Biological Membranes - A. N. Ogurtsov 2012

Electrogenesis of Biomembranes
Secondary Transport by Cotransporters
Gastric Acidity

The mammalian Stomach contains a 0.1 M solution of Hydrochloric acid (HCl). This high acidity kills many pathogenic microorganisms and promotes the Denaturation of numerous Proteins before they are processed by digestive Enzymes such as Pepsin (which, incidentally, is also activated by a decrease in pH). Hydrochloric acid is secreted into The Stomach by specialized epithelial Cells called parietal or oxyntic cells. These cells contain H+/K+-ATPases in their apical membranes, which are capable of maintaining a proton concentration in the gastric lumen that is a million times higher (pH = 1.0) than in The Cell Cytosol (pH = 7.0) (Figure 112).

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Figure 112 - Acidification of the gastric lumen by parietal Cells of the gastric mucosa

These transport proteins are P-type ion pumps, and structurally they are similar to the Na+/K+-ATPases of Plasma Membranes (Figure 66). The ATP molecules required for the operation of these pumps are synthesized by numerous Mitochondria in the parietal cells.

Since [H+]x[OH-] = 10-14 M2 is always a constant, the export of protons by parietal cells into the gastric lumen results in an excess concentration of OH- in the cytosol of these cells. This excess is eliminated by the action of the Cl-/HCO-3 antiporter on the basolateral membrane, which exports hydroxyl OH- ions from the cytosol as part of HCO-3 (recall that HCO-3 = CO2 + OH-). The reaction CO2 + OH- = HCO-3 is catalyzed by the enzyme Carbonic anhydrase.

The Cl-/HCO-3 antiporter removes excess OH- from the cytosol in exchange for Cl- ions, which then exit the cytosol into the lumen through Cl- channels in the apical membrane.

To maintain electroneutrality (avoiding electrostatic effects), a K+ counterion exits from the cytosol into the gastric lumen through a corresponding potassium channel simultaneously with each Cl- ion. The concentration of K+ ions in the cytosol is kept constant by H+/K+-ATPase (P-type) pumps located on the apical surface of the cell, which pump potassium ions into the cell and pump excess protons, remaining after the removal of hydroxyl ions, out of the cytosol into the gastric lumen.

As a result, equal amounts of Cl- and H+ ions (i.e., HCl) are exported from the cell into the gastric lumen, while the pH of the cell cytosol remains neutral, and the generated OH- anions are transported into the Blood in the form of HCO-3.

METABOLISM/35.html">Selection/41.html">Review Questions and Tasks

1. Which transport proteins are called cotransporters?

2. What are the Similarities and differences between cotransporters and symporters?

3. What are the similarities and differences between cotransporters and protein pumps?

4. What are the Two Types of cotransporters distinguished in Biomembranes?

5. What components contribute to the total Free energy gain during the import of one mole of sodium ions into the cell?

6. How does the 2-Na+/1-glucose symporter achieve a glucose concentration in the cell cytosol that is 30,000 times higher than outside the cell?

7. How do the 3-Na+/1-Ca2+ antiporters of the cardiomyocyte Plasma Membrane maintain a calcium concentration in the cell cytosol that is 10,000 times lower than outside the cell?

8. Which metabolic processes lead to an increase in the acidity of the cell cytosol?

9. Which two types of plasma membrane cotransporters reduce cytosolic acidity in animal cells?

10. In what form is the hydroxyl ion transported by the Na+HCO-3/Cl- antiporter?

11. Which cotransporter is used to remove excess hydroxyl ions from the cytosol of animal cells?

12. Which Three types of cotransporters maintain the physiological pH level in the cell cytosol?

13. Which transport proteins maintain the high acidity of The plant cell vacuole?

14. What is the energy source for The transport of sodium, calcium, and sucrose into the plant cell vacuole?

15. Which proteins mediate the transport of Water molecules across the membrane?

16. How many subunits make up a functional aquaporin?

17. What is The Structure of the aquaporin selectivity filter?

18. What are the two types of cell junctions?

19. What are the two possible transepithelial pathways for metabolite transport?

20. What do the apical and basolateral membranes of epithelial cells contact?

21. What constitutes the polarity of intestinal epithelial cells?

22. How is the surface area of the apical membrane significantly increased?

23. Which transporter proteins mediate the transcellular pathway of metabolite Transport from the intestinal lumen?

24. Why is nutrient Transport Across the intestinal epithelium accompanied by a flow of water molecules?

25. Which cotransporters maintain the high acidity of the gastric lumen?

26. Which transport proteins maintain potassium ion balance in the system that maintains gastric acidity?



Last update: 13/08/2026

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