Factors Affecting Enzyme Activity

HSC Biology | Study Notes

This topic matters because enzymes control the rate of important cell reactions, and the syllabus specifically includes modelling enzyme action in cells and investigating how environmental conditions affect enzyme activity.

In this lesson

• how temperature affects enzyme activity

• how pH affects enzyme activity

• how substrate concentration affects enzyme activity

• what denaturation means

• how to explain enzyme trends in exam questions

Why enzyme activity changes

Enzymes are proteins that speed up chemical reactions in cells. Their activity can change depending on the conditions around them.

The main factors on this page are:

• temperature

• pH

• substrate concentration

These factors matter because they affect:

• how often enzyme and substrate collide

• how well the substrate fits into the active site

• whether the enzyme keeps its correct shape

Temperature

Temperature affects the kinetic energy of particles.

Low temperatures

At low temperatures:

• particles move more slowly

• there are fewer successful collisions between enzyme and substrate

• enzyme activity is slower

The enzyme is not damaged, it is just working more slowly.

Rising temperature

As temperature increases:

• particles move faster

• collisions happen more often

• more enzyme-substrate complexes form

• enzyme activity increases

This continues until the enzyme reaches its optimum temperature.

High temperatures

Above the optimum temperature:

• the enzyme begins to lose its specific shape

• the active site changes shape

• the substrate no longer fits properly

• enzyme activity drops quickly

This is because the enzyme becomes denatured.

pH

pH measures how acidic or alkaline a solution is.

Why pH matters

Each enzyme has an optimum pH at which it works best.

If the pH moves too far away from the optimum:

• the shape of the active site may change

• the substrate may not bind as effectively

• enzyme activity decreases

Extreme pH

At very high or very low pH:

• bonds within the enzyme can be disrupted

• the enzyme may denature

• activity may stop

Important point

Different enzymes have different optimum pH values. There is not one ideal pH for all enzymes.

Substrate concentration

Substrate concentration is the amount of substrate available for the enzyme to act on.

Low substrate concentration

When substrate concentration is low:

• there are fewer substrate particles available

• fewer collisions occur

• the reaction rate is low

Increasing substrate concentration

As substrate concentration increases:

• collisions between enzyme and substrate increase

• more active sites are occupied

• reaction rate increases

High substrate concentration

Eventually the rate stops increasing, even if more substrate is added.

This happens because:

• all the enzyme active sites are occupied

• the enzymes are working at their maximum rate

At this point, the enzyme is said to be saturated.

Denaturation

Denaturation is a change in the shape of a protein.

What happens during denaturation

When an enzyme is denatured:

• its three-dimensional shape changes

• the active site changes shape

• the substrate no longer fits properly

• the enzyme can no longer work effectively

Causes of denaturation

For this topic, the main causes are:

• high temperature

• extreme pH

Why denaturation is important

Denaturation explains why enzyme activity can suddenly drop rather than just slowly decline.

Putting the factors together

Temperature trend

• low temperature, low activity

• temperature rises, activity increases

• optimum temperature reached

• above optimum, activity drops quickly due to denaturation

pH trend

• activity is highest at optimum pH

• activity drops on either side of the optimum

• extreme pH may denature the enzyme

Substrate concentration trend

• low substrate concentration, low rate

• increasing substrate concentration, increasing rate

• rate levels off when enzymes become saturated

How to describe enzyme graphs

In exam questions, you often need to describe the trend and then explain it.

Good structure

1. State the trend clearly.

2. Use the correct scientific term, such as optimum, saturation, or denaturation.

3. Explain why the change happens.

Example phrasing

• “As temperature increases, enzyme activity increases until an optimum is reached.”

• “Beyond the optimum temperature, activity decreases because the enzyme denatures.”

• “As substrate concentration increases, the rate increases until it levels off because all active sites are occupied.”

Worked example

Exam-style question

Explain why enzyme activity increases as temperature rises, but then decreases sharply at very high temperatures.

Worked answer

As temperature rises, particles gain kinetic energy, so enzyme and substrate collide more often and more enzyme-substrate complexes form. This increases enzyme activity up to the optimum temperature. At very high temperatures, the enzyme denatures, changing the shape of the active site so the substrate no longer fits, causing activity to decrease sharply.

Why this works

This answer:

• describes the trend

• uses the terms optimum temperature and denatures

• explains both parts of the graph

Common mistakes

• Saying enzymes are killed by low temperatures. Low temperatures usually just slow them down.

• Forgetting that high temperature can denature the enzyme.

• Saying all enzymes have the same optimum pH.

• Confusing denaturation with saturation.

• Saying the reaction rate increases forever as substrate concentration increases.

Quick quiz

1. Why does enzyme activity usually increase as temperature rises at first?

2. What happens to an enzyme above its optimum temperature?

3. Why does pH affect enzyme activity?

4. What does enzyme saturation mean?

5. Which two factors on this page can cause denaturation?

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