When you see, smell or think of food, the salivary glands in your mouth produce saliva, which in turn stimulates the secretion of gastric juices by your stomach. One of the components of gastric juice is hydrochloric acid, which is important for digestion of protein. Protein is a complex nutrient made up of amino acids that are linked together by peptide bonds. Hydrochloric acid initiates the process of breaking down dietary proteins into amino acids, which are then used by the body to produce cells, hormones, enzymes and tissues.
Secretion of HCl
The stomach is a J-shaped sack that can expand to hold about half a gallon of food. The stomach has digestive glands that secrete about 2 liters of gastric juice daily, which contains hydrochloric acid. The stomach cells also secrete pepsinogen, an inactive form of an enzyme that digests protein. Once pepsinogen comes in contact with hydrochloric acid, it converts to its active form, pepsin. A thick mucus lining that coats the inner surface of the stomach protects the stomach from the acidic environment due to the secretion of hydrochloric acid, and from the action of pepsin.
Action on Protein
In addition to converting pepsinogen to pepsin, hydrochloric acid denatures the complex shape of proteins. This unwinds the proteins, allowing pepsin to act on the otherwise hidden peptide bonds of dietary proteins. Additionally, hydrochloric acid provides the optimal low pH of 2 for the maximum activity of pepsin. In the ideal conditions, pepsin starts the process of protein digestion by acting on the peptide bonds of dietary protein and breaking them into smaller amino acid chains called peptides. The acidic contents of the stomach are neutralized in the small intestine.
Digestion of Protein
The breakdown of the smaller peptide chains into the individual amino acids for absorption takes place in the small intestine. Protein digestion enzymes, trypsin and chymotrypsin, are released by the pancreas into the small intestine. They break the longer peptides into chains of two amino acid units called dipeptides, three amino acid units called tripeptides or even six amino acid units. A final pancreatic enzyme, carboxypeptidases, breaks the dipeptides and tripeptides into single amino acid units, which are then absorbed by the cells of the small intestine.
Other Functions of HCl
Although digestion of proteins starts in the stomach, digestion of carbohydrates begins in the mouth by the action of salivary amylase, a carbohydrate-digesting enzyme produced by the salivary glands in the mouth. However, once the masticated food and enzyme mixture reach the stomach, the low pH in the stomach, caused by the secretion of hydrochloric acid, stops the activity of salivary amylase and, therefore, the digestion of carbohydrates. Hydrochloric acid also protects you from bacterial infection by killing the bacteria that may be present or swallowed along with the food.
References
- Encyclopedia Britannica: Human Digestive System: Digestion and Absorption of Specific Nutrients: Proteins
- University of Cincinnati Clermont College; Effects of Antacids on Pepsin; J. Stein Carter; 1998
- National Digestive Diseases Information Clearinghouse: Your Digestive System and How it Works
- Encyclopedia Britannica: Pepsin
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