Cambridge O Level Biology · Syllabus 5090 · Excretion
Waste Product of Metabolism
What is Waste Product of Metabolism?
A substance produced by the chemical reactions taking place inside the cells of an organism for which that organism has no further use, and which must be removed before it accumulates to a harmful concentration. In Cambridge O Level Biology the two required examples are carbon dioxide, produced during aerobic respiration in every living cell and excreted by the lungs in expired air, and urea, produced in the liver from excess amino acids and removed from the blood by the kidneys in urine.
This definition is part of the Excretion chapter in Cambridge O Level Biology.
Common mistakes with Waste Product of Metabolism
- “Excretion means removing faeces from the body.” Why wrong Faeces is undigested and unabsorbed food. It was never inside a cell and no reaction of the body produced it, so it is not a waste product of metabolism. Say instead Excretion is the removal of toxic materials, the waste products of metabolism and substances in excess of requirements. Removing faeces is egestion.
Questions students ask about Waste Product of Metabolism
What is the difference between excretion and egestion?
Excretion removes substances the body's own cells produced, or substances present in excess. Egestion removes undigested and unabsorbed food through the anus. The material removed by egestion never entered a cell and was never produced by a chemical reaction of the body, which is why faeces is not a waste product of metabolism.