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Protein is a complex macromolecule, which consists of carbon, nitrogen, oxygen, hydrogen and sulfur. It is composed of unbranched chains of amino acids. A typical protein consists of about 200-300 amino acids.


Protein is the second most abundant substance in the body second to water. Protein is made up of twenty different aminoacids. The aminoacids are adequately synthesized in the body and these are called non essential aminoacids. Some aminoacids are not synthesized from the body and must be supplied from the diet, they are called essentail aminoacids.

Protein helps in growth. It is essentail for the formation of some hormones, plasma proteins, enzymes, minerals, vitmains and hemoglobin.

Excess of protein is not used by the body, it is converted by the liver in to the fat and stored in body tissues.

Protein-containing foods are grouped as either complete or incomplete proteins.

  • Complete proteins contain all nine essential aminoacids. Complete proteins are found in animal foods such as meat, fish, poultry, eggs, milk, and milk products such as yogurt and cheese. Soybeans are the only plant protein considered to be a complete protein.
  • Incomplete proteins lack one or more of the essential amino acids. Sources of incomplete protein include beans, peas, nuts, seeds, and grain. A small amount of incomplete protein is also found in vegetables.

Plant proteins can be combined to provide all of the essential amino acids and form a complete protein. Examples of combined, complete plant proteins are rice and beans, milk and wheat cereal, and corn and beans.

Functions of protein:

Antibodies - are specialized proteins involved in defending the body from antigens (foreign invaders). One way antibodies destroy antigens is by immobilizing them so that they can be destroyed by white blood cells.

Contractile Proteins - are responsible for movement. Examples include actin and myosin. These proteins are involved in musclecontraction and movement.

Enzymes - are proteins that facilitate biochemical reactions. They are often referred to as catalysts because they speed up chemical reactions. Examples include the enzymes lactase and pepsin. Lactase breaks down the sugar lactose found in milk. Pepsin is a digestive enzyme that works in the stomach to break down proteins in food.

Hormonal Proteins - are messenger proteins which help to coordinate certain bodily activities. Examples include insulin, oxytocin, and somatotropin. Insulin regulates glucose metabolism by controlling the blood-sugar concentration. Oxytocin stimulates contractions in females during childbirth. Somatotropin is a growth hormone that stimulates protein production in muscle cells.

Structural Proteins - are fibrous and stringy and provide support. Examples include keratin, collagen, and elastin. Keratins strengthen protective coverings such as hair, quills, feathers, horns, and beaks. Collagens and elastin provide support for connective tissue such as tendons and ligaments.

Storage Proteins - store amino acids. Examples include ovalbumin and casein. Ovalbumin is found in egg whites and casein is a milk-based protein.

Transport Proteins - are carrier proteins which move molecules from one place to another around the body. Examples include hemoglobin and cytochromes. Hemoglobin transports oxygen through the blood. Cytochromes operate in the process of electron transport as electron carrier proteins.

Sources of proteins:

Complete proteins are found in foods like eggs, poultry, meat, fish, sea foods, milk yoghurt, cream, cheese, soya bean.

Sources of incomplete proteins are seeds, nuts, vegetables, legumes (peas and beans) and grains. Nuts like cashews, almonds, lima beans, lentils, red kidney beans and tofu are rich in proteins.




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