Enzymes

What you'll learn

What enzymes are and how their active sites make them specific.
How temperature, pH and substrate concentration affect enzyme activity.
How to investigate the effect of pH on amylase activity.
How to calculate reaction rates and link enzymes to food molecules.

Enzymes: the basic idea

Living cells carry out thousands of chemical reactions. A chemical reaction is a process where substances are changed into new substances.

Many reactions in the body would happen far too slowly at normal body temperatures, so cells use enzymes to speed them up.

Definition

Enzyme

An enzyme is a biological catalyst: a substance made by living organisms that speeds up a chemical reaction without being used up.

A catalyst is not changed overall by the reaction, so it can be used again. This is why small amounts of enzyme can control lots of reactions.

Key Idea

Why enzymes matter

Enzymes make reactions fast enough for life, but they do not get used up and they do not change what the final products are.

Active sites and enzyme specificity

Enzymes are proteins with a very precise 3D shape. The part of the enzyme where the reaction happens is called the active site.

The substance that fits into the active site is called the substrate. The substances made by the reaction are called the products.

When the substrate fits into the active site, an enzyme-substrate complex forms. The enzyme then helps the substrate change into products, which leave the active site. The enzyme is unchanged and can be used again.

This diagram shows how a matching substrate fits the active site, while a non-matching substrate cannot fit.

Diagram showing enzyme active site, substrate, enzyme-substrate complex, products, and enzyme specificity

Definition

Specificity

Enzyme specificity means that an enzyme usually catalyses only one reaction, because only a substrate with the correct shape can fit its active site.

Analogy

Lock and key

The active site is like a lock and the substrate is like a key: only the correctly shaped key fits well enough for the reaction to happen.

Denaturation: when enzymes stop working

An enzyme’s function depends on the shape of its active site. If the active site changes shape, the substrate may no longer fit.

Definition

Denatured enzyme

A denatured enzyme has had its active site changed so the substrate can no longer bind properly, causing the enzyme to stop working.

High temperatures and extreme pH values can denature enzymes. This happens because the bonds holding the enzyme protein in its correct 3D shape are disrupted.

Low temperatures do not usually denature enzymes. They slow reactions down because particles have less kinetic energy, but the enzyme’s shape is not permanently damaged.

Common Mistake

Low temperature does not denature

Do not say “cold temperatures denature enzymes”. Cold temperatures reduce enzyme activity by reducing particle movement and collisions.

Factors affecting enzyme activity

Enzyme activity means how fast an enzyme-controlled reaction happens. You can think of it as the reaction rate.

The three main factors you need here are temperature, pH, and substrate concentration. The graphs below show the classic GCSE patterns.

Graphs showing effects of temperature, pH and substrate concentration on enzyme activity

Temperature

At low temperatures, enzyme and substrate particles have less kinetic energy, so they move more slowly. There are fewer successful collisions, so fewer enzyme-substrate complexes form each second.

As temperature increases, particles move faster, so collisions happen more often. Enzyme activity increases up to the optimum temperature, where the enzyme works fastest.

Above the optimum temperature, the enzyme starts to denature. The active site changes shape, so the substrate no longer fits properly and the rate falls quickly.

Example

Interpreting a temperature curve

An enzyme has its highest activity at 37 °C. Predict what happens at 20 °C and 70 °C.

Compare 20 °C with the optimum: 20 °C is below 37 °C, so particles have less kinetic energy and form fewer enzyme-substrate complexes.
Compare 70 °C with the optimum: 70 °C is far above 37 °C, so the enzyme is likely to denature and the active site changes shape.
Decide the effect on activity: at 20 °C the enzyme works slowly, but at 70 °C it may stop working because the substrate can no longer fit the active site.

pH

pH is a scale showing how acidic or alkaline a solution is. Low pH is acidic, pH 7 is neutral, and high pH is alkaline.

Each enzyme has an optimum pH where it works best. If the pH is too far from the optimum, the active site can change shape and the enzyme may denature.

For example, an enzyme in the stomach may work best in acidic conditions, while enzymes in the small intestine often work best in neutral or slightly alkaline conditions.

Substrate concentration

Substrate concentration means how much substrate is present in a certain volume.

