Proteins can be just as useful as carbohydrates and lipids in releasing energy. Broken down into their element amino acids, they release energy during respiration. The list below shows important uses of protein:

  • cell-membrane protein carry substances across the membrane for processes such as facilitated circulation and active transport
  • transport protein, e.g. hemoglobin transports oxygen in the blood
  • enzymes catalyze biochemical reactions, e.g. pepsin breaks down protein into polypeptides
  • storage proteins, e.g. aleurone in seeds help germination, and casein in milk helps bring valuable protein to babies
  • hormones are passed through the blood and trigger reactions in other parts of the body, e.g. insulin regulates blood sugar
  • immune-protein, e.g. antibodies are made by lymphocytes and act against antigenic sites on microbes
  • structural protein provide strength to organs, e.g. collagen makes tendons tough
  • contractile protein, e.g. actin and myosin help muscles shorten during contraction
  • buffer protein, e.g. blood proteins, due to their charge, help sustain the pH of plasma.