Define Proteins as Structural Elements and Structural Units?

The liver cell membrane analysis shows that this membrane contains 50-60% protein, 35% lipids and 5% carbohydrates. The carbohydrate present is joined primarily to the protein forming glycoproteins, compounds which constitute the receptor sites of several hormones. The protein portion of the membrane is so oriented that its hydrophilic aspects are also in close proximity to the intracellular and extracellular fluids. The protein molecules are interspersed within the lipids and lend both structural stability and fluidity to the membrane. Membrane function depends on how the proteins are placed in the membrane and on the fluidity, which results from the combination of proteins in a lipid mixture. If the lipid is more saturated, a more rigid crystalline structure will form. Many lipids being fluid and less rigid, allow the proteins to change their shape in response to ionic changes and hence these proteins function as enzymes, carriers, binding or receptor sites or entry ports for a large variety of materials binding, entering or leaving the cell. Thus proteins serve as the structural and functional units of the cell membrane.

Proteins are also important intracellular structural units. Muscle is composed of 20% protein, 75% water and 5% inorganic mate ial, glycogen and other organic compounds. The major proteins in muscle are myosin - a large globular protein, and actin - a smaller globular protein. These two proteins, plus the filamentous tropomyosin and troponin a e the moleculas components of the muscles. The muscle proteins are characterized by their elasticity, which contributes to the contractile power of the tissue. The most important structural function of protein is related Lo skin and connective tissue. The skin is composed of epithelial tissue which covers not only the exterior of the body but also lines the gastrointestinal tract, respiratory tract and the urinary tract. One of the major proteins found in the skin is 'melanin'. Melanin is a tyrosine derivative and. provides the pigmentation or characteristic colour to the skin.

Persons unable to form this pigment are albinos and the disease is called 'albinism.' Keratin is the protein which forms hair, nails, hooves, feathers or horns. Each of these structures is slightly different but all contain keratin. This protein is insoluble in water and is resistant to most digestive enzymes. It has a high percentage of cystine. Connective tissue is that tissue which holds various cells and tissues together. It includes bones and teeth also, since they start with a matrix protein into which various amounts of minerals are deposited. Collagen and elastin are the two distinct types of proteins in the connective tissue. It contains protein, hydroxyproline and glycine. These proteins are not easily degradable and are inert metabolically. Even in protein deficient states, the body will synthesize collagen and elastin and these proteins will not be catabolized for needed amino acids. However, this protein can be degraded to a limited degree by boiling in acid. It is then converted to gelatin. The collagen of bone, skin, cartilage and ligaments differ in chemical composition from that of the white fibrous tissue which holds individual cells together within muscle, liver and other organs. Elastin and chondroalbumoid are two other proteins in the connective tissue. They are present in small amounts and serve as a part of the structural protein.