MPSC 2023 Prelims Paper-I · Question 70 of 100

Correct — option (3). The stem is positive: it asks which statements are correct, and the safe procedure on a list of this kind is to mark each statement true or false on its own before looking at the options at all. Statement a says enzymes are organic catalysts that accelerate biochemical reactions, and that is the standard definition — enzymes are organic molecules produced by living cells which speed up reactions by lowering the activation energy, and which emerge from the reaction unconsumed and unchanged, so that a very small quantity can process a great deal of substrate. Statement b says enzymes are made up from proteins, and that too is correct: enzymes are proteins, chains of amino acids folded into a specific three-dimensional shape whose active site accommodates the substrate, and it is the dependence of function on that folded shape which explains almost everything else about enzyme behaviour. Statement d says amylase is an enzyme that acts on starch, which is correct — amylase hydrolyses starch to smaller sugars, and in the human body it is secreted twice, as salivary amylase which begins the digestion of starch in the mouth and as pancreatic amylase which continues it in the small intestine. Statement c is the one that fails. It asserts that enzyme activity is not dependent upon temperature, and the opposite is true: temperature is among the strongest influences on enzyme activity. As temperature rises from low values the rate increases, because molecules move faster and collide more often, up to an optimum which for human enzymes is about the normal body temperature of 37 °C. Beyond that the rate does not keep rising but falls away sharply, because heat disrupts the weak bonds holding the protein in its folded shape; the enzyme is denatured, the active site loses its geometry, and the catalytic power is destroyed. The asymmetry is worth noting — cooling slows an enzyme without destroying it, and activity returns on warming, whereas heating past the denaturation point is irreversible. Statement c is therefore incorrect, and the correct statements are a, b and d, which is option (3). One warning about statement c: it is itself phrased negatively, so a candidate who is also mis-reading the stem as negative can double back on themselves and count it as correct. The stem here is positive, and statement c is simply false.

• (1)a and b — This option correctly excludes the false statement c but also drops statement d, which is true, and an option that omits a correct statement is as wrong as one that includes an incorrect one. Amylase does act on starch: it is the starch-splitting enzyme of the digestive system, secreted in the saliva by the salivary glands, where it begins breaking starch down into maltose and shorter dextrin chains, and again by the pancreas into the small intestine, where the work is completed. A candidate may hesitate over statement d because the printed English is faulty — the paper prints 'Amylase is an enzyme that act on starch' — but a grammatical slip in the printing is not a factual error, and this paper carries several. The lesson is procedural: having identified the false statement, do not stop, because the option list still requires every true statement to be accounted for.
• (2)a, c and d — This option includes statement c, which is the one false item in the list, and that alone disqualifies it whatever else it contains. Enzyme activity depends very strongly on temperature: the rate rises with temperature up to an optimum, around 37 °C for most human enzymes, and then falls steeply as the heat breaks the weak bonds that maintain the enzyme's folded shape and denatures the protein. The same option also omits statement b, so it is doubly wrong. This is the option that catches the candidate who has misread the direction of the stem, since a and d are true while c is false, making the selection an incoherent mixture rather than the answer to either the positive or the negative version of the question — a useful reminder that a wrong reading of the stem does not usually produce a self-consistent wrong answer, and that noticing the incoherence is a way back.
• (4)all the above — 'All the above' requires every one of the four statements to be correct, and statement c is not. Enzyme activity is dependent upon temperature in the most direct way, which is why the body maintains a stable internal temperature and why a high fever is dangerous, and why enzyme preparations are refrigerated rather than warmed. Escape options of this kind appear ten times across this paper, and being offered is no evidence of being keyed; the discipline they demand is to verify each statement individually and select the escape only when all of them survive. Here the item can in fact be reduced very quickly, because statement c is the only one making a claim that is easy to test against ordinary experience, and once it falls, both this option and option (2) fall with it, leaving only two options to choose between.

Enzymes are protein catalysts made by living cells. They speed up biochemical reactions, often by many orders of magnitude, by binding the substrate at an active site and lowering the activation energy the reaction must overcome; they are not consumed in the process and are free to act again, so a small quantity turns over a large amount of substrate. The active site is a pocket whose shape and chemistry complement the substrate, which accounts for the high specificity of enzyme action — an idea first expressed as the lock-and-key model and refined into the induced-fit model, in which the active site adjusts its shape as the substrate binds. Because catalytic power depends on that three-dimensional shape, and the shape is held together by weak bonds, enzymes are sensitive to their physical conditions. Activity rises with temperature to an optimum, near 37 °C for human enzymes, and falls sharply above it as the protein denatures; the fall is irreversible, whereas the loss of activity at low temperature is not, since cooling merely slows molecular motion. Each enzyme also has an optimum pH, and these differ widely according to where the enzyme works: pepsin acts in the strong acid of the stomach at a pH of about 2, whereas trypsin acts in the alkaline conditions of the small intestine at a pH near 8, and salivary amylase near neutrality. Activity also rises with substrate concentration until every active site is occupied, after which the rate levels off. Enzymes are named and classified by the reaction they catalyse, and the digestive enzymes illustrate the pattern of specificity plainly — amylase on starch, protease on protein, lipase on fats, each acting on one class of substrate and no other. The one important qualification to the statement that enzymes are proteins is the existence of ribozymes, catalytic RNA molecules, which is a recognised exception rather than a challenge to the general rule.

