The protein-digesting enzyme secreted by the stomach wall in case of mammals is called
- (a)chitinase
- (b)amylase
- (c)pepsin
- (d)trypsin
Correct — C, pepsin. Gastric chief cells in the wall of the stomach release an inactive precursor called pepsinogen, and the hydrochloric acid of the gastric juice converts it into active pepsin. Pepsin is an endopeptidase — it cuts long protein chains into shorter peptides rather than finishing the job down to amino acids — and it is most active in the strongly acidic range of about pH 1·5 to 2·5, which is exactly the environment only the stomach provides. Both halves of the question are satisfied only by pepsin: it digests protein, and it is secreted by the stomach wall itself.
- (a)chitinase — Chitinase breaks down chitin, which is a carbohydrate — the tough polysaccharide of fungal cell walls and of insect and crustacean shells. Whatever else it does, it is not a protein-digesting enzyme, so it cannot answer this question.
- (b)amylase — Amylase is a carbohydrate enzyme, not a protease — it splits starch into maltose. In mammals it comes from the salivary glands (ptyalin) and from the pancreas, and the salivary form is in fact switched off by the stomach's acid rather than being made there.
- (d)trypsin — Trypsin is a protease, which makes it the sharpest trap here, but it is a pancreatic enzyme, not a gastric one. The pancreas secretes it as inactive trypsinogen into the duodenum, where enterokinase activates it, and it works in the alkaline intestinal medium, not in stomach acid.
Chemical digestion in mammals is organised by compartment: each region of the alimentary canal has its own enzymes, its own pH and its own class of food molecule to attack. The stomach is the protein compartment. Its gastric glands secrete hydrochloric acid, mucus and pepsinogen; the acid both sterilises the food and converts pepsinogen into pepsin. Secreting the enzyme in an inactive form is a safety device — an active protease stored inside the cells that made it would digest those cells.
Two words in the stem do all the work — 'protein-digesting' and 'secreted by the stomach wall'. Amylase and chitinase fail the first test because they act on carbohydrates. Trypsin passes the first test but fails the second, and that is where most candidates lose the mark, because trypsin is drilled alongside pepsin as a protease and the site of secretion is the only thing separating them. Fix the pairing once — pepsin from the stomach, trypsin and chymotrypsin from the pancreas acting in the small intestine — and the whole family of questions on digestive enzymes becomes routine.
- Pepsin is secreted by gastric chief cells as inactive pepsinogen and is activated by the hydrochloric acid of gastric juice.
- Pepsin works best in an acidic medium of roughly pH 1·5 to 2·5, and it is an endopeptidase that yields peptides rather than free amino acids.
- Trypsin and chymotrypsin are pancreatic proteases, secreted as zymogens and activated in the duodenum.
- Amylase acts on starch, not protein — salivary amylase (ptyalin) in the mouth and pancreatic amylase in the small intestine.
- Theodor Schwann described pepsin in 1836 and named it from the Greek pepsis, meaning digestion.
Only one enzyme is both a protease and a secretion of the stomach wall — pepsin.
- Picking trypsin because it is a protease, without checking that the question asks for the stomach's secretion.
- Assuming salivary amylase keeps working in the stomach — the acid inactivates it.
- Confusing rennin, a milk-clotting gastric enzyme in young mammals, with renin, the kidney enzyme of blood-pressure control.
As a straight one-liner like this, as a match-the-enzyme-to-its-substrate list, or as an optimum-pH comparison between pepsin and trypsin.
Which of the following is not a digestive enzyme in the human system?
- (a) Trypsin
- (b) Gastrin
- (c) Ptyalin
- (d) Pepsin
Answer(b) Gastrin
The same cast of enzymes, sorted a different way. Trypsin, ptyalin and pepsin are all digestive enzymes; gastrin is a hormone that merely stimulates acid and pepsin secretion. Knowing which secretion is an enzyme and which is a hormone is the companion skill to knowing where each enzyme is made.
Match List I (Substance) with List II (Physiological role) and select the correct answer using the codes given below the Lists: I. Ptyalin II. Pepsin III. Renin IV. Oxytocin — A) Converts angiotensinogen in blood into angiotensin B) Digests starch C) Digests proteins D) Hydrolyses fats E) Induces contraction of smooth muscles
- (a) I-B, II-C, III-A, IV-E
- (b) I-C, II-D, III-B, IV-E
- (c) I-B, II-C, III-E, IV-A
- (d) I-C, II-A, III-B, IV-D
Answer(a) I-B, II-C, III-A, IV-E
Pepsin is pinned to protein digestion here exactly as in this NDA item, and the list also plants renin — the kidney enzyme of blood-pressure control — next to the gastric enzymes, which is the classic spelling trap alongside rennin.
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
Answer(a) pH 2·0 and 7·9, respectively
The same pepsin-versus-trypsin distinction, asked from the pH side instead of the site-of-secretion side. Together the two items make the point that the stomach protease and the pancreatic protease live in opposite chemical worlds.
- practice — not a real PYQ
Pepsinogen secreted in the stomach is converted into active pepsin by
- (a)bile salts
- (b)hydrochloric acid
- (c)enterokinase
- (d)sodium bicarbonate
Answer(b) hydrochloric acid — the acid of gastric juice removes part of the pepsinogen molecule and unmasks the active enzyme.
- practice — not a real PYQ
Which one of the following enzymes is NOT secreted by the pancreas?
- (a)Trypsin
- (b)Chymotrypsin
- (c)Pepsin
- (d)Lipase
Answer(c) Pepsin — it comes from the gastric glands of the stomach wall, while the other three are pancreatic.