BPSC 70th CCE Prelims General Studies · Question 13 of 143

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.

• (a)Nitric acid — 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 — 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 — 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.

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.

This question can be settled without any physiology at all, by sorting the options into laboratory chemicals and body chemicals. Nitric acid, sulphuric acid and phosphoric acid are all mineral acids you meet in a chemistry syllabus — reagents, battery electrolytes, industrial inputs. Hydrochloric acid appears in both lists, and it is the only one of the four that any human tissue produces. That elimination is the whole question. It is worth extending the habit, because BPSC and UPSC both run a family of 'which acid' items that reward learning the acids as a set rather than one at a time: hydrochloric acid in the stomach, citric acid in lemon and other citrus fruit, lactic acid in curd and in muscle during hard exercise, acetic acid in vinegar, formic acid in an ant's sting and a nettle's hairs, tartaric acid in tamarind, malic acid in apples and oxalic acid in tomato. Learn them together and a whole class of one-line questions collapses into a single memorised table.

• Hydrochloric acid is secreted by the parietal or oxyntic cells of the gastric glands, giving gastric juice a pH of roughly 1.5 to 3.5 — the most acidic fluid produced in the human body.
• The acid converts pepsinogen, secreted by the chief cells, into pepsin. Pepsin acts only in acid conditions, which is why protein digestion begins in the stomach and not in the small intestine.
• Gastrin, released by the G cells of the stomach, is the hormone that stimulates acid secretion — a hormone, not an enzyme, which is a distinction examiners test directly.
• A layer of alkaline mucus protects the stomach wall from its own secretion; when that barrier fails, peptic ulcers follow, most often from Helicobacter pylori infection or long-term use of NSAID painkillers.
• The chyme leaving the stomach is neutralised in the duodenum by bicarbonate in pancreatic juice, which switches the environment to alkaline so that trypsin, pancreatic lipase and pancreatic amylase can work.
• The wider 'which acid' set worth memorising together: citric acid in citrus fruit, lactic acid in curd and in fatigued muscle, acetic acid in vinegar, formic acid in an ant's sting, tartaric acid in tamarind, malic acid in apples, oxalic acid in tomato.

• Confusing gastrin with an enzyme. Gastrin is the hormone that triggers acid secretion; pepsin is the enzyme the acid activates. UPSC has asked exactly this distinction.
• Believing protein digestion happens only in the small intestine. It begins in the stomach with pepsin, and only continues in the intestine with trypsin.
• Choosing phosphoric acid because phosphorus is important in the body. Phosphate in bone, DNA and ATP is not the same thing as phosphoric acid being secreted as a digestive juice.

BPSC asks human physiology as flat one-line recall from the school syllabus — which acid, which gland, which organ — and pads the option list with strong mineral acids from the chemistry paper, so the elimination is chemical rather than biological. UPSC asks the same anatomy through a mechanism or a consequence: which secretion enters which duct, why an assertion about protein digestion is false, or which item in a list is not a digestive enzyme at all. Prepare the digestive tract as a table of organ, secretion, enzyme and pH, and both styles are covered by the same page of notes.

• practice — not a real PYQ

  The inactive precursor pepsinogen is converted into the active enzyme pepsin in the stomach by

  • (a)bile from the liver
  • (b)hydrochloric acid
  • (c)trypsin from the pancreas
  • (d)salivary amylase

  Answer(b) hydrochloric acid — secreted by the parietal cells. Bile is alkaline and enzyme-free, trypsin acts in the small intestine, and salivary amylase is inactivated by stomach acid.

• practice — not a real PYQ

  Which one of the following secretions of the human digestive system contains no enzymes ?

  • (a)Saliva
  • (b)Gastric juice
  • (c)Bile
  • (d)Pancreatic juice

  Answer(c) Bile — it is produced by the liver and stored in the gall bladder, and it emulsifies fat without digesting it. Saliva carries amylase, gastric juice carries pepsin, and pancreatic juice carries trypsin, lipase and amylase.