Proteins are extremely large molecules composed of many amino acids. Proteins are complex organic compounds rich in carbon, hydrogen, oxygen, nitrogen and sometimes phosphorous and sulphur also. Proteins are needed by the body for:

  • growth and development
  • repair and maintenance
  • synthesis of antibodies, enzymes, and hormones

They can also be used as a source of energy. 1 gram of protein yields about 4 kcal of energy. Building blocks of proteins are the amino acids. Nutritionally, amino acids belong to two categories:

Essential amino acids

These are the amino acids which can not be synthesized in the animal body and must be supplied with food. For example, leucine

Non essential amino acids

These are the amino acids which can be synthesized in the body particularly from carbohydrates and need not be supplied in the diet. For example, alanine

Digestion of Protein

Like fats, proteins can not be absorbed in the tissue until they are broken down into their amino acids. Digestion of proteins occurs in stomach and small intestine where acids and enzymes break up proteins into amino acids.


Animal sources: Milk, egg, fish, bean, meat, and liver

Plant sources: Whole cereals (wheat and maize), pulses, nuts, grams, and legumes

Intake of more than one plant protein in the same meal (dal-roti, sambar-idli) can produce a mixture containing all the essential amino acids.

Proteins are structural components of body. For example, protein keratin is present in hair and nails. Collagen present in the connective tissue is also an example of protein. Actin and myosin are examples of contractile proteins present in the muscles.

Functions of Proteins

  • Proteins are required for building and maintaining body tissues.
  • Proteins are found in all the enzymes. For example Trypsin, pepsin and rennin.
  • Some proteins function as hormones, to regulate many body functions. For example, insulin is a hormone which regulates blood glucose level in the body.
  • Proteins also act as antibodies and protect the body from antigen (foreign agent).
  • Transport protein carries different substances from blood to the tissues in the body. Hemoglobin is a transport protein.