Final answer:
Pepsin is an enzyme specialized for protein digestion in the stomach's acidic environment with an optimal pH of 1.5, which is not present in the mouth or small intestine, where enzymes operate at different pH levels.
Step-by-step explanation:
Why Pepsin Works Best in the Stomach
Pepsin is a digestive enzyme that is critical for the breakdown of proteins in our diet. It works optimally in the acidic environment of the stomach, which is characterized by a pH of about 1.5. In the mouth, the conditions are not as acidic, with saliva containing the enzyme amylase that functions at a pH of 6.9 for starch digestion.
The stomach secretes hydrochloric acid (HCl), creating the perfect environment for pepsin to activate and function. This acidic environment also helps to kill harmful microorganisms that may be present in food, providing a form of protection for the digestive tract.
Once the stomach contents enter the small intestine, the acidity is neutralized by bases like bicarbonate to accommodate the enzymes there, like trypsin, which require a more basic environment (optimum pH around 8) for effective protein digestion.
This is a stark contrast to the stomach's acidic milieu and thus demonstrates the precise pH adaptability of enzymes within the human digestive system. Consequently, pepsin does not work in the mouth or the small intestine because it requires a high level of acidity to maintain its structure and catalytic activity, which is only available in the stomach.