MPSC 2021 Prelims Q62: Which gastric juices are secreted in human stomach

MPSC 2021 Prelims Paper-I · Question 62 of 100

State Services Preliminary Examination 2021, held 23 January 2022. Transcribed from the commission's own booklet (संच A); answer as per the official MPSC final answer key.

Which gastric juices are secreted in human stomach ?

(1)Pepsin and Trypsin
(2)Trypsin and HCl
(3)HCl and Pepsin
(4)Amylopsin and Pepsin

Answer

(3)

Why

Correct — option (3), HCl and Pepsin. Gastric juice is the secretion of the gastric glands in the wall of the stomach, and the question asks only which of the named substances are produced there rather than which act on food at some later stage. Three kinds of cell in the gastric glands do the work. The parietal or oxyntic cells secrete hydrochloric acid, which the English options print as HCl while the Marathi column spells the name out in full; they also secrete intrinsic factor, the protein without which vitamin B12 cannot be absorbed further down the gut. The chief or peptic cells secrete pepsinogen, the inactive precursor of the protein-splitting enzyme pepsin. The mucous neck cells secrete the alkaline mucus that lines the stomach wall and keeps the acid from digesting the stomach itself. So the two substances that belong together in the stomach are exactly the two the keyed option pairs, and the reason they belong together is that neither works without the other. Pepsinogen is secreted in an inactive form precisely so that it cannot digest the cell that makes it, and it is the hydrochloric acid that snips it into active pepsin; once a little pepsin exists it activates more pepsinogen itself, so the process is autocatalytic. The acid does three further things. It brings the contents of the stomach to a pH of roughly 1.5 to 3.5, which is the range in which pepsin is active at all — an enzyme that would be destroyed in the alkaline conditions of the small intestine. It kills most of the bacteria swallowed with food. And it denatures the proteins of the meal, unfolding them so that pepsin can reach the peptide bonds inside them. Pepsin then breaks those long protein chains into shorter fragments, the proteoses and peptones, which pass on to the small intestine for the pancreatic and intestinal enzymes to finish. Gastric juice contains a little else besides — gastric lipase, which makes a small start on fats, and rennin, which curdles milk protein and is important in infants — but hydrochloric acid and pepsin are the two that define it. Everything else in the option set is manufactured somewhere other than the stomach.

Why the others are wrong

(1)Pepsin and Trypsin — Pepsin does belong to the stomach, but trypsin does not, and pairing them is the commonest error on this question because both are proteases and are learnt in the same breath. Trypsin is a pancreatic enzyme. It is secreted by the acinar cells of the pancreas as the inactive trypsinogen, travels down the pancreatic duct into the duodenum, and is activated there by enterokinase, also called enteropeptidase, an enzyme of the intestinal mucosa; the trypsin so formed then activates chymotrypsinogen and the other pancreatic zymogens in turn. It also works in the opposite chemical environment from pepsin: the pancreatic juice is alkaline and neutralises the acid arriving from the stomach, and trypsin needs that alkaline medium, while pepsin would be destroyed by it. Site of production and pH optimum both separate the two.
(2)Trypsin and HCl — Half right and half wrong, which is what makes it attractive. Hydrochloric acid is indeed a gastric secretion, from the parietal cells, but trypsin is not made in the stomach at all — it is the pancreatic protease, secreted as trypsinogen and activated by enterokinase in the duodenum. The option also asks the candidate to believe something chemically impossible: trypsin's optimum is the alkaline pH of the small intestine, so an enzyme sitting in a medium of pH 1.5 to 3.5 alongside hydrochloric acid would simply be inactivated. Whenever an option puts a pancreatic enzyme in the same list as gastric acid, that internal contradiction is the quickest way to reject it.
(4)Amylopsin and Pepsin — Amylopsin is the older name for pancreatic amylase, the starch-splitting enzyme of the pancreatic juice, which acts on starch in the small intestine and converts it to maltose. It is not a gastric secretion, and no carbohydrate-digesting enzyme is secreted by the human stomach at all — starch digestion begins in the mouth with salivary amylase, or ptyalin, and is halted by the acid of the stomach before resuming in the intestine. Pepsin is correctly placed in this option, but its partner is not, so the pair fails. The trio of old names is worth fixing once and for all: ptyalin is salivary amylase, amylopsin is pancreatic amylase, steapsin is pancreatic lipase, and all three of those belong outside the stomach.

