protein

Large, complex, biologically-important molecules composed of amino acids joined by peptide bonds. The number of amino acids used can be many hundreds. There are 20 different amino acids and they can be joined in any order. Proteins are essential to all living organisms. As enzymes they regulate all aspects of metabolism. Structural proteins such as keratin and collagen make up skin, claws, bones, tendons, and ligaments; muscle proteins produce movement; haemoglobin transports oxygen; and membrane proteins regulate the movement of substances into and out of cells. For humans, protein is an essential part of the diet, and is found in greatest quantity in soy beans and other grain legumes, meat, eggs, and cheese. During digestion protein molecules are broken down into amino acids which are then easily absorbed into the body.

Protein synthesis occurs in cells. The information describing the order in which the different amino acids are joined is found in DNA in the form of a code. The part of the DNA that carries the code for making one protein is called a gene. Each protein described in the code has an effect on the appearance and characteristics of the organism.

During digestion in the body, proteases (any enzymes that break down a protein) are produced by the stomach, the pancreas, and the small intestine. These enzymes catalyse the breakdown of protein into amino acids. Pepsin is an enzyme which is produced by the walls of the stomach. It digests large protein molecules into smaller protein molecules (smaller polypeptides). The conditions in the stomach are very acid and pepsin works at its fastest rate in such conditions.

The amino acids produced by the digestion of proteins are absorbed into the blood in the small intestine. They are transported around the body by the blood for use by cells to make the proteins they need. This protein synthesis needs energy and this energy is provided by respiration.