BIOLOGY · Stage 9 · Chapter 10: Excretion in Humans
Biology 10.1 What is excretion? — Removing Metabolic Waste Products — Alhussan International Academy - Biology (Stages 7-9) Revision Notes & Topical Questions
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10.1 What is excretion? — Removing Metabolic Waste Products — Alhussan International Academy — Biology (Stages 7–9) notes: Define excretion, name the main excretory products and organs, and explain how urea is made and why excretion matters.Excretion = the removal of the waste pr…
- Carbon dioxide
- Protein excess
- Deamination
- Daily output
- Meat eaters
- Urine colour
Section 1 of 6What excretion means
1Excretion2 points
= the removal of the waste products of metabolism, and of substances in excess of the body's needs, from the body.
- Metabolism is the sum of all the chemical reactions in cells, and many produce substances that are harmful if they build up.
- Excretion is one of the characteristics of all living organisms.
2Egestion2 points
- the removal of undigested food as faeces, which is not excretion.
- Faeces are materials taken into the body but never absorbed into the blood, such as plant fibre.
Section 2 of 6Main excretory products and organs
| Waste product | Where it comes from | Organ that removes it |
|---|---|---|
| Carbon dioxide | Respiration in all cells | Lungs (breathed out) |
| Urea | Breakdown of excess amino acids in the liver | Kidneys (in urine); some in sweat |
| Excess water and salts | Food and drink | Kidneys; also skin (sweat) and lungs (water vapour) |
- Carbon dioxide
- it dissolves to form an acid and lowers the pH of blood, so large amounts are toxic.
Section 3 of 6How urea is made
- Protein excess
- the body can store excess carbohydrate and lipid as fat but cannot store amino acids or protein.
- Deamination
- in the liver the amino group is removed from the amino acid and converted into urea.
- The remaining carbon-containing part is used for respiration or converted to carbohydrate or fat.
- Transport
- urea is soluble, so it is carried dissolved in the blood plasma to the kidneys.
- Toxicity
- if the urea concentration in the blood gets too high it damages organs including the brain and can kill.
- Daily output
- a healthy adult passes about 1.5 litres of urine containing about 25 g of urea.
- Meat eaters
- an animal such as a lion eating a lot of protein makes a lot of urea.
Section 4 of 6The kidneys and urine
- Kidney
- it is a reddish-brown organ about 10 cm long, against the back at about waist level.
- A thick layer of fat protects its delicate structures from damage.
- Urea
- it forms white crystals that dissolve in water to give a colourless solution.
- Urine colour
- the yellow colour comes from yellow substances excreted along with the urea and water.
Section 5 of 6Why excretion is essential
- It prevents the build-up of toxic substances that would damage cells and enzymes.
- It keeps the chemical composition of blood and tissue fluid steady, as part of homeostasis.
- It controls the water content of the body so that cells do not shrink or swell.
- Kidney failure
- it lets urea and other wastes accumulate, causing illness and eventually death unless treated.
Section 6 of 6Excretion in plants
- Plants excrete without special organs
- oxygen from photosynthesis leaves through stomata, and for example palms release oxygen into the air.
- Stored wastes
- waste products can be stored in leaves that are lost in autumn.
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