Final answer:
Saliva can break down acidic foods such as lemons because it contains the enzyme amylase, which operates optimally at a slightly basic pH that matches the pH of saliva. The capability of enzymes to function at different pH levels allows for the digestion of substances with various acidity or basicity.
Step-by-step explanation:
The question posed by the student is related to the breakdown of acidic foods by saliva, which is slightly basic with a pH of 7.4. This is a chemistry-related question because it involves understanding pH levels and how enzymes in saliva function. The answer to the multiple-choice component of the question is not included in the provided information, and it does not directly contribute to understanding the underlying biochemical process, so we should focus on the explanation of pH and enzyme activity.
Human saliva contains the enzyme amylase, which operates optimally at a pH range of 7-7.5. Enzymes like amylase can break down foods irrespective of their pH because they are designed to work at specific pH levels that are optimal for their function. Acidic foods like lemons with a pH between 2 and 3 can indeed be broken down by saliva because amylase will break down starches within the food, and this activity is not impeded by the acidic nature of lemons.
The breakdown of food by saliva is an example of how organisms adjust to ingesting substances with varying pH levels. The pH scale is an inverse logarithm, and a pH below 7.0 is acidic, while a pH above 7.0 is basic. Enzymes in the human body have adapted to work at different pH levels found in various parts of the digestive system.