Final answer:
Hydrochloric acid (HCl) produced by parietal cells in the stomach activates pepsin, kills bacteria, and denatures proteins, facilitating digestion.
Step-by-step explanation:
The main function of the hydrochloric acid (HCl) released by the parietal cells is to create a highly acidic environment in the stomach with a pH ranging from 1.5 to 3.5. This acidity is crucial for several reasons: it activates the protein-digesting enzyme pepsin from its inactive form pepsinogen, kills ingested bacteria, and helps to denature proteins, which unwinds protein molecules making them more susceptible to enzymatic digestion. The production of HCl in the stomach is also important for maintaining nerve function and osmotic balance within the body.