CDS (I) 2022 General Knowledge · Question 25 of 120

Correct — A, Hydrochloric acid. The stomach lining secretes hydrochloric acid, and the word 'mineral' in the stem does half the work of the question. A mineral acid is an inorganic one — hydrochloric, sulphuric, nitric, phosphoric — and of the four options only hydrochloric acid belongs to that family; the other three are organic acids built on carbon. The acid is produced by specialised cells in the gastric glands and keeps the stomach contents strongly acidic, around pH 1·5 to 3·5. It does two jobs there. It converts the inactive pepsinogen into pepsin, the enzyme that begins the digestion of protein and which only works in acid; and it kills most of the bacteria that arrive with food. The stomach survives its own acid because a thick layer of mucus is secreted over the lining at the same time.

• (b)Lactic acid — An organic acid, and it belongs elsewhere. Lactic acid forms in muscle during anaerobic respiration and in milk turned sour by bacteria; it is not secreted by the stomach and is not a mineral acid.
• (c)Uric acid — Also organic, and it is a waste product rather than a secretion. Uric acid is a nitrogenous waste excreted in urine, and its crystals deposited in joints are what cause gout.
• (d)Methanoic acid — The organic acid of ant stings and nettle hairs, better known as formic acid. It has nothing to do with digestion, and like the other two it fails the 'mineral acid' test in the stem.

Acids divide into mineral acids, which are inorganic and generally strong — hydrochloric, sulphuric, nitric — and organic acids, which contain carbon and are generally weak, such as ethanoic, citric, lactic and methanoic. The stomach's hydrochloric acid comes from the gastric glands in its wall, alongside pepsinogen and mucus. The acid activates pepsinogen into pepsin and provides the acidic medium pepsin needs, while the mucus protects the stomach wall from being digested along with the meal. When the balance fails, the result is acidity or an ulcer, and antacids such as magnesium hydroxide work simply by neutralising the excess.

Two independent routes reach the same answer here, which makes it a comfortable mark. The first is direct recall of gastric juice. The second works even without that: read the adjective in the stem, note that only one of the four is a mineral acid at all, and the question is decided. Two facts hang usefully off this one. The stomach is the only part of the alimentary canal that is strongly acidic — saliva is nearly neutral and the small intestine is made alkaline by bile and pancreatic juice, because the enzymes there need alkaline conditions. And the acid is a defence as much as a digestive aid, which is why suppressing it too heavily has its own costs.

• The gastric glands secrete hydrochloric acid, a mineral acid, keeping the stomach at about pH 1·5 to 3·5.
• The acid converts pepsinogen into pepsin and supplies the acidic medium that pepsin needs to digest protein.
• It also kills most bacteria swallowed with food.
• Mucus secreted over the stomach lining prevents the wall from being digested by its own acid and enzyme.
• The small intestine is alkaline, made so by bile and pancreatic juice, because the enzymes acting there need alkaline conditions.

• Reaching for lactic acid because it is associated with sourness.
• Ignoring the word 'mineral' in the stem, which by itself removes three options.
• Assuming the whole digestive tract is acidic when only the stomach is.

As a direct question on the acid in gastric juice, or as part of a broader digestion item on which enzyme acts where and under what conditions.

• practice — not a real PYQ

  Pepsin, the protein-digesting enzyme of the stomach, works best in

  • (a)a strongly acidic medium
  • (b)a neutral medium
  • (c)a strongly alkaline medium
  • (d)the absence of water

  Answer(a) a strongly acidic medium — the hydrochloric acid of gastric juice both activates pepsinogen and supplies the conditions pepsin needs.

• practice — not a real PYQ

  An antacid such as magnesium hydroxide relieves acidity in the stomach by

  • (a)coating the stomach wall with a waterproof layer
  • (b)neutralising the excess hydrochloric acid
  • (c)stopping the secretion of mucus
  • (d)converting pepsin back into pepsinogen

  Answer(b) neutralising the excess hydrochloric acid — a mild base reacts with the acid and raises the pH towards a comfortable level.