Which of the following pH values is the best for the action of the enzymes - renin and pepsin in the stomach?

Analyze the question: The question asks for the pH at which the stomach enzymes renin and pepsin work best.

Explain the concept: Every enzyme has an optimum pH, the pH at which it works fastest. This depends on where the enzyme works:

• Stomach: The stomach secretes hydrochloric acid, so the environment is strongly acidic (about pH 1.5-2.5). Pepsin and renin are adapted to this.
• Mouth (salivary amylase): about pH 6.5-7 (slightly acidic to neutral).
• Small intestine (trypsin, lipase, pancreatic amylase): about pH 8-9 (alkaline).

Solution:

1. Pepsin breaks down proteins into peptides, and renin curdles milk protein (casein). Both work in the stomach.
2. The stomach contains HCl, so the pH is very low (acidic).
3. Compare the options: • A. pH 2 is strongly acidic, which matches the stomach. • B. pH 7 is neutral, which is more suited to saliva. • C. pH 8 is alkaline, which suits the small intestine. • D. pH 9 is more strongly alkaline, so it is also not suited to the stomach.

At alkaline or neutral pH, pepsin would work poorly or become inactive.

Answer: A. pH 2

(A small note: in humans, renin is mostly found in infants, and in adults it is largely replaced by pepsin. Either way, the stomach's acidic conditions are what matter here. Some textbooks also call this enzyme "rennin.")

Exam tip: Remember the pattern "Stomach = Sour = Small pH number." Stomach enzymes like acid, and intestinal enzymes like alkali. If you see "stomach" in a question, pick the most acidic pH option.