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Carbon, Hydrogen, Oxyge and Nitrogen (main elecments).

Notes
1. Proteins may be divided into two classes:
    (a) Animal proteins.
    (b) Vegetable proteins.
2. All proteins are made up of amino acide.
3. There are at least 22 different amino acids in proteins.
4. The body can change some amino acids into others.
5. Essential amino acids are necessary for growth and repair. There are 10 of these. The body cannot make them. It has to get them from food.
6. Animal proteins generally contain all the essential amino acide. Vegetable proteins generally lack one or more essential amino acide.
Here are 2 exceptions:
    (a) Gelatine is an animal protein, but lacks 4 essential amino acide.
    (b) Soya bean flour is a vegetable protein, but contains all the essential amino acids.
7. The body needs both animal and vegetable proteins.

Sources.
Animal: meat, milk, eggs, fish, cheese.
Vegetable: cereals, pulses, nuts.