Biology, Eighth Edition (Raven)

Chapter 48: The Digestive System

Hydrochloric Acid Production... of the Stomach

How can digestive tissue produce a concentrated acid without damage to cells and molecules exposed to the acid? Tissues associated with the stomach produce not only digestive enzymes but also hydrochloric acid. One type of cell, the parietal cell, secretes HCl which can have a pH as low as 1.5. The HCl helps to chemically break down the food in the stomach. The digestive enzymes have evolved a molecular structure that functions well in a very acid environment. The cells lining the stomach and associated ducts are coated with a protective mucus. The cells are also replaced at a rapid rate.

View the animation below, then complete the quiz to test your knowledge of the concept.


In the parietal cells, the enzyme carbonic anhydrase causes a reaction between...
A)carbon dioxide and water.
B)carbonic acid and water.
C)bicarbonate ions and hydrogen ions.
D)hydrogen ions and potassium ions.
E)hydrogen ions and chloride ions.

Ion exchange molecules in the plasma membrane of the parietal cells...
A)exchange a hydrogen ion coming in for a potassium ion going out.
B)exchange bicarbonate ions going out for hydrogen ions coming in.
C)exchange chloride ions going out for potassium ions coming in.
D)exchange hydrogen ions going out for chloride ions coming in.
E)exchange bicarbonate ions going out for chloride ions coming in.

Hydrochloric acid is formed when...
A)hydrogen ions and chloride ions join in the parietal cell cytoplasm.
B)hydrogen ions and chloride ions join in the parietal cell nucleus.
C)hydrogen ions and chloride ions join in the stomach lumen.
D)hydrogen ions and chloride ions join in the duct of the gastric gland.

In the parietal cell, carbonic acid dissociates into a bicarbonate ion and a hydrogen ion.

The carbonic acid ion is then joined to a hydrogen ion by the enzyme carbonic anhydrase.
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