Protein are amino acids polymers which make up about 50% organic content of living organisms. Each protein has its own unique three-dimensional conformation and performs a specific function. Proteins are the most complex biological macromolecules, which are involved in every activity in living organisms.
(1) Catalyst: Almost all enzymes are proteins. Enzymes selectively accelerate biochemical reactions, but they are not consumed in these reactions. For example, pepsins are responsible for breaking down the food proteins into peptides.
(2) Structure support: Collagen and elastin form the network under the skin and epidermis is attached to it. Keratin is one of the main components of hair, cuticles and feathers.
(3) Movement: actin and myosin are responsible for muscle contraction.
(4) Recognition: The unique glycoproteins(antigens) on the cell membrane serve as their ID card by which the immune system can recognize intruders.
(5) Defense: The immune system recognizes the antigens of invaders and produces some specific antibodies (immunoglobulins). The antibodies bind to the invaders' antigens to render them inactive.
(6) Transport: Hemoglobins transport oxygen to tissues and carry away carbon dioxide during circulation; The proteins on cell membrane transport glucose into the cell.
(7) Communication: Growth hormone is a protein that promotes bone production and protein synthesis; Some neuropeptides act as neurotransmitters to transmit electrical impulses between nerve cells.
(8) Storage: The protein of egg white supplies amino acids to the developing embryo. The proteins in plant seeds provide nutrients for their germination, and are also a protein source for humans.