- Pepsin which digest protein donot digest the cells of intestine because-
(1) Intestine cells do not have proteins
(2) Half life of pepsin is very less
(3) Pepsin acts only in acidic pH
(4) Pepsin do not digest intestine proteins
Pepsin is a key digestive enzyme that initiates the breakdown of proteins in the stomach. It is secreted as an inactive precursor called pepsinogen by chief cells in the stomach and activated in the acidic environment of the stomach. Pepsin plays an essential role in protein digestion by cleaving peptide bonds in dietary proteins to produce smaller peptides. However, one interesting aspect about pepsin is that it does not digest the cells lining the small intestine, even though these cells contain proteins as well.
Why Does Pepsin Not Digest Intestinal Cells?
The primary reason pepsin does not digest the cells of the intestine lies in its requirement for an acidic environment to remain active. Pepsin’s enzymatic activity is highly dependent on the pH of its surroundings and shows optimal function at a pH between 1.5 and 2.5, typical of the stomach’s acidic gastric juice.
pH Sensitivity of Pepsin
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Active only in acidic pH: Pepsin is activated and functions optimally only in the acidic environment of the stomach. When the stomach contents, including pepsin, move into the duodenum and intestines, the pH changes significantly, becoming less acidic and more alkaline (pH around 6 to 7.5).
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Inactivation at neutral or alkaline pH: Once the pH rises above approximately 5.0 to 6.0, pepsin’s structure changes, and the enzyme becomes inactive. This loss of activity prevents it from digesting proteins in the intestinal walls or any other proteins present in the more neutral intestinal environment.
Additional Protections for Intestinal Cells
Besides pH-dependent inactivation, the small intestine also produces a protective mucus lining and bicarbonate secretions that help neutralize stomach acid, further raising the pH and protecting intestinal cells from acid and enzymatic digestion.
Furthermore, the intestinal lining is structurally designed to resist enzymatic digestion and rapidly renews itself to maintain integrity against digestive enzymes.
Other Considerations
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Half-life of pepsin: Pepsin has a relatively short half-life and is diluted as it moves through the digestive tract, further reducing its chances of affecting intestinal cells.
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Specificity: Although pepsin digests proteins, it has specificity for dietary proteins and does not target the host’s living cells actively.
Summary of the Mechanism
Factor Effect on Pepsin Activity/Intestine Protection Optimal pH (1.5-2.5) Pepsin active only in stomach acidic environment pH in intestines (~6-7.5) Pepsin becomes inactive, preventing intestinal protein digestion Mucus and bicarbonate secreted Neutralize acid, protect mucosal lining Short half-life Pepsin reduces rapidly as it moves towards intestines Structural resistance Intestinal epithelial cells resist enzymatic digestion Conclusion
Pepsin does not digest the cells of the intestine primarily because it requires an acidic pH to be active. The higher pH in the intestinal environment leads to inactivation of pepsin, protecting intestinal cells from enzymatic digestion. This ensures that pepsin’s proteolytic activity is confined to the stomach, where it initiates protein digestion optimally.
Therefore, the correct answer is:
(3) Pepsin acts only in acidic pH -