Statement-list questions are the largest single family in this paper, with thirty-eight of the hundred questions built on them, so the technique for handling them is worth more than any individual fact. The method is fixed: read the stem and note its direction, judge each statement true or false in isolation, and only then look for the option that matches your verdicts exactly. Reversing that order — scanning the options first and testing statements against them — is what produces the characteristic failure, in which a candidate finds an option containing the one statement they are sure about and stops there. This item adds a specific hazard the Commission uses often: statement c is itself phrased in the negative, and a negatively worded statement inside a positively worded stem invites a double negative that reverses the answer. There is no way to read your way out of that except by settling the truth of the statement on its own terms first, quite apart from the stem. A second point concerns the printed English, which is defective in three of the four statements — 'organic catalyst that accelerate biochemical reaction', 'is not depends upon temperature', 'an enzyme that act on starch' — and reproduced here exactly as printed. Bad grammar in a statement is not evidence that it is factually false, and a candidate who treats printing errors as planted clues will misjudge items that are simply carelessly typeset.

• Enzymes are organic catalysts produced by living cells which accelerate biochemical reactions by lowering the activation energy and are not consumed in the reaction, so a small quantity can process a large amount of substrate.
• Enzymes are proteins whose catalytic power depends on the three-dimensional shape of the active site; the only significant exception is the class of ribozymes, which are catalytic RNA molecules.
• Enzyme activity is strongly dependent on temperature: it rises to an optimum near 37 °C for human enzymes and then falls sharply as heat denatures the protein and destroys the geometry of the active site.
• Loss of activity through heating is irreversible because the protein is denatured, whereas loss of activity through cooling is reversible, since low temperature only slows molecular motion without unfolding the enzyme.
• Amylase hydrolyses starch to maltose and shorter dextrins, and is secreted both by the salivary glands, where starch digestion begins in the mouth, and by the pancreas, where it continues in the small intestine.

• Double-negating a negatively worded statement inside a positively worded stem, which reverses the verdict on statement c and produces a confidently wrong answer
• Stopping once the false statement has been found, without confirming that every remaining true statement is present in the option selected
• Treating a grammatical error in a printed statement as a signal that the statement is factually wrong, when this paper's English column carries many such slips
• Selecting 'all the above' without testing each statement separately, when a single false item is enough to defeat it
• Assuming that because low temperature reduces enzyme activity it also destroys the enzyme, when cooling is reversible and only heating past the optimum causes denaturation

Enzymes appear in MPSC science sections both as definition questions and, more often, as statement lists of the kind used here, in which three or four assertions about enzyme nature, composition, specificity and conditions are offered and the candidate must sort the true from the false. The false statement is almost always constructed by negating a true one or by reversing a relationship — activity does not depend on temperature, enzymes are consumed in the reaction, enzymes are made of carbohydrates — so a candidate who has learnt the properties as positive statements can identify the planted error quickly. The companion family pairs digestive enzymes with their substrates and secreting organs, and the two are best prepared together, since the same short list of enzymes serves both. The Commission also likes to place a negatively worded statement in a positively worded list, as here, and to place two questions on the same topic side by side, as it has done with the preceding question on zymogens.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which of the following statements about the effect of temperature on enzyme activity is correct ?

  • (a)Enzyme activity increases indefinitely as the temperature is raised
  • (b)Enzyme activity rises to an optimum temperature and then falls sharply as the enzyme is denatured
  • (c)Enzyme activity is unaffected by temperature
  • (d)Enzymes are permanently destroyed by low temperature

  Answer(b) Enzyme activity rises to an optimum temperature and then falls sharply as the enzyme is denatured — the rise happens because warmth increases molecular motion and the frequency of collisions, and the fall happens because heat breaks the weak bonds maintaining the enzyme's folded shape, destroying the active site. Low temperature only slows the enzyme and the effect is reversible on warming, so the loss of activity from cold is not the same in kind as the loss from heat.

• practice — not a real PYQ

  Salivary amylase acts upon which of the following substrates ?

  • (a)Proteins
  • (b)Fats
  • (c)Starch
  • (d)Nucleic acids

  Answer(c) Starch — salivary amylase, secreted by the salivary glands, begins the digestion of starch in the mouth by hydrolysing it to maltose and shorter dextrin chains, and pancreatic amylase completes the work in the small intestine. Proteins are digested by proteases such as pepsin and trypsin, fats by lipases, and nucleic acids by nucleases, each enzyme acting on one class of substrate and no other, which is the specificity that follows from the shape of the active site.