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A protein is a biologically functional molecule made up of polymers of amino acids. Proteins may also contain sugars (glycoproteins) and lipids (lipoproteins).

Proteins are an essential part of all living organisms, especially as structural components of body tissues such as muscle, hair, etc., and as enzymes and antibodies. Proteins account for more than 50% of the dry mass of most cells.


There are 2 main types of proteins.

Fibrous proteins - insoluble form of proteins which form the structural basis of many body tissues.

Globular proteins - soluble form of proteins which include all enzymes, hormones, etc…

Based on function, proteins are classified into 8 groups:

Enzymes - act as catalysts in biochemical reactions. e.g. Digestive enzymes.

Receptors - these proteins mediate between a chemical agent that acts on nervous tissue and the physiological response. All organs having nerve endings, which respond to stimuli with the help of receptors.

Hormones - These are regulatory proteins which control the physiological processes such as growth and development, metabolic rates(insulin) etc…

Defensive proteins - These are components of immune system which protect the organism from external agents such as virus and bacteria. e.g. antibodies

Transport proteins - these are special type of proteins capable of transporting substances throughout the circulatory system. e.g. Haemoglobin carries oxygen, lipoprotein carries lipids etc…

Structural proteins - These proteins form a structural part of organism and provide protection along with response and stimuli against external agents.

e.g. Keratin(hair, horns, feathers), connective tissues(Collagen, elastin) etc…

Storage proteins - Proteins stored for consumption of offspring. e.g. protein in eggs(albumin), milk(casein), seeds etc…

Contractile and motor proteins - These proteins are essential for locomotion. e.g. Actin and myosin.


Before absorption, proteins are broken down into their constituent amino acids. In the absence of carbohydrates, proteins are consumed for energy.

Human body requires a mixture of  eight amino acids to maintain nitrogenous equilibrium. These amino acids are also known as essential(indispensable) amino acids. These include isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. In addition,  infants require histidine.

Dietary sources Meat, nuts, legumes, egg, milk etc…
Deficiency diseases


Cause: malnutrition or semi-starvation conditions. It normally affects the children below 3 years.

Effects: stunted growth, emaciation, brittle hair, dehydration and loose folds of skin on the limbs and buttocks due to loss of muscle and fat. Persistent marasmus can cause mental handicap and impaired growth.

Treatment: providing a high-energy, protein rich diet.

Kwashiorkor: It is one of the most important causes of ill health and death among children in the tropics. It affects typically the small child weaned from the breast and not yet able to cope with an adult diet, or for whom an adequate amount of first-class protein is not available.

Cause: malnutrition.

Effects: stunted growth and a puffy appearance due to oedema, enlargement of liver, dehydration, and the child loses resistance to infection, which may have fatal consequences. The more advanced stages are marked by jaundice, drowsiness, and a fall in body temperature.

Treatment: Child is kept warm and frequently fed with first-class proteins such as small amounts of milk, and vitamin and mineral tablets. A nutritious diet is then gradually introduced. Most treated children recover, but those less than 2 years old may suffer from  permanently stunted growth.