Question

Using both H ⁺ ions and Cl ^- ions, why does food digests completely?

Answer 1

Gastric glands produce hydrochloric acid (HCl), enzymes, and intrinsic factor. HCl is responsible for the high acidity (pH 1.5 to 3.5) of the stomach contents. The acidity directly kills a lot of the bacteria we ingest with food and helps denature proteins and substances found in plants. In the presence of gastric juice’s low pH, the inactive enzyme, pepsinogen, is modified to become the active protein enzyme, pepsin. Protein enzymes of the digestive system are produced and secreted in an inactive form, to be activated only in the lumen, thus preventing digestion of the cells themselves. The glycoprotein intrinsic factor is necessary for the absorption of vitamin B12 later in the small intestine.

It is amazing that cells of the body can produce acids that normally would destroy the cell itself. Strong acids are also produced in cells in the immune system (to break down foreign organisms) and the skeletal system (to break down mineralized bone). High levels of acidity in the wrong places can be very destructive to living cells. Our body has several mechanisms to control pH.

In the stomach cell, the enzyme carbonic anhydrase converts one molecule of carbon dioxide and one molecule of water indirectly into a bicarbonate ion (HCO3-) and a hydrogen ion (H+). In the stomach ion exchange is used to move H+ ions out the cells and into the lumen of the stomach.

The bicarbonate ion (HCO3-) is then exchanged for a chloride ion (Cl-) on the basal side (away from the lumen) of the cell. Potassium (K+) and chloride (Cl-) ions diffuse into the secretory region of the cell called the canaliculi. Then the potassium is exchanged in this region for hydrogen ions via a H+/K+ ATPase. The hydrogen ions, which are millions of times more concentrated in this region of the cell than any other, are then pumped from the canaliculi into the lumen of the stomach.