Which of the following acid is secreted in the stomach ?

Question

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Correct — C, Hydrochloric acid. The stomach lining is folded into gastric glands, and inside them the parietal cells, also called oxyntic cells, pump hydrochloric acid into the stomach cavity. They release it in sufficient strength to bring gastric juice to a pH of roughly 1.5 to 3.5, which makes it the most acidic fluid the human body produces. The acid is doing three distinct jobs, and each is examinable in its own right. First, it activates the enzyme. The chief cells of the same glands secrete pepsinogen, an inactive precursor, and hydrochloric acid converts it into pepsin, the protein-splitting enzyme that works only in a strongly acid environment — which is why protein digestion begins in the stomach rather than in the small intestine. Second, it sterilises. Most bacteria and other microorganisms swallowed with food are killed outright by that pH, so the acid is a chemical barrier as much as a digestive agent. Third, it denatures protein, unfolding the long folded molecules so that pepsin can reach the peptide bonds inside them, and it softens the connective tissue in meat. The stomach survives its own acid because mucous cells coat the lining with alkaline mucus; when that defence is breached the result is a peptic ulcer, most commonly caused by infection with the bacterium Helicobacter pylori or by prolonged use of painkillers of the NSAID class. None of the other three acids listed is secreted anywhere in the human body.

Why the other options are wrong — (a) Nitric acid — a strong oxidising mineral acid of industry, used in making fertilisers, dyes and explosives, and severely corrosive to living tissue. No human gland secretes it. The name is close enough to cause trouble, because the body does produce nitric oxide, a gas that acts as a signalling molecule and relaxes blood vessels, and a candidate who half-remembers 'the body makes something nitric' can be pulled across to this option.  (b) Sulphuric acid — the acid whose name is most strongly associated with corrosiveness, which is precisely why it appears here. It is a laboratory and industrial reagent, the electrolyte of the lead-acid car battery and a bulk input to the fertiliser industry, and it has no place in human physiology. Nothing in the digestive tract, or anywhere else in the body, secretes it.  (d) Phosphoric acid — the most sophisticated of the three wrong options, because phosphorus genuinely is essential to the body. Phosphate is a structural component of bone and teeth, forms the backbone of DNA and RNA, and is the currency of energy transfer in ATP. But that is phosphate locked into compounds, not free phosphoric acid secreted as a digestive juice. Outside the body its familiar use is as the acidulant that gives cola drinks their sharpness.

Concept: The alimentary canal works as a sequence of chemical environments, each tuned to a different pH so that a different class of enzyme can operate, and most questions on digestion are really questions about that sequence. In the mouth, saliva is close to neutral and salivary amylase, also called ptyalin, begins the breakdown of starch. In the stomach, hydrochloric acid from the parietal cells drops the pH to about 1.5 to 3.5, pepsin is activated, and protein digestion begins; the acid also destroys swallowed microbes. When the semi-digested chyme passes into the duodenum the environment reverses completely. Bile from the liver, stored and concentrated in the gall bladder, contains no enzymes at all but emulsifies fat into fine droplets, and pancreatic juice brings bicarbonate that neutralises the acid along with trypsin for protein, pancreatic lipase for fat and pancreatic amylase for starch. The pH swing matters because enzymes work only within narrow windows: salivary amylase is destroyed by stomach acid, and pepsin in turn stops working once the alkaline pancreatic secretions arrive. Absorption then happens across the villi of the small intestine, while the large intestine mainly reclaims water.

State PCS 2024 Q13 — Science & Technology PYQ with Answer

State PCS 2024 Q13 — Science & Technology PYQ with Answer