Proteins are made up of smaller building blocks called amino acids, joined together in chains. There are 20 different amino acids. Some proteins are just a few amino acids long, while others are made up of several thousands. These chains of amino acids fold up in complex ways, giving each protein a unique 3D shape. Some, like keratin, the hair protein, form long fibres. Others, like haemoglobin, the protein that carries oxygen around your body, are roughly spherical.