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Enzymes: Question 1

Syllabus 3.1

Multiple choice AS 1 mark

Enzymes are described as biological catalysts because they speed up the rate of intracellular reactions without themselves being permanently changed.

Which statement correctly explains how an enzyme increases the rate of a reaction?

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

Step 1: Recall what a catalyst does

A catalyst, including an enzyme, increases the rate of a reaction without being used up or permanently changed itself. This immediately rules out any option in which the enzyme is consumed, converted into a new substance, or otherwise altered by the reaction. The enzyme molecule is chemically the same before and after catalysing a reaction, ready to bind another substrate molecule.

Step 2: Recall how an enzyme actually lowers activation energy

Every reaction has an activation energy (the minimum energy that reacting molecules need before they can collide successfully and start converting into products. An enzyme’s active site is a region with a shape (and chemical environment) complementary to its substrate. When the substrate binds, an enzyme-substrate complex forms, either because the active site was already the right shape (the lock-and-key model) or because the active site moulds slightly around the substrate as it binds (the induced-fit model). Forming this complex holds the substrate in the correct orientation and can strain particular bonds within it, providing an alternative reaction pathway that needs less energy to get started than the uncatalysed reaction would. This is what is meant by “lowering the activation energy”) the enzyme does not change the difference in energy between the reactants and the products, only how easily that difference can be reached.

Step 3: Evaluate each option

  • A: incorrect. The enzyme is a catalyst, not a reactant; it is not consumed or broken down, so it cannot release energy in this way.
  • B: correct. This is exactly how enzymes work: forming a temporary enzyme-substrate complex that lowers the activation energy needed for the reaction.
  • C: incorrect. An enzyme does not generate heat or raise the temperature of the solution; its effect is chemical (lowering activation energy), not thermal.
  • D: incorrect. The enzyme itself is not converted into a new substance; it is the substrate that is chemically changed into product, while the enzyme is released unchanged and reused.

Final answer

  • The enzyme increases the rate of reaction by forming an enzyme-substrate complex that lowers the activation energy, option B.