Edexcel iGCSE Biology-2.11 temperature changes and enzyme function- Study Notes- New Syllabus

Effect of Temperature on Enzyme Function

📝 Introduction

• Enzymes are proteins that control and speed up chemical reactions inside living organisms.
• Every enzyme has a specially shaped region called the active site.
• The substrate fits into the active site, and the enzyme helps convert it into products.
• Temperature is one of the most important factors that affects how well an enzyme works.
• A small change in temperature can change the speed of the reaction, while a very high temperature can permanently damage the enzyme.
• Every enzyme has an optimum temperature, where it works at its fastest rate.

🧬 Active Site and Enzyme Function

• The active site is the part of an enzyme where the substrate binds.
• The shape of the active site is very specific.
• Only the correct substrate can fit into the active site.
• This is explained by the Lock and Key Model:
  • Enzyme = Lock 
  • Substrate = Key
• If the active site changes shape, the substrate cannot fit properly.
• As a result, the enzyme cannot carry out the reaction.

🌡️ Why Does Temperature Affect Enzymes?

• Temperature changes affect enzyme activity because they change the movement of molecules.

When temperature changes:

• The kinetic energy of enzyme and substrate molecules changes.
• The speed at which molecules move changes.
• The number of collisions between enzymes and substrates changes.
• Very high temperatures can also change the shape of the enzyme’s active site.

❄️ Effect of Low Temperature (Below the Optimum Temperature)

• At low temperatures, enzymes still work, but much more slowly.

What happens?

• Molecules have low kinetic energy.
• Enzyme molecules move slowly.
• Substrate molecules also move slowly.
• Because they move slowly, they collide less often.

Effect on Enzyme Activity

• Fewer enzyme-substrate complexes are formed.
• Less product is produced in a given time.
• The reaction becomes slow.

Active Site

• The active site keeps its normal shape.
• The substrate can still fit into the active site.
• The enzyme is not damaged.

Important Points

• Low temperature does not denature the enzyme.
• The enzyme simply becomes less active.
• If the temperature is increased again, the enzyme starts working faster.

🌟 Effect at the Optimum Temperature

What is Optimum Temperature?

• The optimum temperature is the temperature at which an enzyme works most efficiently.
• At this temperature, the rate of reaction is at its highest.

In Human Enzymes

• Most human enzymes have an optimum temperature of about 37°C.
• This is the normal body temperature.

What happens at the Optimum Temperature?

• Enzyme molecules move quickly.
• Substrate molecules also move quickly.
• Collisions between enzymes and substrates happen very frequently.
• More enzyme-substrate complexes are formed.
• The active site keeps its correct shape.
• The substrate fits perfectly into the active site.
• The maximum amount of product is produced.

Result

• This is the fastest rate of enzyme activity.

🔥 Effect of High Temperature (Above the Optimum Temperature)

• As the temperature continues to rise above the optimum, enzyme activity begins to decrease.

What happens first?

• Molecules gain a lot of kinetic energy.
• They move and vibrate very rapidly.

What happens to the enzyme?

• The weak bonds that hold the enzyme’s shape begin to break.
• The enzyme loses its original three-dimensional shape.

What happens to the active site?

• The active site changes shape.
• It is no longer complementary to the substrate.
• The substrate cannot fit into the active site.

Result

• Enzyme-substrate complexes cannot form.
• The reaction slows down quickly.
• Eventually, the reaction stops completely.

🚫 Denaturation

• When an enzyme loses its normal shape because of high temperature, it is said to be denatured.

What is Denaturation?

• Denaturation is the permanent change in the shape of an enzyme’s active site.
• It usually happens when the enzyme is exposed to temperatures above its optimum.

What happens after denaturation?

• The substrate can no longer bind to the active site.
• No enzyme-substrate complex can form.
• No product is produced.
• The enzyme can no longer perform its function.

Important Points

• Denaturation is usually permanent.
• Cooling the enzyme afterwards does not restore its original shape.
• Once denatured, the enzyme cannot work again.

🔬 Effect of Temperature on the Active Site

At Low Temperature

• Active site keeps its normal shape.
• Substrate fits correctly.
• Reaction is slow because molecules move slowly.

At Optimum Temperature

• Active site has the perfect shape.
• Substrate fits easily.
• Maximum enzyme activity occurs.

At High Temperature

• Active site changes shape.
• Substrate cannot fit.
• Reaction stops because the enzyme is denatured.

📈 Temperature and Enzyme Activity Graph

• The graph of enzyme activity against temperature has a characteristic pattern.

Low Temperature

• Enzyme activity starts at a low level.
• As temperature increases, activity increases gradually.

Near the Optimum Temperature

• Activity increases rapidly.
• The graph reaches its highest point.
• This is where the enzyme works best.

Above the Optimum Temperature

• Activity falls sharply.
• This happens because enzymes become denatured.
• The graph drops quickly to a very low level.

Remember

• The graph rises gradually.
• The graph falls suddenly.
• This gives the graph an asymmetrical shape.

🧪 Examples

Human Amylase

• Human amylase works best at about 37°C.
• This matches normal human body temperature.
• At temperatures much higher than this, amylase becomes denatured.

Enzymes in Thermophilic Bacteria

• Thermophilic bacteria live in hot environments such as hot springs.
• Their enzymes have a much higher optimum temperature, often around 70°C.
• These enzymes remain stable at temperatures that would denature human enzymes.
• They are useful in many industrial processes that require high temperatures.

💡 Important Points

• Temperature changes the kinetic energy of molecules.
• Low temperature slows down enzyme activity but does not damage the enzyme.
• At the optimum temperature, enzyme activity is at its maximum.
• High temperature changes the shape of the active site.
• A changed active site prevents the substrate from binding.
• Denatured enzymes stop working permanently.
• Different organisms have enzymes with different optimum temperatures depending on where they live.

📊 Summary Table – Effect of Temperature on Enzymes

⚡ Quick Recap
✔ Enzymes are proteins that act as biological catalysts.
✔ Every enzyme has a specific active site where the substrate binds.
✔ Low temperature slows the movement of molecules, so fewer collisions occur and the reaction is slow.
✔ The enzyme is not damaged at low temperatures and works normally again when warmed.
✔ Optimum temperature is the temperature at which the enzyme works at its fastest rate.
✔ Most human enzymes have an optimum temperature of 37°C.
✔ High temperature causes the enzyme to vibrate strongly, breaking the weak bonds that maintain its shape.
✔ This changes the shape of the active site, so the substrate can no longer fit.
✔ The enzyme becomes denatured, and the reaction stops permanently.
✔ The enzyme activity graph shows a gradual increase, reaches a peak at the optimum temperature, and then falls sharply due to denaturation.