NDA (II) 2018 General Ability Test · Question 83 of 146

Correct — D, bile juice. Food leaves the stomach as chyme, soaked in hydrochloric acid and around pH 2, and the enzymes waiting for it in the small intestine work only in an alkaline medium. NCERT's Class 10 chapter on life processes puts the job squarely on bile — the food coming from the stomach is acidic and has to be made alkaline for the pancreatic enzymes to act, and bile juice from the liver accomplishes this in addition to acting on fats. Bile is made in the liver, stored in the gall bladder and released through the common bile duct into the duodenum, and it is alkaline because it carries bicarbonate along with the bile salts that emulsify fat.

• (a)pancreatic juice — Pancreatic juice is itself alkaline and does carry bicarbonate, so this is the strongest challenger. But the school syllabus this question is set from assigns the neutralising role to bile and casts the pancreatic secretion as the enzyme supply — trypsin for protein, pancreatic amylase for starch, lipase for emulsified fat — that can only work once bile has raised the pH.
• (b)duodenal secretion — The duodenum is the place where the neutralisation happens, not the source of the fluid that does it. Its own wall secretes mucus and hormones such as secretin and cholecystokinin, whose function is to signal the liver and pancreas to send their juices in.
• (c)large intestine secretion — Far too late in the tract. By the time material reaches the large intestine, digestion and absorption of nutrients are essentially over; the colon's business is absorbing water and salts and forming faeces, and its secretion is mucus for lubrication.

Digestion runs on a pH switch. The stomach works acidic, because hydrochloric acid activates pepsinogen into pepsin and kills swallowed microbes; the small intestine works alkaline, because trypsin, amylase and lipase are all denatured by acid. The switch is thrown in the duodenum, where two ducts open together at the same point — the common bile duct bringing alkaline bile from the liver and gall bladder, and the pancreatic duct bringing enzymes and bicarbonate from the pancreas. Bile also does a second, purely mechanical job: its bile salts break large fat globules into a fine emulsion, multiplying the surface area on which lipase can work.

The tempting error is to answer with the organ whose juice is most obviously alkaline rather than the one the syllabus credits. Both bile and pancreatic juice are alkaline, and in a physiology textbook the bulk of the bicarbonate that neutralises chyme comes from the pancreas. The Class 10 text this NDA item follows states the point plainly and gives the role to bile, and the official key marks (d) accordingly, so author your answer to bile and carry the nuance separately. The safe way to hold both is by function: bile changes the conditions, the pancreas supplies the workers.

• Chyme leaves the stomach strongly acidic, at roughly pH 2, because of gastric hydrochloric acid.
• Bile is produced by the liver, stored and concentrated in the gall bladder, and released into the duodenum through the common bile duct.
• Bile contains no digestive enzyme; it works by alkalinising the chyme and by emulsifying fats into smaller droplets.
• Pancreatic juice supplies trypsin, pancreatic amylase and lipase, all of which need an alkaline medium to act.

• Thinking bile contains digestive enzymes. It contains none — its work is alkalinisation and emulsification.
• Confusing the site of an event with its cause; the duodenum is where neutralisation happens, not what does it.
• Assuming the whole gut is acidic because the stomach is. The small intestine is alkaline, and its enzymes fail if it is not.

NDA asks which secretion neutralises chyme, which gland makes bile, or which enzyme acts on which food in which part of the tract.

• practice — not a real PYQ

  Bile is produced in the human body by the

  • (a)gall bladder
  • (b)liver
  • (c)pancreas
  • (d)duodenum

  Answer(b) liver — the gall bladder only stores and concentrates the bile that the liver makes.

• practice — not a real PYQ

  The chief role of bile salts in the digestion of food is to

  • (a)break down proteins into amino acids
  • (b)emulsify fats into small droplets
  • (c)convert starch into maltose
  • (d)absorb water from the residue

  Answer(b) emulsify fats into small droplets — this multiplies the surface area available to lipase.