Final answer:
Protein digestion primarily occurs in the small intestine, starting in the stomach with the enzyme pepsin and completed by enzymes like trypsin in the small intestine.
Step-by-step explanation:
The majority of protein digestion takes place in the small intestine, where final digestion occurs. This process begins in the stomach, where gastric juices, primarily consisting of hydrochloric acid (HCl) and the active enzyme pepsin, start to hydrolyze about 10% of peptide bonds. The enzyme pepsin is secreted by the chief cells in an inactive form known as pepsinogen, which is then converted to its active form in the stomach's acidic environment.
Once proteins are broken down into smaller polypeptides, they pass into the small intestine. Here, further digestion is facilitated by pancreatic enzymes such as trypsin and chymotrypsin, as well as enzymes secreted by the cells of the brush border like aminopeptidase and dipeptidase. This multi-step process efficiently reduces proteins into amino acids, which are then absorbed into the bloodstream.