Pepsin & Digestion

Pepsin is a protein-digesting enzyme (a chemical that breaks down proteins into smaller pieces).

Pepsin works only in an acidic medium (a condition where the pH is low, like in the stomach).
In the stomach, hydrochloric acid (HCl) makes the environment strongly acidic.
This acidic environment activates pepsin and allows it to break proteins into smaller peptides (small chains of amino acids).
When food leaves the stomach, it is called chyme (semi-digested food mixed with gastric juice).
This chyme enters the small intestine.
The small intestine does not have an acidic environment.
Instead, it has an alkaline medium (a condition where the pH is high, opposite of acidic).
Bile juice, produced by the liver, enters the small intestine.
Bile juice is alkaline in nature.
Its main function is to neutralise (cancel the effect of) the acid present in chyme.
Pancreatic juice is also released into the small intestine.
Pancreatic juice contains bicarbonate ions (alkaline substances that reduce acidity).
Because of bile and pancreatic juice, the pH in the small intestine becomes alkaline.
Pepsin cannot function in an alkaline medium.
Enzymes are very sensitive to pH (measure of acidity or alkalinity).
When the pH changes too much, the shape of the enzyme changes.
If the shape changes, the enzyme becomes inactive (unable to perform its function).
Therefore, pepsin stops working in the small intestine.
Protein digestion does not stop completely.
Another enzyme called trypsin takes over.
Trypsin is present in pancreatic juice.
Trypsin works best in an alkaline medium.
It continues breaking proteins into smaller peptides and amino acids (basic units of proteins).
Thus, pepsin works only in the stomach, and trypsin continues protein digestion in the small intestine.