Final answer:
The stomach secretes hydrochloric acid and the enzyme pepsin to begin protein digestion, breaking proteins into smaller polypeptides and amino acids.
Step-by-step explanation:
The stomach begins the breakdown of ingested proteins by secreting hydrochloric acid (HCl) and the protein-digesting enzyme pepsin. Pepsin is secreted in an inactive form known as pepsinogen, which is activated by HCl. This acidic environment denatures proteins, allowing pepsin to break them into smaller polypeptides and amino acids. These processes are essential for protein digestion, and as the chyme moves into the small intestine, other enzymes and digestive hormones like secretin and CCK continue the breakdown into individual amino acids that the body can absorb and use.