Proteins are complex structure of amino acids. They are essential building blocks in an organism caring out a vast array of mechanical, structural and biochemical functions in living organism. There are thousands of different protein in plants, animals, human body, micro-organism and other living sources and they differ from each other in shape and function as hundreds of amino acid string sequence fold to form a complex protein structure. Proteins are degradable and have short life span.
In human, 20 amino acids permutate to form thousands of different proteins that are required for growth and repair of tissues and cell functions. They serve as fuel source or as nutrient to living organisms. Out of the 20 amino acids human cannot synthesize 9 amino acids and these are to be obtained from diet. Grains, legumes, nuts, meat, milk, fish and eggs provide us with protein source. Our essential amino acid requirement can be met by diet containing proteins in required quantity.
The quality of protein (i.e to know which protein is better) can be determined from its nutritive value, which is further determined by their essential amino acids composition and their digestibility.