At low substrate concentration, increasing the substrate concentration usually increases the rate. This is because enzyme active sites are more likely to meet substrate molecules.

At high substrate concentration, the rate levels off. This happens because all the enzyme active sites are occupied. Adding more substrate cannot increase the rate unless more enzyme is added.

Definition

Limiting factor

A limiting factor is the factor that is preventing the reaction rate from increasing further.

Example

Identifying the limiting factor

A student doubles the substrate concentration, but the reaction rate stays the same.

Compare the change: substrate concentration increased, but the rate did not increase.
Apply the active site idea: the enzyme active sites were probably already fully occupied.
Identify the limiting factor: substrate concentration is no longer limiting; enzyme concentration is likely to be the limiting factor.

Core Practical: investigating pH and enzyme activity

In this practical, you investigate how pH affects the activity of amylase. Amylase is an enzyme that breaks down starch into sugars.

You use iodine solution to test for starch. Iodine is orange-brown when starch is absent and turns blue-black when starch is present.

A typical method is:

Place drops of iodine solution into the wells of a spotting tile.
Add amylase solution, starch solution and a buffer solution of a chosen pH to a test tube.
Start a timer as soon as the enzyme and substrate are mixed.
Every set time interval, place a drop of the reaction mixture onto a drop of iodine.
Record the time when iodine no longer turns blue-black. This means the starch has been broken down.
Repeat using buffer solutions with different pH values.

A buffer solution keeps the pH constant. Control variables include temperature, volumes of solutions, enzyme concentration and starch concentration.

Tip

What the colour change means

Blue-black means starch is still present. Orange-brown means the starch has been broken down, so the enzyme has completed the reaction.

Common Mistake

Time is not the same as rate

A shorter time means a faster reaction. If you are comparing enzyme activity, convert time into rate using a reciprocal.

Rate calculations for enzyme activity

If you measure the time taken for a reaction to finish, a simple estimate of rate is:

taken\text{rate} = \frac{1}{\text{time taken}}

So if the reaction takes less time, the rate is bigger.

Example

Calculating enzyme activity

In an amylase practical, starch disappears after 30 s at pH 7 and after 120 s at pH 4. Compare the rates.

Use the reciprocal because the measurement is time taken for the starch to disappear:

taken\text{rate} = \frac{1}{\text{time taken}}

Calculate the rate at pH 7:

s−1\begin{aligned} \text{rate} &= \frac{1}{30\ \text{s}} \\ &= 0.033\ \text{s}^{-1} \end{aligned}

Calculate the rate at pH 4 and compare:

s−1\begin{aligned} \text{rate} &= \frac{1}{120\ \text{s}} \\ &= 0.0083\ \text{s}^{-1} \end{aligned}

The enzyme is faster at pH 7 because 0.033 per second is greater than 0.0083 per second.

Enzymes in synthesis and breakdown

Enzymes are important in both building large molecules and breaking them down.

In synthesis, small molecules are joined together to make larger molecules:

Small molecules	Larger molecule made
Sugars	Carbohydrates
Amino acids	Proteins
Fatty acids and glycerol	Lipids

In breakdown, large molecules are broken into smaller molecules. This is especially important in digestion:

Enzyme type	Molecule broken down	Products
Carbohydrase	Carbohydrates	Sugars
Protease	Proteins	Amino acids
Lipase	Lipids	Fatty acids and glycerol

Key Idea

Enzymes can build or break

Enzymes are not just for digestion. They also help cells build carbohydrates, proteins and lipids for growth, repair and storage.

Extra links you may meet in Biology

If you are doing Separate Biology, this section also links to testing foods and measuring energy in food.

Food tests use chemical reagents to identify biological molecules:

Starch: iodine solution changes from orange-brown to blue-black.
Reducing sugars: Benedict’s solution is heated and changes from blue to green, yellow, orange or brick-red.
Protein: Biuret reagent changes from blue to lilac or purple.
Fats/lipids: ethanol emulsion test gives a cloudy white emulsion if lipids are present.

Calorimetry estimates the energy in food. A known mass of food is burned, and the heat released warms a known volume of water. The temperature rise of the water is used to estimate energy transferred.

rise\text{energy transferred} = \text{mass of water} \times 4.2 \times \text{temperature rise}

Example

Calculating energy in food

A 0.50 g crisp heats 20 g of water from 20 °C to 34 °C. Estimate the energy released per gram.

