Stomach Acid

From Chempedia

The stomach, as a member of the digestive system (also known as the alimentary canal or gastrointestinal tract), has the assigned duty of breaking down food into a substance called chyme. The nutrients in chyme are readily absorbed by the intestinal lining. This process is efficiently accomplished by muscle contractions and the stomach’s gastric juice or gastric acid. Gastric acid is mainly composed of four components: hydrochloric acid (HCl), 2 enzymes (pepsin and rennin) and mucus. When food is masticated, a hormone, gastrin, is released from G cells triggering the secretion of a gastric acid containing the necessary enzymes (pepsin and rennin) for breaking down food in the stomach.

In the lining of the stomach are small glandular pits, called gastric pits, lined with one or more types of secretory epithelial cells. These cells include chief cells, parietal cells, mucous cells, and G cells; respectively, the secreted substances are pepsin and rennin, HCl and water, alkaline mucus, and the hormone gastrin. These gastric pits are located mainly in the fundus and body of the stomach.

Contents 1 Acid 2 Enzymes 3 Mucus 4 Why doesn’t the stomach digest itself? 5 Works Cited 6 Researched and written by

Acid

HCl is an acid that dissolves minerals in food and kills bacteria. It breaks down the proteins from the food by breaking bonds and unfolding the protein structures in preparation of denaturization by pepsin. Hydrogen and chlorine ions are released from the parietal cells into a canal where they combine to form HCl which drains into the lumen. Along with HCl, water is secreted to form a solution of about 0.1 M and with a pH of 1. Simultaneously, pepsinogen is being pumped out of the chief cells. Peptide, a pepsin inhibitor, remains attached to the pepsinogen molecules at pH values of 5.0 or more, preventing activation of the enzyme. When the HCl and water are released, the pH in the stomach drops to between 1 and 3(optimal is 1.8), thereby releasing the peptide and activating the pepsinogen, creating pepsin.

Enzymes

Pepsin converts proteins into simpler, more easily absorbed substances by breaking the peptide bonds between the amino acids. These broken down proteins are more accessible to the intestinal lining. Rennin is the other active enzyme in gastric juice. This enzyme is most active the first few days after birth and proceeds to reduce in importance as pepsin increases. Rennin, similarly to pepsin, is synthesized in an inactive form in chief cells, becomes active through exposure to HCl, and operates best under very acidic conditions. Rennin aids in the digestion of milk products, by clotting and curdling the milk, preventing it from flowing right through the stomach and skipping initial digestion of proteins. Once the food has been completely converted into chyme, it continues on to the duodenum, the beginning of the small intestine, for the next stages of digestion.

Mucus

Of the epithelial cells lining the lumen of the stomach, the majority are mucous glands. Mucus serves multiple purposes in the stomach: it acts as a barrier for the stomach lining against noxious acid and it also acts as a lubricant to minimize stresses. Mucus is a member of the Mucin family, and as such is a heavily glycosylated protein. A dense "sugar coating" composed of mainly O-linked oligosaccharides and some N-linked oligosaccharides bestows a considerable water-holding capacity and a resistance to proteolysis, important in sustaining the mucosal barrier.

Why doesn’t the stomach digest itself?

The gastric juice could potentially be quite harmful to the lining of the stomach were it not for the mucus secreting epithelial cells, which protect the stomach from the acid. To further protect the stomach cells, pepsin is synthesized in an inactive form, pepsinogen. Otherwise, the chief cells would be killed due to the destructive nature of pepsin. Epithelial cells are shed at a rate of 500,000 cells/minute, and are completely replaced every three days. If the epithelial cell lining is breached by the gastric acid, a peptic ulcer or other medical conditions could result.

Works Cited

Emanuel Rubin, "Ulcer", in AccessScience@McGraw-Hill, [1], DOI 10.1036/1097-8542.718800, last modified: April 10, 2000. accessed Sept. 20, 2005

J Biol Chem. 2003 Sep 19;278(38):36470-5. Epub 2003 Jul 14. Related Articles, Links

Parker, S. Food and Digestion. Ed. S. Parker. New York, NY: Franklin Watts Inc., 1990.

Role of ADP-ribosylation factor 6 (ARF6) in gastric acid secretion. Matsukawa J, Nakayama K, Nagao T, Ichijo H, Urushidani T. Laboratory of Cell Signaling, Graduate School of Pharmaceutical Sciences, The University of Tokyo, Tokyo 113-0033, Japan.

Simon, Seymour. Ed. Shannon Orr. 1st ed. New York, NY: HarperCollins Publishers, 2005.10.

"Stomach." The Concise Encyclopedia of the Human Body. First American Edition. 1995

Thomas S. Parsons, "Stomach", in AccessScience@McGraw-Hill, [2], DOI 10.1036/1097-8542.657200, last modified: April 10, 2000. accessed Sept. 20, 2005

All images provided by: [3] accessed on November 9, 2005.

Researched and written by

Lindsay Werkmeister, Tess Tibbs, Mike Tran, and David Yusefzadeh