Final answer:
Hydrochloric acid converts pepsinogen into pepsin, the principal enzyme in the stomach for protein digestion. The gastric acidic environment is crucial for this conversion and the subsequent breakdown of proteins into smaller polypeptides.
Step-by-step explanation:
Hydrochloric acid is necessary in the stomach for the conversion of pepsinogen to pepsin (C).
The stomach is instrumental in the process of protein digestion, primarily utilising the enzyme pepsin. Pepsin is secreted by the chief cells in the stomach's gastric glands as an inactive enzyme known as pepsinogen. Once in the stomach, hydrochloric acid (HCl), which is secreted by parietal cells, creates a highly acidic environment with a pH ranging from 1.0 to 2.5. This acidic setting is crucial for two reasons; first, it helps to denature the protein molecules in food, making them more susceptible to enzymatic action. More importantly, it's essential for converting pepsinogen into its active form, pepsin. Pepsin then catalyzes the breakdown of proteins by cleaving peptide bonds, particularly those involving aromatic amino acids. After the initial protein digestion in the stomach, the resulting chyme (a mixture of partially digested food and gastric juice) continues into the small intestine, where digestion is completed by other enzymes.