Describe protein digestion in the stomach.

Protein digestion begins in the stomach. The stomach secretes hydrochloric acid (HCl), which denatures proteins, unfolding their complex structures. HCl also activates the inactive enzyme pepsinogen into its active form, pepsin. Pepsin then hydrolyzes proteins into smaller polypeptides.

As the chyme (partially digested food) moves into the small intestine, the pancreas releases proteolytic enzymes such as trypsin and chymotrypsin (secreted as inactive zymogens, trypsinogen and chymotrypsinogen, and activated in the small intestine). These enzymes further break down polypeptides into smaller peptides. The brush border of the small intestine contains peptidases (e.g., aminopeptidases, dipeptidases) that cleave the remaining small peptides into individual amino acids.

The amino acids are then absorbed by the intestinal cells and transported into the bloodstream.