X-ray of intestine and pelvis
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Protein is a large, complex molecule that must undergo a series of processes during digestion. During digestion and absorption, protein passes through many organs. Once protein is digested, the body can utilize its nutrients to build and repair many of the cells in the body. The body also uses the calories from protein which are released during the digestion process for energy when carbohydrates and fats are not available.

Protein Digestion in the Stomach

Once protein is chewed and swallowed, hydrochloric acid and pepsin begin protein digestion in the stomach. HCl helps to kill bacteria in food that could cause infection. It also makes the stomach very acidic with a pH of 1.5. This acidic environment is necessary for HCl to react with pepsinogen to form pepsin so that it can break the central peptide bond in proteins. Rennin is an enzyme that is present in infants to help break down milk protein.

Protein Digestion in the Small Intestine

The pancreas releases digestive enzymes into the small intestine. In the duodenum, the first section of the small intestine, trypsin breaks down proteins into single amino acids by a process called hydrolysis. During hydrolysis, a water molecule is placed between two amino acids, breaking the bond. Trypsin also activates the enzymes chymotrypsin, carboxypeptidase and elastase that are released into the small intestine for amino acid chain breakdown.

Protein Absorption

Protein absorption takes place in the jejunum and ileum portions of the small intestine. This process requires energy. Adenosine triphosphate is the energy source the body utilizes during protein absorption. The body uses the carrier protein transport system to absorb amino acids. Each amino acid group has a carrier protein that is responsible for transporting it from the intestines to the mucosa cells. Sodium and potassium are minerals needed for the amino acids to pass from the intestines through the villi and into the bloodstream.

Protein Metabolism and Synthesis

The single molecule amino acids, or free amino acids, that are absorbed through the wall of the small intestine are now used for the last part of protein metabolism, protein synthesis. The proteins ingested from animal and plant protein sources are made into new tissues or used for tissue repairs in the body (hair, skin, nails, muscle) or they are broken down and used for energy. When there is too much protein in the body, the excess gets converted into fat for storage.