Biology · Excretion. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Excretion is the removal from an organism of toxic materials and the waste products of metabolism. Metabolism means the chemical reactions that take place inside cells. If these wastes stayed in the body they would build up and damage the cells.
Two excretory products must be learned here. Carbon dioxide is made in cells by aerobic respiration. The blood carries it to the lungs and it is breathed out. Urea is made in the liver when excess amino acids are broken down. Urea is toxic. The blood carries it to the kidneys, which remove it in urine.
Faeces are mostly undigested food that was never absorbed into cells, so removing them is egestion, not excretion.
Students often say urea is made in the kidneys. It is made in the liver; the kidneys only remove it from the blood.
Answer: Yes. Carbon dioxide is a waste product of metabolism (respiration) and it is removed from the body through the lungs, so this is excretion.
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