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.