Final answer:
The mouth contains saliva with salivary amylase for carbohydrate digestion, which is then inactivated in the stomach's acidic environment where proteins are digested by pepsin.
Step-by-step explanation:
The key chemicals that the mouth and stomach contain play vital roles in the process of digestion. In the mouth, saliva contains an enzyme known as salivary amylase, which begins the chemical breakdown of starches into simpler sugars like maltose. Salivary amylase requires a neutral pH and is aided by ions such as Ca++ and Cl¯. However, because food does not remain in the mouth for long, salivary amylase continues its action in the stomach until it is inactivated by the acidic environment.
The stomach has a highly acidic pH between 1.5 and 2.5, due to the secretion of hydrochloric acid (HCl), which is necessary for the activation of pepsinogen into pepsin, an enzyme that digests proteins. The acidic environment also provides protection against pathogens. Carbohydrate digestion ceases in the stomach as the necessary enzymes for this process are not found in gastric secretions. Instead, the breakdown of carbohydrates resumes and is completed in the small intestine.
Additionally, saliva also contains other compounds such as bicarbonate and phosphate ions that maintain its pH, immunoglobulin A for preventing microbial infection, lysozyme with antimicrobial properties, and epidermal growth factor.