5.2 Effects of Temperature and pH

2026 📋 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Understand that the progress of enzyme-catalysed reactions can be followed by measuring the concentrations of reactants and products.
  2. Investigate and describe the effects of temperature and pH on enzyme activity.
  3. Explain the effect of changes in temperature and pH on enzyme activity in terms of kinetic energy, shape and fit, denaturation, and the frequency of effective collisions.

📖 Main Content

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1. How Do We Follow the Progress of an Enzyme-Catalysed Reaction?

Before we look at how temperature and pH affect enzymes, it is important to understand how scientists measure what enzymes are actually doing.

An enzyme-catalysed reaction is a chemical reaction that is sped up by an enzyme. In these reactions:

  • Reactants (also called substrates) are the starting chemicals that get broken down or changed.
  • Products are the new chemicals that are made at the end of the reaction.

As the reaction moves forward:

  • The concentration of the reactant decreases — because it is being used up.
  • The concentration of the product increases — because more of it is being made.

Scientists can track (follow) the progress of the reaction by regularly measuring either:

  • How much reactant is left over time, or
  • How much product has been made over time.

Example: Imagine an enzyme breaks down hydrogen peroxide into water and oxygen gas. You can follow the reaction by measuring how much oxygen gas is produced — the more oxygen collected, the further the reaction has gone. Alternatively, you could measure how much hydrogen peroxide remains. Both tell you how fast the enzyme is working.

Key idea: If the enzyme is working well (high activity), the reactant disappears quickly and the product appears quickly. If the enzyme is working poorly (low activity), both changes happen slowly.

2. What is Enzyme Activity?

Enzyme activity simply means how well and how fast an enzyme is doing its job — catalysing (speeding up) a reaction.

Enzyme activity depends on the active site — a specially shaped region on the enzyme where the substrate (reactant) fits in, like a key fitting into a lock. When the substrate fits into the active site, the reaction happens. This is known as the "lock and key" model.

Two main factors that affect enzyme activity (and are on your syllabus) are:

  • Temperature
  • pH

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