Which gastric juices are secreted in human stomach ?

Question

UnlockIAS · Accepted Answer

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 other options are wrong — (a) 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.  (b) 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.  (d) 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.

State PCS 2021 Q62 — Science & Technology PYQ with Answer

State PCS 2021 Q62 — Science & Technology PYQ with Answer