Proteins are composed of carbon, hydrogen, oxygen, and nitrogen; it is the presence of the last of these which distinguishes proteins from the other materials of life. There is often also a little sulphur and phosphorus. Proteins are highly complex substances with huge molecules containing thousands or millions of atoms. Each molecule is a long chain made up of smaller molecules known as amino acids. These are joined by a link called a ‘peptide bond’, which is fairly weak and easy to break.