Calculate the temperature rise:

34∘C−20∘C=14∘C34^\circ\text{C} - 20^\circ\text{C} = 14^\circ\text{C}

Substitute into the energy equation:

J\begin{aligned} \text{energy} &= 20\ \text{g} \times 4.2 \times 14^\circ\text{C} \\ &= 1176\ \text{J} \end{aligned}

Divide by the mass of food burned:

J/g\begin{aligned} \text{energy per gram} &= \frac{1176\ \text{J}}{0.50\ \text{g}} \\ &= 2352\ \text{J/g} \end{aligned}

Common Mistake

Calorimetry is an estimate

Not all the heat from burning food transfers to the water. Some is lost to the air, so school calorimetry often underestimates the true energy content.

Exam technique

In the exam

Always link enzyme activity to the active site: if the shape changes, the substrate no longer fits.
When explaining temperature, separate low temperature effects from high temperature denaturation.
For practical questions, name the independent variable, dependent variable and at least two control variables.
If time is measured, remember that shorter time means higher rate, and use the reciprocal if asked for enzyme activity.

Self review

Check yourself

Why does an enzyme usually work on only one substrate?
How can high temperature and extreme pH reduce enzyme activity?
In the amylase practical, what does it mean when iodine stays orange-brown?

What you'll learn

What enzymes are and how their active sites make them specific.
How temperature, pH and substrate concentration affect enzyme activity.
How to investigate the effect of pH on amylase activity.
How to calculate reaction rates and link enzymes to food molecules.

Enzymes: the basic idea

Living cells carry out thousands of chemical reactions. A chemical reaction is a process where substances are changed into new substances.

Many reactions in the body would happen far too slowly at normal body temperatures, so cells use enzymes to speed them up.

Definition

Enzyme

An enzyme is a biological catalyst: a substance made by living organisms that speeds up a chemical reaction without being used up.

A catalyst is not changed overall by the reaction, so it can be used again. This is why small amounts of enzyme can control lots of reactions.

Key Idea

Why enzymes matter

Enzymes make reactions fast enough for life, but they do not get used up and they do not change what the final products are.

Active sites and enzyme specificity

Enzymes are proteins with a very precise 3D shape. The part of the enzyme where the reaction happens is called the active site.

The substance that fits into the active site is called the substrate. The substances made by the reaction are called the products.

This diagram shows how a matching substrate fits the active site, while a non-matching substrate cannot fit.

Diagram showing enzyme active site, substrate, enzyme-substrate complex, products, and enzyme specificity

Definition

Specificity

Enzyme specificity means that an enzyme usually catalyses only one reaction, because only a substrate with the correct shape can fit its active site.

Analogy

Lock and key

The active site is like a lock and the substrate is like a key: only the correctly shaped key fits well enough for the reaction to happen.

Denaturation: when enzymes stop working

An enzyme’s function depends on the shape of its active site. If the active site changes shape, the substrate may no longer fit.

Definition

Denatured enzyme

A denatured enzyme has had its active site changed so the substrate can no longer bind properly, causing the enzyme to stop working.

High temperatures and extreme pH values can denature enzymes. This happens because the bonds holding the enzyme protein in its correct 3D shape are disrupted.

Low temperatures do not usually denature enzymes. They slow reactions down because particles have less kinetic energy, but the enzyme’s shape is not permanently damaged.

Common Mistake

Low temperature does not denature

Do not say “cold temperatures denature enzymes”. Cold temperatures reduce enzyme activity by reducing particle movement and collisions.

Factors affecting enzyme activity

Enzyme activity means how fast an enzyme-controlled reaction happens. You can think of it as the reaction rate.

The three main factors you need here are temperature, pH, and substrate concentration. The graphs below show the classic GCSE patterns.

Graphs showing effects of temperature, pH and substrate concentration on enzyme activity

Temperature

At low temperatures, enzyme and substrate particles have less kinetic energy, so they move more slowly. There are fewer successful collisions, so fewer enzyme-substrate complexes form each second.

