Proteins are polymers and their monomers are amino acids. There are about twenty different amino acids. In synthesis, cells join amino acids together in a long chain. The order of the amino acids decides how the chain folds, and so decides the shape and the function of the protein.
Different proteins do different jobs. Enzymes speed up reactions, hormones carry messages, and an antibody helps to fight infection, and antibodies are made in large numbers during an illness. Haemoglobin is a protein that carries oxygen.
In breakdown, protease enzymes split proteins into amino acids. This happens in digestion, where proteins from food are broken down so that the amino acids can be absorbed. The cells then use these amino acids to make the body's own proteins.
The biuret test shows protein. Biuret solution is added to the sample and it turns from blue to purple if protein is present.