CDS (I) 2020 General Knowledge · Question 5 of 120

Correct — D, Hydrochloric acid. The gastric glands in the stomach wall release three things — hydrochloric acid, the protein-splitting enzyme pepsin, and mucus. The acid does two jobs. It creates the strongly acidic medium in which pepsin can work at all, and it kills much of what arrives with the food. Mucus is what keeps the acid from digesting the stomach lining itself; when that balance tips, the result is the pain we call acidity, and the remedy is a mild base such as magnesium hydroxide.

• (a)Formic acid — Formic acid, also called methanoic acid, is what an ant injects when it stings and what makes a nettle leaf burn. It has no role in human digestion.
• (b)Sulphuric acid — A strong mineral acid of the laboratory and of lead-acid batteries. Nothing in the human body secretes it.
• (c)Nitric acid — Another laboratory mineral acid, used among other things by goldsmiths to clean gold and silver. It is not produced anywhere in the body.

Digestion in the stomach is chemical as well as mechanical. Gastric glands in the stomach wall secrete hydrochloric acid, pepsin and mucus together. The acid brings the stomach contents to a pH of roughly 1 to 2; pepsin is a protein-digesting enzyme that only works in that range, so the acid is a precondition for protein digestion rather than a digestive agent in itself.

All four options are acids, and three of them are the familiar strong mineral acids of a chemistry laboratory, so the item is really asking whether you have connected a chemistry fact to a biology one. The bridge is the antacid: people take a mild base for indigestion, which means the stomach's own secretion must be an acid, and the acid the textbooks name is hydrochloric. Formic acid is the deliberate distractor here because it does appear in the body's story — but as something injected by an ant or a nettle, not secreted by us.

• Gastric glands in the stomach wall release hydrochloric acid, pepsin and mucus.
• The acid creates the acidic medium that lets pepsin act; mucus protects the stomach lining from the acid.
• Stomach contents typically sit near pH 1 to 2, among the most acidic environments in the body.
• Antacids such as magnesium hydroxide, a mild base sold as milk of magnesia, neutralise the excess acid of indigestion.
• Formic (methanoic) acid is the acid of ant stings and nettle hairs, not of the stomach.

• Picking formic acid because it is the one acid on the list associated with a living thing.
• Believing the acid itself digests protein; pepsin does that, and the acid only makes it possible.
• Forgetting mucus, which is the reason the acid does not eat the stomach.

As 'which acid is present in the stomach', or through the antacid — which substance relieves indigestion and why.

• practice — not a real PYQ

  The main function of the hydrochloric acid secreted in the human stomach is to

  • (a)digest proteins directly
  • (b)provide the acidic medium needed by pepsin
  • (c)emulsify fats before absorption
  • (d)neutralise the alkaline food coming from the mouth

  Answer(b) provide the acidic medium needed by pepsin — the acid sets the conditions, and pepsin does the protein digestion.

• practice — not a real PYQ

  A person suffering from acidity is given milk of magnesia. This works because milk of magnesia is

  • (a)a mild acid
  • (b)a mild base
  • (c)a neutral salt
  • (d)a strong acid

  Answer(b) a mild base — magnesium hydroxide neutralises the excess hydrochloric acid in the stomach.