Final answer:
Pepsin is the enzyme that digests proteins in the stomach's acidic environment, optimally functioning at a pH of around 1.5. Hydrochloric acid in the stomach activates pepsin and helps maintain the necessary acidic conditions. In the small intestine, the acidic contents are neutralized to allow other enzymes like trypsin to continue digestion in an alkaline environment.
Step-by-step explanation:
The enzyme pepsin is crucial for protein digestion within the stomach's highly acidic environment. Pepsin is secreted by the chief cells in the form of pepsinogen and becomes active when it comes in contact with hydrochloric acid (HCl), which is released by parietal cells in the stomach. This acidic milieu, with a pH of around 1.5, is optimal for pepsin's activity, allowing it to cleave proteins into smaller polypeptides effectively. As these proteins are broken down into peptides, they are further digested into amino acids by other enzymes, like peptidases, in the small intestine. The transition from the stomach's acidic environment to the alkaline conditions in the small intestine (pH of about 8) involves the neutralization of acids by bicarbonate, which is secreted by the pancreas. This ensures that other digestive enzymes, such as trypsin, can continue protein digestion under their optimal alkaline conditions.