Which statement best explains why proteins are not chemically digested in the mouth but begin to be chemically digested in the stomach?

• A. Because the mouth secretes strong bases that neutralize proteins making them resistant to enzymatic action until they reach the acidic stomach where pepsin hydrolyzes peptide bonds.
• B. Salivary glands produce abundant proteases, but the mechanical action of chewing inhibits those enzymes so chemical digestion is delayed until stomach enzymes take over.
• C. Although chewing disrupts protein tertiary structure and some minor chemical hydrolysis may occur, salivary secretions contain protease precursors that only become active after swallowing due to temperature and pH changes in the stomach that allow pepsinogen conversion and protein denaturation.
• D. There is no active protease in saliva and oral pH does not denature proteins, so only mechanical breakdown occurs until acidic stomach conditions activate pepsin.

Answer: (D)