As temperature increases, particles move faster, so collisions happen more often. Enzyme activity increases up to the optimum temperature, where the enzyme works fastest.

Above the optimum temperature, the enzyme starts to denature. The active site changes shape, so the substrate no longer fits properly and the rate falls quickly.

Example

Interpreting a temperature curve

An enzyme has its highest activity at 37 °C. Predict what happens at 20 °C and 70 °C.

Compare 20 °C with the optimum: 20 °C is below 37 °C, so particles have less kinetic energy and form fewer enzyme-substrate complexes.
Compare 70 °C with the optimum: 70 °C is far above 37 °C, so the enzyme is likely to denature and the active site changes shape.
Decide the effect on activity: at 20 °C the enzyme works slowly, but at 70 °C it may stop working because the substrate can no longer fit the active site.

pH

pH is a scale showing how acidic or alkaline a solution is. Low pH is acidic, pH 7 is neutral, and high pH is alkaline.

Each enzyme has an optimum pH where it works best. If the pH is too far from the optimum, the active site can change shape and the enzyme may denature.

For example, an enzyme in the stomach may work best in acidic conditions, while enzymes in the small intestine often work best in neutral or slightly alkaline conditions.

Substrate concentration

Substrate concentration means how much substrate is present in a certain volume.

At low substrate concentration, increasing the substrate concentration usually increases the rate. This is because enzyme active sites are more likely to meet substrate molecules.

At high substrate concentration, the rate levels off. This happens because all the enzyme active sites are occupied. Adding more substrate cannot increase the rate unless more enzyme is added.

Definition

Limiting factor

A limiting factor is the factor that is preventing the reaction rate from increasing further.

Example

Identifying the limiting factor

A student doubles the substrate concentration, but the reaction rate stays the same.

Compare the change: substrate concentration increased, but the rate did not increase.
Apply the active site idea: the enzyme active sites were probably already fully occupied.
Identify the limiting factor: substrate concentration is no longer limiting; enzyme concentration is likely to be the limiting factor.

Core Practical: investigating pH and enzyme activity

In this practical, you investigate how pH affects the activity of amylase. Amylase is an enzyme that breaks down starch into sugars.

You use iodine solution to test for starch. Iodine is orange-brown when starch is absent and turns blue-black when starch is present.

A typical method is:

Place drops of iodine solution into the wells of a spotting tile.
Add amylase solution, starch solution and a buffer solution of a chosen pH to a test tube.
Start a timer as soon as the enzyme and substrate are mixed.
Every set time interval, place a drop of the reaction mixture onto a drop of iodine.
Record the time when iodine no longer turns blue-black. This means the starch has been broken down.
Repeat using buffer solutions with different pH values.

A buffer solution keeps the pH constant. Control variables include temperature, volumes of solutions, enzyme concentration and starch concentration.

Tip

What the colour change means

Blue-black means starch is still present. Orange-brown means the starch has been broken down, so the enzyme has completed the reaction.

Common Mistake

Time is not the same as rate

A shorter time means a faster reaction. If you are comparing enzyme activity, convert time into rate using a reciprocal.

Rate calculations for enzyme activity

If you measure the time taken for a reaction to finish, a simple estimate of rate is:

taken\text{rate} = \frac{1}{\text{time taken}}

So if the reaction takes less time, the rate is bigger.

Example

Calculating enzyme activity

In an amylase practical, starch disappears after 30 s at pH 7 and after 120 s at pH 4. Compare the rates.

Use the reciprocal because the measurement is time taken for the starch to disappear:

taken\text{rate} = \frac{1}{\text{time taken}}

Calculate the rate at pH 7:

s−1\begin{aligned} \text{rate} &= \frac{1}{30\ \text{s}} \\ &= 0.033\ \text{s}^{-1} \end{aligned}

Calculate the rate at pH 4 and compare:

s−1\begin{aligned} \text{rate} &= \frac{1}{120\ \text{s}} \\ &= 0.0083\ \text{s}^{-1} \end{aligned}

The enzyme is faster at pH 7 because 0.033 per second is greater than 0.0083 per second.

Enzymes in synthesis and breakdown

Enzymes are important in both building large molecules and breaking them down.

