keto acid can be used in respiration.

3. Amino acid + oxygen = keto acid + ammonia
Ammonia + carbon dioxide = urea water
2NH CO = CO(NH2)2 + H2O

4. Urea then dissolves in blood plasma and is removed by the kidneys in urine.

5. Carbon dioxide
Carbon dioxide is made as a by-product of respiration. In excess, it is toxic.
CO2 + H2O = H2CO3 (carbonic acid)
H2CO3 = H+ + HCO3- (hydrogencarbonate ions)