1. Proteins are essential structure components of cell membranes, organelles, cytoplasm, extracellular matrices and fibres. Keratin is the major constituent of hair, skin, nails, horns, feathers and wool. Cartilage is made of collagen.
2. Many proteins function as enzymes to catalyse specific chemical reactions.
3. Some proteins act as carriers, which bind and transport specific molecules across a membrane or in a body fluid. Haemoglobin (in blood) transports and myoglobin (in muscle) stores oxygen. In plants P-protein is involved in the transport of organic compounds through phloem.
4. Some proteins function as receptor molecules. These bind with specific informational molecules like hormones reaching the cell and mediate in their cellular effects.
5. Hormones, such as insulin and parathyroid hormone, are proteins that regulate metabolism.
6. Contractile proteins like actin and myosin participate in cellular movements and locomotion.
7. Some proteins act as antibodies that participate in the defence mechanism of the body.
8. Storage proteins include albumin of egg and glutelin of wheat.
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