The human body, and everything else, is made up of atoms.
There are about 26 different sorts of atom in the human body, combined into numerous different sorts of molecules.
Amino acids contain carbon (C), nitrogen (N), oxygen (O) and hydrogen (H) atoms, and some contain an atom of sulfur (S).
There are about 20 different amino acids, with different side chains (R groups).
Amino acids are linked via peptide bonds to make polypeptides and proteins.
Each protein molecule can be hundreds of amino acids long and the amino acids must be joined in a precise order, which is specified by a code in the DNA in the chromosomes.
The side-chains (R groups) of the amino acids can interact with one another to fold the protein into a particular shape which is essential for the protein to function correctly.
When protein food is eaten, the amino acids are released by the activity of peptidase enzymes during digestion. The amino acids are then absorbed into the blood and used to build up the body's own proteins.
The amount of protein needed in a balanced diet differs according to age and gender. Insufficient or excess protein in the diet can cause health problems.