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By the end of this topic, you should be able to:
Before we can understand excretion, we need to understand metabolism.
Metabolism means all the chemical reactions that happen inside the cells of a living organism.
Your cells are constantly carrying out chemical reactions. These reactions do important jobs — they release energy, build new materials, and keep you alive. However, just like a factory produces waste alongside its useful products, your cells also produce waste products as a result of these reactions.
Some of these waste products are toxic — meaning they are poisonous and harmful to the body if they are allowed to build up. Others are simply substances that the body has too much of and does not need.
Excretion is the removal of toxic materials and the waste products of metabolism from an organism.
In simple terms: excretion is how the body gets rid of the harmful and unwanted waste made during its chemical reactions.
Important things to understand about excretion:
These two words are very commonly confused in exams, so learn this carefully:
| Term | What it means |
|---|---|
| Excretion | Removal of waste products made by the body's own chemical reactions (e.g. carbon dioxide, urea) |
| Egestion | Removal of undigested food from the body through the anus (this food was never actually absorbed into the body's cells) |
Egestion is not excretion because the undigested food was never involved in metabolism — it was never part of the body's chemical reactions.
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