Final answer:
Pepsin is active in the stomach, where it initiates protein digestion by breaking down protein into smaller polypeptides. The stomach's acidic environment, with a pH between 1.5 and 2.5, is essential for converting the inactive pepsinogen into active pepsin.
Step-by-step explanation:
Pepsin is an enzyme that plays a crucial role in protein digestion within the gastrointestinal (GI) tract. Specifically, pepsin is active in the stomach, where it begins the process of breaking down protein molecules. Pepsin is secreted by the chief cells in the stomach wall in the form of an inactive enzyme called pepsinogen. The highly acidic environment, maintained by hydrochloric acid (HCl) secreted by parietal cells, is essential for the conversion of pepsinogen to its active form, pepsin.
This acidic pH, which ranges from about 1.5 to 2.5, not only activates pepsin but also helps to denature food proteins, exposing the peptide bonds for efficient enzymatic action. Pepsin preferentially cleaves peptide linkages that involve aromatic amino acids such as tryptophan, tyrosine, and phenylalanine. This catabolic process results in the breakdown of protein into smaller polypeptides and amino acids. Later, as the partially digested food mixture known as chyme moves into the small intestine, protein digestion is completed.