Concept

Digestion in the human alimentary canal proceeds region by region, and each region has its own secretion, its own enzymes and its own pH. In the mouth the salivary glands supply saliva containing salivary amylase, or ptyalin, which begins the breakdown of starch to maltose in a slightly acidic to neutral medium, and lysozyme, which is antibacterial. In the stomach the gastric glands supply gastric juice — hydrochloric acid from the parietal or oxyntic cells, pepsinogen from the chief or peptic cells, mucus from the mucous neck cells, and intrinsic factor, also from the parietal cells, for the later absorption of vitamin B12. The acid activates pepsinogen to pepsin, provides the strongly acidic pH of about 1.5 to 3.5 at which pepsin works, denatures dietary protein and kills swallowed bacteria; pepsin then cleaves proteins into proteoses and peptones. Gastric lipase makes a small start on fats and rennin curdles milk protein in infants. In the duodenum, pancreatic juice arrives with trypsinogen and chymotrypsinogen, which are activated to trypsin and chymotrypsin by enterokinase from the intestinal mucosa, with pancreatic amylase or amylopsin for starch, and with pancreatic lipase or steapsin for fats; bile from the liver, stored in the gall bladder, emulsifies fat and neutralises the acid, and pancreatic juice is itself alkaline. The intestinal juice and the brush-border enzymes — maltase, sucrase, lactase, peptidases — finish the job, and absorption takes place through the villi of the small intestine.

The human digestive system is one of the most reliably examined areas of MPSC's biology section, because a single well-made table of secretions answers a large family of questions: which gland makes what, which enzyme acts on which substrate, what each substrate becomes, and in which part of the canal each step occurs. The Commission's favourite device is exactly the one used here — offering an enzyme from the right region paired with an enzyme from the wrong one, so that a candidate who remembers that pepsin is gastric will still be caught if he has not fixed where trypsin comes from. The second favourite device is the older nomenclature: ptyalin, amylopsin and steapsin sit in Indian school textbooks alongside the modern names, and a question can be made hard simply by printing the old name. The habit worth building is to store every digestive enzyme with three attributes rather than one — where it is secreted, what it acts on, and in what pH — since all three are examinable and the third often exposes an impossible pairing by itself.

Key facts

Gastric juice is secreted by the gastric glands of the stomach and its principal constituents are hydrochloric acid from the parietal or oxyntic cells, pepsinogen from the chief or peptic cells, mucus from the mucous neck cells, and intrinsic factor for the absorption of vitamin B12.
Pepsinogen is inactive when secreted and is converted to the active enzyme pepsin by hydrochloric acid; once formed, pepsin activates further pepsinogen itself, and it digests proteins into proteoses and peptones.
Hydrochloric acid maintains a gastric pH of roughly 1.5 to 3.5, which is the range in which pepsin is active, denatures dietary proteins so that pepsin can reach their peptide bonds, and kills most bacteria swallowed with food.
Trypsin is a pancreatic enzyme, secreted as trypsinogen and activated in the duodenum by enterokinase (enteropeptidase); it works in the alkaline medium of pancreatic juice, and it in turn activates the other pancreatic zymogens.
The older names commonly printed in question papers are ptyalin for salivary amylase, amylopsin for pancreatic amylase and steapsin for pancreatic lipase — none of the three is a gastric secretion, and no carbohydrate-digesting enzyme is secreted by the human stomach.

Place every substance in the option set before you accept a single pair — the stem asks what the STOMACH secretes, not what digests the meal later on

Pepsin — secreted as pepsinogen by the chief or peptic cells→STOMACH. It is secreted inactive precisely so that it cannot digest the cell that made it, and it is the hydrochloric acid alongside it that snips pepsinogen into active pepsin; once a little pepsin exists it activates more pepsinogen itself. It then cleaves proteins into the shorter proteoses and peptones

Hydrochloric acid — secreted by the parietal or oxyntic cells→STOMACH. It holds the contents at a pH of roughly 1.5 to 3.5, which is the only range in which pepsin is active at all; it denatures dietary protein so that pepsin can reach the peptide bonds inside it; and it kills most bacteria swallowed with the food. The same cells also secrete intrinsic factor, without which vitamin B12 cannot be absorbed further down the gut

