The optimum pH at which the enzymes pepsin and trypsin of the human alimentary canal works are
- A.pH 2.0 and 7.9, respectively
- B.pH 4.5 and 3.5, respectively
- C.pH 9.5 and 4.5, respectively
- D.pH 7.0 and 8.0, respectively
Correct answer
Explanation
The correct answer is A, pH 2.0 and 7.9, respectively. Pepsin works in the strongly acidic stomach, while trypsin works in the mildly alkaline small intestine.
Pepsin is secreted by the chief cells of the stomach as inactive pepsinogen, which hydrochloric acid from the parietal cells turns into pepsin. It breaks proteins into peptides best at a pH of about 2. Trypsin comes from the pancreas as trypsinogen and is activated in the duodenum by the enzyme enterokinase. Bile and pancreatic juice, rich in bicarbonate, make the intestine slightly alkaline, and trypsin works best at a pH of about 7.6 to 8. Each enzyme loses its activity away from its own pH range.
B is wrong because a pH of 3.5 would be far too acidic for trypsin. C is wrong because pepsin stops working in alkaline conditions such as pH 9.5. D is wrong because a neutral pH of 7.0 is too high for pepsin, which needs strong acid.
Exam tip: pepsin in the stomach at pH about 2; trypsin from the pancreas in the intestine at pH about 8; enterokinase activates trypsinogen.
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