Proteins

Proteins are compounds of carbon, hydrogen, oxygen, nitrogen and sometimes sulphur and phosphorus. The presence of nitrogen differentiates protein from carbohydrate and fat. Amino acids are small units which build up to form large protein molecules. There are twenty different amino acids in our body which can be joined in any particular order to make a protein molecule. Amino acids are divided into two categories-----essential and non-essential. Essential amino acids are those which must be present in the diet since they cannot be synthesized in human body. Non-essential amino acids can be synthesized from other amino acids in human body. Proteins are present in milk and milk products, eggs, meat, fish, beans and nuts.

Food which contains proteins

Functions:

It provide the amino acids needed for forming cell membranes and cytoplasm in new cells.

It is necessary for the growth and repair of body cells.

The cells of muscles, tendons, and ligaments are maintained with protein.

It is essential for making haemoglobin in blood, antibodies, enzymes and hormones.

It also provides energy. One gram of protein gives about 18kJ of energy. It serves as a source of energy and heat when there is insufficient carbohydrate in the diet.

Digestion of proteins:

Proteins are broken down into peptides by an enzyme called protease which is found in gastric juice in stomach and pancreatic juice in duodenum. When the peptides reach the small intestine, it is further digested into amino acids by proteases in the intestinal juice.

The amino acids is simple enough to be absorbed by human body. It passes through the walls of villi on the surface of ileum by diffusion and active transport and then into the blood capillaries.

A diet high in meat could cause an excessive saturated fat and cholesterol intake. Another potential problem is that a high-protein diet may put a strain on the kidneys. The excess amino acids are deaminated in the liver. The eliminated amino group is converted into urea while the remaining carbon skeleton is changed into carbohydrates or fat. The urea is then excreted by the kidney as urine.