Studyledge

Enzymes: Question 1

Syllabus 5.1

Multiple choice Core 1 mark

A student extracts a protease enzyme from kiwi fruit and adds a small, equal sample of the enzyme to three separate test tubes:

  • test tube 1 contains egg white (a protein)
  • test tube 2 contains vegetable oil (a fat)
  • test tube 3 contains a paste made from cornflour (a carbohydrate)

All three test tubes are left at the same temperature for 30 minutes. Only the contents of test tube 1 show any sign of the protein being broken down into smaller, soluble fragments. Test tubes 2 and 3 show no change at all.

Which statement best explains why the enzyme has this effect only on the egg white?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Compare what happened in each test tube

All three test tubes contained the same enzyme, at the same temperature, for the same length of time. The only thing that differed between the tubes was the type of substrate: protein (egg white), fat (vegetable oil) or carbohydrate (cornflour paste). Since a change only occurred in the tube containing protein, the explanation must be about which substrate the enzyme can act on, not about temperature or the enzyme running out.

Step 2: Recall why enzymes act on only one type of substrate

An enzyme’s active site has a shape that is complementary to the shape of one particular substrate. Only a substrate that fits this shape can bind to the active site and be converted into products. Egg white protein fits the active site of this enzyme, but the fat molecules in the oil and the carbohydrate molecules in the cornflour paste have different shapes, so they cannot bind to it.

Why the other options are wrong

  • A: denaturation is caused by extreme temperature or extreme pH permanently changing the shape of the active site, not by an enzyme simply coming into contact with a different type of substrate.
  • C: a catalyst, including an enzyme, is not changed or used up by the reaction it speeds up, so the enzyme is still available to act on the other test tubes.
  • D: the question states that all three test tubes were kept at the same temperature, so a temperature difference cannot explain why only the protein was broken down.

Final answer

The enzyme only breaks down the egg white because its active site shape is complementary only to the protein substrate, option B.