Which one of the following pairs of enzymes do not work at acidic pH?
- (a)Trypsin and Pepsin
- (b)Chymotrypsin and Pepsin
- (c)Trypsin and Amylase
- (d)Pepsin and Amylase
Correct — C, Trypsin and Amylase. Neither of these two works in acid. Trypsin is a pancreatic protease that acts in the small intestine, and NCERT is explicit that 'the food coming from the stomach is acidic and has to be made alkaline for the pancreatic enzymes to act' — bile juice does that neutralising. Amylase acts at the two ends of the tract where conditions are near neutral or alkaline: salivary amylase in the mouth, at a pH close to 7, and pancreatic amylase in the duodenum. Neither survives a trip through the stomach in working order. Every other pair on the list contains pepsin, and pepsin is the one enzyme in this question that needs acid — the gastric glands 'release hydrochloric acid, a protein digesting enzyme called pepsin, and mucus', and the acid 'creates an acidic medium which facilitates the action of the enzyme pepsin'.
- (a)Trypsin and Pepsin — Contains pepsin, which works only in acid, at a pH around 2. Trypsin does belong on the answer side, but a pair is only correct if both members qualify.
- (b)Chymotrypsin and Pepsin — Same flaw. Chymotrypsin is pancreatic and alkaline-working like trypsin, but pepsin's presence spoils the pair.
- (d)Pepsin and Amylase — Pepsin again, this time alongside an enzyme that genuinely fails in acid. Two enzymes with opposite pH requirements can never be the answer to a question asking for a pair that shares one.
Every enzyme has a pH at which its shape and therefore its activity is at a maximum, and it works because the digestive tract is divided into compartments with different chemistry. The mouth is near neutral, so salivary amylase can begin on starch. The stomach is strongly acidic, which activates pepsinogen into pepsin and lets it attack protein. The small intestine is made alkaline by bile so that the pancreatic enzymes — trypsin, chymotrypsin, lipase and pancreatic amylase — can finish the job.
The fastest route through this question is to find the enzyme that behaves differently from the rest. Pepsin is the only acid-working enzyme in the option list, so any pair containing it is out, and that eliminates three options in a single stroke. Notice the shape of the trap: three of the four pairs mix an acid enzyme with an alkaline one, which is what a candidate reading quickly will miss. Underneath the trick is a fact worth holding for its own sake — the site where an enzyme is made and the site where it acts are often different. Trypsin and chymotrypsin are produced in the pancreas as inactive precursors and only become active in the duodenum, which is the body's way of not digesting the organ that makes them.
- Pepsin is secreted by the gastric glands and requires the acidic medium that hydrochloric acid creates.
- Food leaving the stomach is acidic and must be made alkaline by bile before the pancreatic enzymes can act.
- Trypsin and chymotrypsin are pancreatic proteases that work in the alkaline small intestine.
- Salivary amylase acts in the mouth at close to neutral pH; pancreatic amylase acts in the duodenum.
- Trypsin and chymotrypsin are secreted as inactive precursors and activated only after they reach the intestine.
- Assuming all protein-digesting enzymes need acid because pepsin does.
- Forgetting that amylase appears twice, in saliva and in pancreatic juice, and that both act away from acid.
- Choosing a pair on the strength of one familiar member without checking the other.
As a pair-selection question on pH, as an enzyme-to-site match, or as an assertion and reason item on where protein digestion is completed.
Assertion (A): All the proteins in our food are digested in small intestine only. Reason (R): The protein-digesting enzymes from pancreas are released into small intestine.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is not the correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer(d) A is false but R is true
The same division of labour, tested as an assertion and reason. Protein digestion starts in the acid stomach with pepsin and is completed in the alkaline intestine by the pancreatic proteases, which is exactly the split the CDS pair turns on.
Match List I with List II and select the correct answer using the codes given below the Lists: List I (Substance) I. Ptyalin II. Pepsin III. Renin IV. Oxytocin List II (Physiological role) A) Converts angiotensinogen in blood into angiotensin B) Digests starch C) Digests proteins D) Hydrolyses fats E) Induces contraction of smooth muscles
- (a) I-B, II-C, III-A, IV-E
- (b) I-C, II-D, III-B, IV-E
- (c) I-B, II-C, III-E, IV-A
- (d) I-C, II-A, III-B, IV-D
Answer(a) I-B, II-C, III-A, IV-E
Ptyalin is salivary amylase, so this item pairs the same two enzymes with their substrates. Knowing that ptyalin works on starch in the mouth places amylase away from the acid compartment without any further recall.
- practice — not a real PYQ
The enzyme pepsin acts best at a pH of about
- (a)2
- (b)7
- (c)8
- (d)11
Answer(a) 2 — pepsin is a gastric enzyme and depends on the strongly acidic medium created by hydrochloric acid.
- practice — not a real PYQ
Which secretion makes the food leaving the stomach alkaline so that pancreatic enzymes can act on it?
- (a)Gastric juice
- (b)Saliva
- (c)Bile juice
- (d)Intestinal mucus
Answer(c) Bile juice — it comes from the liver, neutralises the acidic chyme and also emulsifies fats.