Factors Affecting Enzyme Activity

This lesson explores the factors that affect the rate of enzyme-catalysed reactions as required by the Edexcel GCSE Combined Science specification (1SC0). You need to explain how temperature, pH and substrate concentration affect enzyme activity, and describe practical investigations to test these factors.

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Overview of Factors

Three main factors affect the rate of enzyme activity:

1. Temperature
2. pH
3. Substrate concentration

Each factor influences how often the enzyme and substrate molecules collide and how well the substrate fits into the active site.

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Factor 1: Temperature

How Temperature Affects Enzyme Activity

As temperature increases from a low value:

• Molecules gain kinetic energy and move faster.
• There are more frequent collisions between enzyme and substrate molecules.
• More enzyme-substrate complexes form per unit time.
• The rate of reaction increases.

At the optimum temperature, the rate of reaction is at its maximum.

Above the optimum temperature:

• Excess heat energy causes bonds within the enzyme (hydrogen bonds and other weak interactions) to break.
• The active site changes shape — the enzyme is denatured.
• The substrate can no longer fit, so no enzyme-substrate complexes form.
• The rate of reaction drops rapidly and eventually reaches zero.

graph LR
    A[Low temp: slow movement, few collisions] --> B[Increasing temp: more kinetic energy, more collisions]
    B --> C[Optimum temp: maximum rate]
    C --> D[Above optimum: denaturation, rate drops to zero]

Exam Tip: The graph of rate vs temperature is not symmetrical — it rises gradually, peaks at the optimum, then drops sharply. This sharp drop reflects the rapid denaturation of the enzyme.

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Factor 2: pH

How pH Affects Enzyme Activity

• Every enzyme has an optimum pH at which it works fastest.
• Small changes in pH away from the optimum reduce the rate of reaction.
• Large changes in pH (too acidic or too alkaline) cause denaturation — the active site changes shape permanently.
• This is because pH affects the charges on the amino acids in the active site, breaking ionic and hydrogen bonds.

Examples of Optimum pH

Enzyme	Optimum pH	Location
Pepsin	~2	Stomach (acidic environment due to HCl)
Salivary amylase	~7	Mouth (neutral)
Pancreatic lipase	~8	Small intestine (alkaline, aided by bile)

Exam Tip: If a question asks you to explain why an enzyme stops working at extreme pH, state that the bonds holding the active site shape are broken, the active site changes shape, and the substrate can no longer fit — the enzyme is denatured.

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Factor 3: Substrate Concentration

How Substrate Concentration Affects Enzyme Activity

• At low substrate concentration, there are many free active sites. Increasing the substrate concentration means more substrate molecules are available to collide with enzymes, so the rate of reaction increases.
• At high substrate concentration, all active sites are occupied — the enzyme is saturated. Adding more substrate has no effect because there are no free active sites available.
• The rate of reaction plateaus (levels off).

To increase the rate beyond this point, you would need to add more enzyme molecules.

graph LR
    A[Low substrate: many free active sites, rate increases] --> B[More substrate: rate continues to rise]
    B --> C[All active sites occupied: enzyme saturated, rate plateaus]

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Practical Investigation: Effect of Temperature on Amylase Activity

This is a common practical you need to be able to describe.

Method

1. Add starch solution and amylase solution to separate test tubes.
2. Place both test tubes in a water bath at the desired temperature (e.g. 20 °C) and leave for 5 minutes to equilibrate.
3. Mix the amylase and starch solutions together and start a timer.
4. Every 30 seconds, remove a drop of the mixture and add it to a drop of iodine solution on a spotting tile.
5. Iodine turns blue-black in the presence of starch. When the iodine stays brown/orange, all the starch has been digested.
6. Record the time taken for the starch to be fully digested.
7. Repeat at different temperatures (e.g. 20, 30, 40, 50, 60, 70 °C).

Variables

Variable	Detail
Independent	Temperature of the water bath
Dependent	Time taken for starch to be digested (iodine stays orange)
Control variables	Volume and concentration of starch solution, volume and concentration of amylase solution, volume of iodine

Expected Results

• The time decreases as temperature rises toward the optimum (~37 °C for amylase).
• Above the optimum, the time increases (or starch is never fully digested) because the enzyme denatures.

Exam Tip: In this practical, a shorter time means a faster rate of reaction. If asked to calculate rate, use: Rate = 1 ÷ time. Always include repeats and calculate a mean for reliability.

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Practical Investigation: Effect of pH on Amylase Activity

Method

1. Set up spotting tiles with iodine solution.
2. Prepare test tubes containing starch solution mixed with buffer solutions at different pH values (e.g. pH 2, 4, 6, 7, 8, 10).
3. Place all test tubes in a water bath at 37 °C (to keep temperature constant).
4. Add amylase to each test tube and start the timer.
5. Test samples with iodine every 30 seconds. Record the time for starch digestion.

Expected Results

• Amylase works fastest at pH 7 (optimum).
• At extreme pH values (very acidic or very alkaline), the starch is not digested because the amylase is denatured.

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Summary

• Temperature: Increasing temperature increases the rate up to the optimum (more kinetic energy, more collisions). Above the optimum, the enzyme denatures and the rate drops to zero.
• pH: Each enzyme has an optimum pH. Extreme pH values cause denaturation.
• Substrate concentration: Increasing substrate concentration increases the rate until the enzyme is saturated (all active sites occupied), then the rate plateaus.
• The amylase practical uses iodine to test for the presence of starch. A shorter digestion time means a faster rate.
• Always identify independent, dependent and control variables in practical questions.

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Deeper Look: Explaining Each Factor Mechanistically

It is not enough in a GCSE exam to say "higher temperature gives a faster rate". You need to explain why. Each factor affects either the frequency of successful collisions between enzyme and substrate, or the shape of the active site itself.

The Collision Theory of Enzyme Action

Chemical reactions require molecules to collide with the correct orientation and with enough energy. For enzyme-catalysed reactions:

• The substrate must collide with the active site.
• The substrate must have the complementary shape to fit.
• The collision must have enough energy to form the enzyme-substrate complex.