Proteins are present in all living plant and animal tissues
Composition
Proteins, like fats and carbohydrates, contain carbon hydrogen and oxygen. In addition they contain about sixteen per cent nitrogen which distinguishes the proteins for carbohydrates and fats.
Proteins are built from simpler compounds called amino acids. The amino acids contain a basic (amino – NH2) and an acidic (carboxyl –COOH) group in their molecules.
Some of amino acids cannot be synthesized in the body, but are essential for maintenance in human beings. These amino acids, which have to be supplied in the food, are called essential amino acids.
The remaining amino acids, which can be synthesized from others in the body, are termed as non essential amino acid, because our body does not have to depend for their supply in the foods we eat.
Structure of Proteins
The structure of a protein molecule is dependent on the number and kind of amino acids in it. The sequence of amino acids and the manner on which these are linked indicates the structure of protein.
Early experiments on protein quality ere carried out with purified proteins. On the basis of these experiments, proteins were classified as complete, partially complete and incomplete proteins depending on their ability to maintain life and promote growth.
Thus, animal proteins from milk, eggs and meat were all included in the list of complete proteins.
Gliadin, one of the proteins from wheat was found able to maintain life, but lack sufficient amounts of some amino acids necessary for growth.
Therefore, it was reported to be partially incomplete protein.
Zein one of the proteins from corn and gelatin are the examples of proteins, which are incapable of maintaining tissues or supporting growth. Therefore, these are totally incomplete proteins.
However, later experiments with diets made up of mixed protein from plants sources were found to maintain life sand support growth of animals.
Proteins in Food
