NDA (I) 2018 General Ability Test · Question 93 of 150

Correct — A, Protein only. The acid-secreting cells of the stomach wall produce hydrochloric acid, and the acid has one crucial digestive job: it converts pepsinogen, the inactive form released by the neighbouring chief cells, into pepsin, the enzyme that begins the breakdown of protein. Lose the acid and pepsinogen is never switched on, so protein digestion in the stomach effectively stops. Nothing else in the stomach is affected in the same way. Fats are hardly touched there at all — they are emulsified by bile and split by pancreatic lipase further down, in the small intestine — and starch is dealt with by salivary amylase in the mouth and by pancreatic amylase in the small intestine. So the one biomolecule whose digestion suffers is protein.

• (b)Lipid — Fat digestion barely begins in the stomach. It depends on bile from the liver to break the fat into fine droplets and on pancreatic lipase to split them, both of which act in the small intestine and neither of which needs gastric acid.
• (c)Carbohydrate only — Starch digestion starts in the mouth with salivary amylase and is completed in the small intestine by pancreatic amylase. Gastric acid actually halts salivary amylase by making the surroundings too acidic for it, so removing the acid does not hinder carbohydrate digestion — if anything it lets the mouth's enzyme work a little longer.
• (d)Protein and Carbohydrate — Half right, and the most attractive wrong answer. Protein does belong on the list, but carbohydrate does not, for the reason given above. The word 'only' in option (a) is what separates the two, so it repays close reading.

The stomach lining contains several kinds of secretory cell working together. Parietal or oxyntic cells release hydrochloric acid, which drives the pH of the stomach contents down to about 2. Chief cells release pepsinogen, which that acidity converts into pepsin; pepsin then works best precisely at the low pH the acid creates, so the acid both activates the enzyme and provides its optimum conditions. Mucus cells coat the lining with a protective layer so that the stomach does not digest itself. The acid has two further duties beyond digestion — it kills most bacteria swallowed with food, and its arrival in the duodenum triggers the release of the hormone secretin, which sets the next stage of digestion going.

The stem says 'to a greater extent' rather than 'entirely', and that wording is careful and correct. Protein digestion is not abolished by the loss of gastric acid, because trypsin and chymotrypsin from the pancreas continue the job in the small intestine; what is lost is the first and substantial stomach stage. Read a question of this kind by asking which enzyme depends on the damaged secretion and which nutrient that enzyme handles, then check whether the same nutrient has a backup elsewhere. Doing so also explains the real-world version of this scenario: people with very low stomach acid, whether from disease or from long-term acid-suppressing medication, are prone to poorer protein breakdown, to weaker absorption of iron and vitamin B12, and to more gut infections, since the acid barrier against swallowed microbes is gone.

• Parietal cells of the stomach wall secrete hydrochloric acid, taking the gastric pH down to about 2.
• The acid converts pepsinogen into pepsin, the stomach's protein-digesting enzyme.
• Pepsin works best in strongly acidic conditions, so it needs the acid twice over.
• Fat digestion depends on bile and pancreatic lipase in the small intestine, not on gastric acid.
• Starch is digested by salivary amylase in the mouth and pancreatic amylase in the small intestine.

• Adding carbohydrate to the answer; starch digestion is handled outside the stomach and does not need acid.
• Assuming fat digestion begins in the stomach when it is essentially an intestinal process.
• Overlooking the word 'only', which is what separates the correct option from the tempting combined one.

NDA likes cause-and-consequence items in physiology — remove one secretion or organ and ask what fails. The same format has been used for bile, the pancreas and the salivary glands.

• practice — not a real PYQ

  Pepsinogen secreted by the stomach is converted into active pepsin by

  • (a)bile
  • (b)hydrochloric acid
  • (c)mucus
  • (d)pancreatic amylase

  Answer(b) hydrochloric acid — which is why damage to the acid-secreting cells stops protein digestion in the stomach.

• practice — not a real PYQ

  If bile production stopped, the digestion of which nutrient would be most affected?

  • (a)Protein
  • (b)Starch
  • (c)Fat
  • (d)Vitamin C

  Answer(c) Fat — bile emulsifies fat into fine droplets so that pancreatic lipase can act on it.