In synthesis, small molecules are joined together to make larger molecules:

Small molecules	Larger molecule made
Sugars	Carbohydrates
Amino acids	Proteins
Fatty acids and glycerol	Lipids

In breakdown, large molecules are broken into smaller molecules. This is especially important in digestion:

Enzyme type	Molecule broken down	Products
Carbohydrase	Carbohydrates	Sugars
Protease	Proteins	Amino acids
Lipase	Lipids	Fatty acids and glycerol

Key Idea

Enzymes can build or break

Enzymes are not just for digestion. They also help cells build carbohydrates, proteins and lipids for growth, repair and storage.

Extra links you may meet in Biology

If you are doing Separate Biology, this section also links to testing foods and measuring energy in food.

Food tests use chemical reagents to identify biological molecules:

Starch: iodine solution changes from orange-brown to blue-black.
Reducing sugars: Benedict’s solution is heated and changes from blue to green, yellow, orange or brick-red.
Protein: Biuret reagent changes from blue to lilac or purple.
Fats/lipids: ethanol emulsion test gives a cloudy white emulsion if lipids are present.

rise\text{energy transferred} = \text{mass of water} \times 4.2 \times \text{temperature rise}

Example

Calculating energy in food

A 0.50 g crisp heats 20 g of water from 20 °C to 34 °C. Estimate the energy released per gram.

Calculate the temperature rise:

34∘C−20∘C=14∘C34^\circ\text{C} - 20^\circ\text{C} = 14^\circ\text{C}

Substitute into the energy equation:

J\begin{aligned} \text{energy} &= 20\ \text{g} \times 4.2 \times 14^\circ\text{C} \\ &= 1176\ \text{J} \end{aligned}

Divide by the mass of food burned:

J/g\begin{aligned} \text{energy per gram} &= \frac{1176\ \text{J}}{0.50\ \text{g}} \\ &= 2352\ \text{J/g} \end{aligned}

Common Mistake

Calorimetry is an estimate

Not all the heat from burning food transfers to the water. Some is lost to the air, so school calorimetry often underestimates the true energy content.

Exam technique

In the exam

Always link enzyme activity to the active site: if the shape changes, the substrate no longer fits.
When explaining temperature, separate low temperature effects from high temperature denaturation.
For practical questions, name the independent variable, dependent variable and at least two control variables.
If time is measured, remember that shorter time means higher rate, and use the reciprocal if asked for enzyme activity.

Self review

Check yourself

Why does an enzyme usually work on only one substrate?
How can high temperature and extreme pH reduce enzyme activity?
In the amylase practical, what does it mean when iodine stays orange-brown?

What you'll learn

Enzymes: the basic idea

Active sites and enzyme specificity

Denaturation: when enzymes stop working

Factors affecting enzyme activity

Temperature

pH

Substrate concentration

Core Practical: investigating pH and enzyme activity

Rate calculations for enzyme activity

Enzymes in synthesis and breakdown

Extra links you may meet in Biology

Recap questions

Review Enzymes by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Enzymes

What you'll learn

Enzymes: the basic idea

Active sites and enzyme specificity

Denaturation: when enzymes stop working

Factors affecting enzyme activity

Temperature

pH

Substrate concentration

Core Practical: investigating pH and enzyme activity

Rate calculations for enzyme activity

Enzymes in synthesis and breakdown

Extra links you may meet in Biology

Recap questions

Review Enzymes by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Enzymes

What you'll learn

Enzymes: the basic idea

Active sites and enzyme specificity

Denaturation: when enzymes stop working

Factors affecting enzyme activity

Temperature

pH

Substrate concentration

Core Practical: investigating pH and enzyme activity

Rate calculations for enzyme activity

Enzymes in synthesis and breakdown

Extra links you may meet in Biology

Recap questions

Review Enzymes by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Enzymes Revision Guide

Enzymes

What you'll learn

Enzymes: the basic idea

Active sites and enzyme specificity

Denaturation: when enzymes stop working

Factors affecting enzyme activity

Temperature

pH

Substrate concentration

Core Practical: investigating pH and enzyme activity

Rate calculations for enzyme activity

Enzymes in synthesis and breakdown

Extra links you may meet in Biology

Recap questions

Review Enzymes by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Enzymes Revision Guide