Editing Parietal cell (section)

===Hydrochloric acid secretion===
Hydrochloric acid is formed in the following manner:
* Hydrogen ions are formed from the dissociation of carbonic acid. Water is a very minor source of hydrogen ions in comparison to carbonic acid. Carbonic acid is formed from [[carbon dioxide]] and water by [[carbonic anhydrase]].
* The bicarbonate ion (HCO<sub>3</sub><sup>−</sup>) is exchanged for a chloride ion (Cl<sup>−</sup>) on the basal side of the cell and the bicarbonate diffuses into the venous blood, leading to an [[alkaline tide]] phenomenon.
* [[Potassium]] (K<sup>+</sup>) and chloride (Cl<sup>−</sup>) ions diffuse into the [[Parietal cell#Canaliculus|canaliculi]].
* Hydrogen ions are pumped out of the cell into the canaliculi in exchange for potassium ions, via the [[hydrogen potassium ATPase|H<sup>+</sup>/K<sup>+</sup>-ATPase]]. These pumps are increased in number on luminal side by fusion of tubulovesicles during activation of parietal cells and removed during deactivation. This pump maintains a million-fold difference in proton concentration.<ref>{{cite journal |last1=Abe |first1=Kazuhiro |last2=Irie |first2=Katsumasa |last3=Nakanishi |first3=Hanayo |last4=Suzuki |first4=Hiroshi |last5=Fujiyoshi |first5=Yoshinori |title=Crystal structures of the gastric proton pump |journal=Nature |date=April 2018 |volume=556 |issue=7700 |pages=214–218 |doi=10.1038/s41586-018-0003-8}}</ref> ATP is provided by the numerous mitochondria.

[[File:Parietal cells.jpg|thumb|Human parietal cells (pink staining) – stomach.]]
As a result of the cellular export of hydrogen ions, the gastric lumen is maintained as a highly acidic environment. The acidity aids in digestion of food by promoting the unfolding (or [[Denaturation (biochemistry)|denaturing]]) of ingested [[protein]]s.  As proteins unfold, the [[peptide bonds]] linking component [[amino acids]] are exposed.  Gastric HCl simultaneously cleaves [[pepsinogen]], a [[zymogen]], into active [[pepsin]], an endopeptidase that advances the digestive process by breaking the now-exposed peptide bonds, a process known as [[proteolysis]].