NDA (I) 2017 General Ability Test · Question 96 of 149

Correct — A, All proteins are enzymes. This is the statement that is not correct, and the question asks for exactly that. Enzymes are one job that proteins do, but proteins do many others — keratin and collagen are structural, haemoglobin transports oxygen, insulin is a hormone, antibodies defend the body, and actin and myosin drive muscle contraction. None of those is a catalyst, so the class of proteins is far wider than the class of enzymes.

• (b)Mostly enzymes are proteins — This statement is correct, so it cannot be the answer. It is also carefully worded: almost every enzyme is a protein, but a few catalytic RNA molecules also act as enzymes, which is why the sentence says mostly rather than all.
• (c)All fats are energy rich compounds — Correct as it stands. Fats yield about nine kilocalories per gram against roughly four for carbohydrates and proteins, so they are the body's densest energy store.
• (d)Glucose is a common carbohydrate — Correct as it stands. Glucose is the simple six-carbon sugar that cells respire for energy and the unit from which starch, glycogen and cellulose are built.

Proteins are polymers of amino acids joined by peptide bonds, and their folded shape decides what they do. Enzymes are the subset of proteins that catalyse biochemical reactions, speeding them up enormously without being consumed and acting on specific substrates at an active site. The wider protein family also supplies structure, transport, hormones, antibodies and movement. So the relationship runs one way only — almost all enzymes are proteins, but the reverse claim is false.

A 'not correct' question rewards checking each statement on its own instead of hunting for the best-sounding one. The one-way relationship in statements (a) and (b) is the whole point of the item, and it is the same logical shape as 'all squares are rectangles'. The careful word in statement (b) is mostly, which leaves room for ribozymes — RNA molecules with catalytic activity — the reason biologists no longer say every enzyme is a protein.

• Proteins are polymers of amino acids linked by peptide bonds.
• Enzymes are biological catalysts and are a subset of proteins, not the whole of them.
• Proteins also serve as structural material, transporters, hormones, antibodies and contractile fibres.
• Fats supply about nine kilocalories per gram, roughly twice what carbohydrates and proteins yield.
• Some RNA molecules, called ribozymes, also show catalytic activity, which is why not every enzyme is a protein.

• Reversing a one-way relationship — all enzymes are proteins does not make all proteins enzymes.
• Missing that the question asks for the statement that is NOT correct.
• Assuming every enzyme is a protein without exception; catalytic RNA is the exception the wording allows for.

NDA sets a 'which statement is not correct' item across the biomolecules, or asks for the identity of a named enzyme or hormone.

• practice — not a real PYQ

  Which one of the following is a protein but not an enzyme?

  • (a)Pepsin
  • (b)Amylase
  • (c)Haemoglobin
  • (d)Trypsin

  Answer(c) Haemoglobin — an oxygen-carrying protein, while the other three are digestive enzymes.

• practice — not a real PYQ

  Compared with carbohydrates, fats provide energy per gram that is roughly

  • (a)half as much
  • (b)the same
  • (c)twice as much
  • (d)ten times as much

  Answer(c) twice as much — about nine kilocalories per gram against roughly four for carbohydrates.