Each amino acid contains a different R group specific to the amino acid. Describe how the R group of an amino acid contributes to the tertiary structure of a protein. (4 marks)

Cysteine contains an S atom in its R group. Where 2 molecules of cysteine are found together, strong covalent bonds or "disulfide bridges" can form between the two S atoms (S=S). R groups may have a positive or negative charge. These charges can either attract or repel. Where two oppositely charged R groups are found together, ionic bonds can form between the R groups. Amino acids with a hydrophilic R group will be positioned in contact with water, towards the outside of the molecule. Amino acids with a hydrophobic R group will be found on the inside of the molecule.