Final answer:
The stomach secretes pepsin to begin the chemical breakdown of proteins, converting the food into a mixture called chyme. Pepsin is crucial for cleaving proteins into smaller polypeptides and stimulating further pepsin activation for digestion.
Step-by-step explanation:
The stomach continues the physical breakdown of food initiated by the mouth and commences the chemical breakdown of proteins by secreting an enzyme known as pepsin. The highly acidic environment, with a pH created primarily by hydrochloric acid (HCl), kills many microorganisms present in the food and, together with the action of pepsin, breaks down proteins into smaller polypeptides. This digestion process transforms the food into a semi-fluid mixture called chyme, which is subsequently moved into the small intestine for further digestion.
Pepsin's role is crucial as it sets off a chain reaction that not only cleaves proteins but also activates more pepsinogen, the inactive form of pepsin, into active pepsin, thereby accelerating the digestion process. After a meal, it typically takes about two to six hours for the stomach to empty its contents into the small intestine, which is facilitated by the pyloric sphincter. The digestive activity continues with the aid of other enzymes in the small intestine such as trypsin and chymotrypsin, which contribute to breaking down proteins into individual amino acids that can be absorbed in the bloodstream.