The activity of ptyalin is likely to decrease with an increase in the concentration of

Analyze the question: Ptyalin is salivary amylase, an enzyme in saliva. The question asks which substance, when its concentration increases, is likely to lower ptyalin's activity.

Concept:

• Ptyalin digests starch into maltose.
• Like all enzymes, it works best at a particular pH. For ptyalin, the optimum is slightly acidic to neutral, about pH 6.8 to 7.
• Increasing the concentration of an acid lowers the pH. This changes the shape of the enzyme's active site (denaturation), so the enzyme works poorly or stops working. This is why ptyalin stops acting once food reaches the acidic stomach.

Going through the options:

• A. Oxygen: It has no direct effect on ptyalin activity.
• B. Starch: This is the substrate. More substrate usually increases the reaction rate until the enzyme is saturated. It does not decrease activity.
• C. Protein: It is not the substrate and does not inhibit ptyalin.
• D. Acid: It lowers the pH below the optimum and denatures the enzyme, so activity decreases.

Answer: D. Acid

Tip: Link each enzyme to its pH. Ptyalin works in a nearly neutral mouth, pepsin works in the acidic stomach, and trypsin works in the alkaline small intestine. If a question mentions acid or alkali, think about pH and enzyme denaturation.