Question

Action of salivary amylase stops when it mixes with gastric juice in stomach because ________ .

• A. digestion of starch completes in mouth
• B. amylase becomes inactive in pH 1.8
• C. amylase becomes inactive in alkaline medium
• D. maltose is digested by pepsin

Hint:

Enzyme rule: Each enzyme has an optimum pH. Salivary amylase works in neutral pH and gets inactivated in the acidic stomach environment.

Answer 1

The Correct Option is B

Concept:
Salivary amylase (ptyalin) is an enzyme that begins the digestion of starch in the mouth. Enzymes function optimally at specific pH levels, and drastic changes in pH can denature or inactivate them.

Step 1: Understand the working conditions of salivary amylase. Salivary amylase works best in a neutral to slightly alkaline pH (around 6.7–7.0) present in the oral cavity.

Step 2: Analyze what happens in the stomach. When food reaches the stomach, it mixes with gastric juice containing hydrochloric acid (HCl), which creates a highly acidic environment (pH $\approx$ 1.5–2). This acidic pH inactivates salivary amylase.

Step 3: Analyze the given options.
• (A) Digestion of starch completes in mouth → Incorrect, it continues in small intestine $\times$
• (B) Amylase becomes inactive in pH 1.8 → Correct, acidic pH denatures enzyme \checkmark
• (C) Amylase becomes inactive in alkaline medium → Incorrect, it works best in neutral/slightly alkaline $\times$
• (D) Maltose is digested by pepsin → Incorrect, pepsin digests proteins $\times$

Step 4: Conclusion. Thus, salivary amylase stops functioning in the stomach because: \[ \text{It becomes inactive in highly acidic pH (around 1.8)} \]