Proteins are the building blocks of any living organism. They are composed of chains of 20 different kinds of amino acids. Proteins constitute life in the form of enzymes, antibodies, hormones, hair, nail, bone, teeth, hemoglobin (blood protein), neurotransmitters, skin or as the basic components of any cell in our body. The smallest units of proteins (amino acids) fall in either of the following types:

Essential amino acids:

Essential amino acids are amino acids human body can not synthesize and there fore they have to be supplied in the diet. List of essential amino acids: Valine, threonine, leucine, lysine, isoleucine, methionine, phenyalanine, and tryptophan.

Non-essential amino acids:

Non-essential amino acids can be produced in the body as long as the carbon, hydrogen, oxygen (from fats or carbohydrates) and nitrogen components are supplied in the diet. List of non-essential amino acids: Alanine, cysteine, glycine, serine, histidine, tyrosine, cystine, proline, arginine, aspartic acid, glutamic acid, and glutamine.

Digestion of protein

Protein digestion begins in the stomach. Acids secreted in the stomach denature protein and make it susceptible to degradation by enzymes. Digestion is completed in the small intestine where single amino acids and peptides are absorbed into the blood stream. Whole protein, not digested, are also absorbed to the blood stream.

Benefits of protein

Proteins are indispensable for growth and maintenance of every kind of cell in our body. Body development, replenishment of lost blood, healing of wounds and scars, replacement of dead cells, and hair and nail growth need proteins. Proteins in the form of enzymes, hormones and antibodies promote healthy metabolic and physiological processes, and boost our nervous and immunity systems. In situations when fat and carbohydrate food sources are not providing adequate calories, proteins are degraded to generate the calories our body needs.

Deficiency of protein

In developed countries, protein deficiency is not a serious problem. In developing countries, however, protein deficiency, called kwashiorkor, is prevalent and particularly in children. Children deficient in protein show an array of symptoms that include: retarded growth, loss of hair and skin color, slow healing of wounds, poor digestibility, diminishing of blood protein, draining of fluid from blood (edema), liver damage, and poor immunity.

For information on foods rich in protein, follow these links:
Food sources of protein: animal and vegetable protein sources and content.
Protein requirement: for kids, adults and pregnant and lactating women.
Soybean/tofu: nutritional value and health benefits
Essential and non essential amino acids: Definitions and Functions
L-Arginine: Food Sources, Health Benefits and Side Effects

For characterization of protein from animal nutrition perspective, please read this article: Crude Protein.