UPPSC 2019 Prelims GS Paper-I · Question 91 of 150

Correct — C, both 1 and 2. Statement 1 is beyond argument: enzymes are biological catalysts. They speed up a reaction by lowering its activation energy, are not consumed or permanently changed by it, act in very small amounts, and are almost all globular proteins (the exceptions are ribozymes, which are catalytic RNA). Statement 2 is the one the commission's key treats as true, and it holds for the largest classes of enzymes a student meets. Intracellular enzymes — the glycolytic enzymes of the cytoplasm, the respiratory enzymes of the mitochondrion, the hydrolases of the lysosome — act inside the very cell that synthesised them. The intestinal brush-border enzymes (maltase, sucrase, lactase) are made by intestinal epithelial cells and act on the surface of those same cells. Salivary amylase acts in the mouth, into which the salivary glands empty; pepsin acts in the stomach lumen, where the gastric chief cells that make it discharge it. HONEST NOTE, because this item is a known key conflict: statement 2 is broadly true rather than universally true. The standard exception is the pancreatic digestive enzymes — trypsin, chymotrypsin, pancreatic amylase and lipase are synthesised in the acinar cells of the pancreas but do their work in the duodenum, a different organ altogether. On a strict reading of 'the site where they are produced', that exception makes option (a) defensible, and many candidates argued for it. The official UPPSC SET-D key marks (c), and the key is what decides marks — so (c) is the answer to carry into the exam, while the pancreatic exception is the fact to carry into your notes.

• (a)1 only — This is the defensible dissent, not a careless error: if you insist that statement 2 must hold for EVERY enzyme, the pancreatic enzymes (made in pancreatic acinar cells, active in the duodenum) break it. The commission nevertheless marked statement 2 correct, reading it in the looser textbook sense that an enzyme acts at or around its site of secretion. Marks follow the key, so (a) scores zero — but understand why it was argued rather than dismissing it.
• (b)2 only — Rejects statement 1, which is the single least controversial claim in the whole question. 'Biocatalyst' is the textbook one-word definition of an enzyme. No reading of enzyme biology makes statement 1 false.
• (d)Neither 1 nor 2 — A blanket rejection cannot stand, because statement 1 is definitionally true. This option only attracts candidates who spot the pancreatic problem in statement 2 and then over-correct by discarding both statements.

Enzymes are biological catalysts — overwhelmingly proteins, plus a small class of catalytic RNAs called ribozymes — that accelerate specific biochemical reactions by lowering the activation energy, without themselves being used up. Each enzyme is highly specific to its substrate (the lock-and-key idea, refined into the induced-fit model), works only within a narrow band of temperature and pH, and is often dependent on a cofactor or coenzyme (many of which are vitamin derivatives). Where an enzyme acts is a separate question from what it does: some enzymes never leave the cell that made them, while others are secreted and travel to their site of action.

This is a two-statement question in which one statement is a giveaway and the entire decision rests on the other. Once you accept 'biocatalysts', only (a) and (c) survive, so the question reduces to: does an enzyme always act where it is made? The school-textbook answer, and the commission's, is yes — because the examples usually taught (cellular respiration, lysosomal digestion, brush-border enzymes, salivary amylase in the mouth) all fit. The exception that a careful student will remember is the pancreas: its acinar cells manufacture trypsinogen, chymotrypsinogen, pancreatic amylase and lipase, which are then piped down the pancreatic duct and activated in the duodenum. That is why this item is flagged as a key conflict. In the exam hall the practical rule is to answer to the setter's evident intention; in your notes, keep the exception.

• Enzymes are biocatalysts: they lower activation energy, are needed only in trace amounts, are not consumed by the reaction, and are almost all proteins — the exceptions being ribozymes (catalytic RNA).
• Intracellular enzymes (glycolytic, mitochondrial, lysosomal) act inside the very cell that produces them, and the intestinal brush-border enzymes maltase, sucrase and lactase are made by, and act on, the same intestinal epithelial cells — these are the cases statement 2 describes accurately.
• The standard exception: pancreatic trypsin, chymotrypsin, amylase and lipase are produced in pancreatic acinar cells but act in the duodenum. Pepsinogen likewise is made by gastric chief cells and acts in the stomach lumen, outside the cell that made it.
• Many digestive enzymes are secreted as inactive precursors (zymogens) — pepsinogen becomes pepsin in gastric acid, trypsinogen becomes trypsin under intestinal enterokinase — which is precisely how the body avoids digesting the organ that makes them.
• Enzymes are pH- and temperature-sensitive: pepsin works best in the strongly acidic stomach (around pH 2), trypsin in the alkaline small intestine (around pH 8); high heat denatures them.

• Confusing enzymes with hormones: gastrin, secretin and insulin are hormones, yet they are the commonest wrong picks in 'which of these is a digestive enzyme' questions.
• Assuming all enzymes are proteins — ribozymes (catalytic RNA) are the standing exception.
• Assuming every digestive enzyme acts where it is secreted — the pancreatic enzymes are the counterexample, and this very question turns on that point.

UPPSC and UPSC both ask enzymes as one-line identification ('which of the following is NOT a digestive enzyme'), as assertion-reason on pancreatic secretion into the small intestine, or — as here — as a two-statement correctness item where one statement is a definitional giveaway and the second carries all the difficulty.

• practice — not a real PYQ

  Enzymes accelerate biochemical reactions mainly by

  • (a)raising the temperature of the reaction mixture
  • (b)lowering the activation energy of the reaction
  • (c)shifting the equilibrium of the reaction towards the products
  • (d)being consumed in stoichiometric amounts along with the substrate

  Answer(b) lowering the activation energy of the reaction — a catalyst provides an easier route to the same products; it does not change the equilibrium position and is not consumed.

• practice — not a real PYQ

  Trypsin, chymotrypsin and pancreatic lipase are synthesised in the pancreas but carry out their digestive action mainly in the

  • (a)Mouth
  • (b)Stomach
  • (c)Duodenum (small intestine)
  • (d)Large intestine

  Answer(c) Duodenum (small intestine) — pancreatic juice is discharged through the pancreatic duct into the duodenum, where trypsinogen is activated to trypsin by intestinal enterokinase.