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 — A, pH 2·0 and 7·9. Pepsin is the protein-digesting enzyme of the stomach, where hydrochloric acid keeps the contents strongly acidic, so pepsin is active at a very low pH of about 2. Trypsin works further down, in the small intestine, whose contents are made alkaline by pancreatic juice and bile, so its best activity is near pH 8. The pairing 'about 2 for pepsin, about 8 for trypsin' is exactly what option (a) gives.
- (b)pH 4·5 and 3·5, respectively — Both values are mildly acidic. Pepsin needs a far more acidic environment (about pH 2), and trypsin needs an alkaline one (about pH 8), so neither figure fits.
- (c)pH 9·5 and 4·5, respectively — This reverses the chemistry: it makes pepsin work in an alkaline medium and trypsin in an acidic one, the opposite of the stomach-versus-intestine reality.
- (d)pH 7·0 and 8·0, respectively — Trypsin's value (about 8) is right, but pepsin at neutral pH 7 is wrong — pepsin is virtually inactive at neutral pH and needs the acidic stomach (about pH 2).
Every enzyme works fastest within a narrow band of pH called its optimum, set by the environment in which it normally acts. Pepsin, secreted in the stomach, is an acid-loving protease with an optimum near pH 2; trypsin, delivered by pancreatic juice into the small intestine, is an alkali-loving protease with an optimum near pH 8.
The question tests whether you can match each digestive enzyme to the acidity of its home organ. The stomach is acidic (hydrochloric acid), the small intestine is alkaline (bicarbonate from pancreas and bile), so the correct answer must pair a low pH with pepsin and a high pH with trypsin. Options that make both values acidic, or that swap acid and alkali, can be eliminated on that logic alone.
- Pepsin acts in the stomach and has an optimum pH of about 2 (strongly acidic).
- Trypsin acts in the small intestine and has an optimum pH of about 8 (alkaline).
- Stomach acidity comes from hydrochloric acid; intestinal alkalinity from pancreatic bicarbonate and bile.
- Both pepsin and trypsin digest proteins, but in opposite pH environments.
Pepsin is acid-loving (pH ~2), trypsin is alkali-loving (pH ~8); only option (a) matches both.
- Pepsin needs a very acidic pH (about 2), not a neutral or alkaline one — do not pick a near-7 value for it.
- Trypsin works in the alkaline small intestine, so its optimum is above 7, not below it.
Asked to state the optimum pH of a named digestive enzyme, or to match enzymes with the acidic or alkaline part of the gut where they act.
Assertion (A): All the proteins in our food are digested in small intestine only. Reason (R): The protein-digesting enzymes from pancreas are released into small intestine.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is not the correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer(d) A is false but R is true — protein digestion begins in the stomach with pepsin and is completed in the small intestine by pancreatic proteases such as trypsin, so it is not done in the small intestine 'only'.
Rests on the same pepsin-in-the-stomach versus trypsin-in-the-intestine split that Q133 tests through their optimum pH values.
In the human digestive system, which one among the following is the role of the pancreas ?
- (a) Secretion of surfactants to break up lipid droplets
- (b) Storage and regulated release of bile
- (c) Secretion of lipase, amylase and protease
- (d) Neutralizing stomach acids
Answer(c) Secretion of lipase, amylase and protease
Same digestive-enzyme theme — it asks for the role of the pancreas, whose juice supplies trypsin (and lipase, amylase) to the alkaline small intestine, the very enzyme paired with pepsin in Q133. Answer: (c) secretion of lipase, amylase and protease.
- practice — not a real PYQ
The protein-digesting enzyme pepsin is most active in which part of the human alimentary canal?
- (a)Mouth
- (b)Stomach
- (c)Small intestine
- (d)Large intestine
Answer(b) Stomach — pepsin needs the acidic medium (about pH 2) created by hydrochloric acid in the stomach.
- practice — not a real PYQ
The optimum pH for the activity of the enzyme trypsin in humans is closest to
- (a)pH 2
- (b)pH 5
- (c)pH 8
- (d)pH 11
Answer(c) pH 8 — trypsin acts in the alkaline small intestine, so its optimum is around pH 8.