Protein digestion begins in the:

Protein digestion begins in the stomach primarily due to the presence of gastric juices that include hydrochloric acid and the enzyme pepsin. Once food enters the stomach, the acidic environment activates pepsinogen, which is secreted by the stomach lining and transforms into pepsin. This enzyme initiates the breakdown of proteins into smaller peptides.

Though some minor digestion of proteins can occur in the mouth as a result of mechanical breakdown, the enzymatic action that significantly contributes to protein digestion starts in the stomach. In the small intestine, further digestion occurs with the help of pancreatic enzymes such as trypsin and chymotrypsin, which continue to break down the peptide chains into even smaller units, but the initial process starts in the stomach itself. Therefore, identifying the stomach as the location where protein digestion begins aligns with the physiological processes involved in digestion.