Describe the structure of proteins, and the bonds involved.

Proteins have three or four levels of structure, in increasing complexity. Primary structure is the sequence of amino acids that the protein is made up of; secondary structure is either alpha-helices or beta pleated sheets that are held together by hydrogen bonds between the N and H atoms of the amino acid backbone; tertiary structure describes more complicated motifs held together by disulphide bridges, ionic bonds, hydrogen bonds between the R groups and van der Waals forces. Quaternary structure is not seen in all proteins, but consists of multiple polypeptide units brought together to form an overall more complex structure.