Trypsin — secreted as trypsinogen by the acinar cells of the pancreas→PANCREAS, and settling this one row disposes of two of the four printed pairs. Trypsinogen travels down the pancreatic duct into the duodenum and is activated there by enterokinase, and the trypsin so formed activates the other pancreatic zymogens in turn. It needs the alkaline medium of pancreatic juice, so an option that seats it beside hydrochloric acid at pH 1.5 contradicts itself before any recall is needed

Amylopsin — the older textbook name for pancreatic amylase→PANCREAS. It acts on starch in the small intestine and converts it to maltose. No carbohydrate-splitting enzyme is secreted by the human stomach at all: starch digestion begins in the mouth with salivary amylase and is halted by the gastric acid before resuming in the intestine. Fix the three old names once and for all — ptyalin is salivary amylase, amylopsin is pancreatic amylase, steapsin is pancreatic lipase, and every one of them belongs outside the stomach

Two of the four named substances are gastric and two are pancreatic, so exactly one printed pair puts both of its members in the stomach, and that is option (3). Gastric juice carries a little else besides — gastric lipase, which makes a small start on fats, and rennin, which curdles milk protein and matters in infants — but hydrochloric acid and pepsin are the two that define it. The habit worth building is to store every digestive enzyme with three attributes rather than one: where it is secreted, what it acts on, and at what pH. All three are examinable, and the third exposes an impossible pairing by itself.

Study next

The complete table of digestive secretions — gland, enzyme, substrate, product and pH — from the salivary glands to the intestinal brush borderThe structure of the human alimentary canal and the associated glands, with the function of the liver, gall bladder and pancreasAbsorption of the digested products through the villi, and the special route taken by fats through the lacteals and the lymphatic systemVitamins and deficiency diseases, including the link between intrinsic factor, vitamin B12 and pernicious anaemiaHormonal control of digestion — gastrin, secretin and cholecystokinin — and why a hormone is not the same thing as an enzyme

Common traps

Pairing pepsin with trypsin because both are proteases, when one is gastric and the other pancreatic and they work at opposite ends of the pH scale
Forgetting that the older textbook names ptyalin, amylopsin and steapsin denote salivary amylase, pancreatic amylase and pancreatic lipase respectively
Treating gastrin as a gastric enzyme when it is a hormone secreted by the G cells of the stomach that stimulates the secretion of acid
Assuming that starch digestion continues in the stomach, when the acid halts salivary amylase and no carbohydrate-splitting enzyme is secreted there

Digestive enzymes reach MPSC papers as one-line recall items — which enzyme digests which nutrient, where a named enzyme is secreted, which secretion contains hydrochloric acid — and as pairing questions of the kind asked here, in which two substances are offered together and only one of them belongs. The Commission also asks the inactive precursor forms, since pepsinogen and trypsinogen are exactly the sort of detail that separates a candidate who has read the chapter from one who has memorised a list, and it asks the activators, enterokinase being a standing favourite. Related questions come from the accessory glands: the largest gland in the body, the functions of bile, why bile has no enzyme yet is essential to fat digestion, and where each vitamin is absorbed. A three-column table learnt once — secretion, source, action — carries nearly all of them.

Related PYQs

No directly related past PYQ was found.

Practice

practice — not a real PYQ
The enzyme trypsin is secreted in an inactive form as trypsinogen and is activated in the small intestine by
- (a)hydrochloric acid
- (b)enterokinase
- (c)bile salts
- (d)pepsin
Answer(b) Enterokinase — also called enteropeptidase, it is secreted by the mucosa of the small intestine and converts pancreatic trypsinogen into active trypsin, which then activates chymotrypsinogen and the other pancreatic zymogens in a cascade. Hydrochloric acid is the activator of the other great zymogen, pepsinogen, but it belongs to the stomach and is neutralised by the alkaline pancreatic juice before trypsinogen arrives. Bile emulsifies fat and contains no enzyme at all.
practice — not a real PYQ
Which one of the following is NOT a constituent of human gastric juice ?
- (a)Hydrochloric acid
- (b)Pepsinogen
- (c)Intrinsic factor
- (d)Pancreatic amylase
Answer(d) Pancreatic amylase — known in older textbooks as amylopsin, it is secreted by the pancreas and acts on starch in the small intestine, not in the stomach. The other three are all products of the gastric glands: hydrochloric acid and intrinsic factor from the parietal or oxyntic cells, and pepsinogen from the chief or peptic cells. No carbohydrate-digesting enzyme is secreted by the human stomach; salivary amylase begins starch digestion in the mouth and is stopped by the gastric acid.