What are the main biological functions of proteins? Give relevant examples.

Structural function. Proteins are part of all cells and tissues of living organisms. The proteins are used to construct the elements of the cytoskeleton, contractile elements of muscle fibers. Cartilage and tendon are predominantly composed of proteins. They include collagen protein. The most important structural component of feathers, hair, nails, claws, horns, hooves in animals is keratin protein.
Enzymatic (catalytic) function. Enzymes are biological catalysts, that is, substances that accelerate the flow of chemical reactions in living organisms. Enzymes are involved in the synthesis and breakdown of various substances. They provide carbon fixation during photosynthesis, the breakdown of nutrients in the digestive tract, etc.
Transport function. Many proteins are able to attach and transfer various substances. Hemoglobin binds and carries oxygen and carbon dioxide.
Contractile (motor) function. Contractile proteins provide the ability of cells, tissues, organs and whole organisms to change shape, move. So, actin and myosin provide muscle function and non-muscle intracellular contractions.
Regulatory function. Some peptides and proteins are hormones. They affect various physiological processes. For example, insulin and glucagon regulate blood glucose, and somatotropin (growth hormone) regulates the processes of growth and physical development.
Signal function. Some proteins of cell membranes are able to change their structure in response to the action of external factors. Using these proteins, signals are received from the external environment and information is transmitted to the cell.
Protective function. Proteins protect the body from the invasion of foreign organisms and from damage.
Toxic function. Many living organisms secrete toxin proteins, which are poisons for other organisms. Toxins are synthesized in the body of a number of animals, fungi, plants, microorganisms.
Energy function. After cleavage to amino acids, proteins can serve as a source of energy in the cell. With complete oxidation of 1 g of protein, 17.6 kJ of energy is released.
Storage function. Reserve seeds are stored in the plant seeds, which are used during germination by the embryo and then by the seedling as a source of nitrogen.

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