EdPlace's GCSE Home Learning Biology Lesson: Enzymes

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Now...onto the lesson!

Enzymes: Controlling all Reactions in Living Organisms

Biology is often seen as the easier of the sciences by many students, but this is a mistake that leads some students to not learn content in enough detail. During assessments, this can result in students not being able to fully apply and explain their understanding when presented with new examples. Enzymes are vital in Biology and students should ensure they learn them in detail as they're likely to appear in any terminal Biology GCSE examination.

Whilst initially enzymes may appear to be a small and straight forward topic, they're one of the most commonly assessed application and practical questions for GCSE Biology courses. This means that students must know how an enzyme works, and be able to link this to data on the effects of temperature or pH on the rate of reactions. 

As a parent, this may seem daunting but all temperature practical questions for enzymes will be looking for students to apply the same basic understanding of enzymes. This is also important information that needs to be taken forward by those planning on studying A-Level Biology.

At EdPlace we’re surrounded by a team of experts who communicate these concepts with children on a day-to-day basis, and we’re ready to share their teaching gems with you. We're confident if you follow this step by step approach your child will:

1) Understand how enzymes work via Lock and Key Theory

2) Apply Lock and Key Theory to explain how temperature affects the rate of enzyme-controlled reactions 

3) Analyse and Explain the temperature-rate of a reaction graph

Step 1: Breaking Down the Terminology

Before we look at how temperature affects the rate of enzyme-controlled reactions, we must first understand some key scientific terminology. These terms will be reinforced during this step-by-step approach, so that your child is able to recall their meanings.

An enzyme is a biological catalyst. This means that they speed up the rate of chemical reactions in cells.

The active site is where the reaction happens in an enzyme. It has a specific shape.

The substrate is the molecule involved in the reaction which fits into the active site of the enzyme. 

Enzymes can become denatured which is effectively when the enzyme changes shape permanently and no longer works.

The rate of reaction is how fast a reaction happens.

Step 2: Prior Knowledge Needed

For all living things to stay alive they must continually complete chemical reactions in their cells. All reactions happen in the cytoplasm of the cell and are controlled by enzymes.

Enzymes are biological catalysts. This means they speed up the rate of reaction and do so by lowering the activation energy of a reaction (the energy needed to start a reaction).

Students should already know that enzymes are an important part of digestion breaking large insoluble molecules down to smaller soluble molecules. They should be able to recall the three following digestive enzymes:

Step 3: Introducing the New Skill

Enzymes are specific to the substrate they work with. So, for example in the table above, only the enzyme protease will digest proteins into amino acids. Protease cannot digest starch or lipids.

This can be explained by looking a little closer at how an enzyme works:

Enzymes have a specific shaped area called the active site. 

This is where the substrate (B) attaches to the enzyme allowing the reaction to happen.

As you will see in the diagram the substrate fits into the active site as they have a complementary shape. A differently shaped substrate will not fit into the active site and reaction will not continue.

This is called the Lock and Key Theory which states that enzymes are specific to one type of substrate as they have an active site that has a complementary shape to one substrate type only.

Once the substrate has fitted perfectly into the active site it forms an enzyme-substrate complex (C). Here, the enzyme causes the reaction to happen, in this example pulling the large substrate molecule into two smaller molecules by breaking the bonds holding it together.

The new products formed will now have a new shape and are released from the active site as a result (D). The enzyme is then ready to accept another substrate and can be constantly reused.

The effect of temperature on the rate of enzyme-controlled reactions:

Students need to understand how temperature affects the rate of enzyme-controlled reactions. All enzymes have an optimum temperature, which is the temperature at which the most enzyme-substrate complexes form. As you move away from this temperature the rate of reaction falls.

Students need to be able to explain what they can see on a graph, plotting the temperature against the rate of reaction for any practical. This may sound daunting, but the graph is always identical.

This graph is used to assess a student’s understanding of enzyme structure and the effect of temperature on it. Most students understand it, but often only half answer a question based upon this.

Imagine the student is given the following exam question:

Use the graph to describe and explain the effect of temperature on the rate of enzyme activity (6 marks).

To gain the full six marks students must talk about the graph both before and after the optimum temperature. Often students only remember to explain the effect of higher temperatures and while they may do this in detail, it'll only give them 3 of the possible 6 marks.

Effect of increasing temperature from 0°C to 40°C:

As the temperature increases so does the rate of enzyme activity.

This is because the enzymes and the substrates gain more kinetic energy and move at a faster rate.

Therefore, they collide more frequently and form more enzyme-substrate complexes. 

The optimum temperature for this enzyme is 40°C which is where the rate of making new enzyme-substrate complexes is highest.

Effect of increasing temperature above 40°C (optimum temperature):

Above the optimum temperature, the rate of enzyme activity rapidly decreases.

This is because the particles make the enzyme vibrate which breaks bonds, changing the shape of the active site.

The active site is no longer a complementary shape to the substrate and the substrate no longer fits into the enzyme, so the reaction stops. The enzyme is denatured, and this is permanent.

Step 4: Give it a Go!

Why not apply what you've learnt from this step-by-step lesson to attempt to the following questions?

a) Explain what Lock and Key Theory is (3 marks)

b) Why does the rate of an enzyme-controlled reaction increase up to the optimum temperature? (3 marks)

c) Why does the rate of reaction drop above the optimum temperature of an enzyme? (3 marks)

Stretch and challenge:

d) Why is human body temperature important to maintain at about 37°C? (4 marks)

e) Enzymes also have a specific pH. What may happen to the structure of an enzyme in the wrong pH which will decrease the rate of enzyme activity? (3 marks)

Step 5 - Putting it into Practice

Now, you’ve covered this together why not put this to the test and assign your child the following 5 activities in this order.

All activities are created by teachers and automatically marked. Plus, with an EdPlace subscription, we can automatically progress your child at a level that's right for them. Sending you progress reports along the way so you can track and measure progress, together - brilliant!

Activity 1 - Identify and Describe Key Features of Eukaryotes and Prokaryotes

Activity 2 - Describe Enzyme Function

Activity 3 - Understand Enzyme Function

Activity 4 - Describe the Enzymes in the Digestive System

Activity 5 - Understand Hormones and Homeostasis

Answers

a) The Lock and Key Theory explains why enzymes increase the rate of reaction for one substrate only (specific). The active site of the enzyme has a complementary shape in which the substrate fits perfectly. This forms an enzyme-substrate complex and then the reaction can proceed.

b) The rate of an enzyme-controlled reaction increases up to the optimum temperature because as the temperature increases, the enzymes and the substrates gain more kinetic energy. As a result, they collide more frequently and form more enzyme-substrate complexes. 

c) The rate of reaction decreases above the optimum temperature of an enzyme because the energy gains too much energy and its particles vibrate to the point where they break bonds in the enzyme. This causes the active site to change shape, so it's no longer a complementary shape to the substrate. The substrate can no longer fit into the active site and no more enzyme-substrate complexes can form. Therefore, the reaction stops.

Stretch and challenge:

d) Human body temperature needs to be maintained at about 37°C because this is the optimum temperature for the enzymes in our cells. If the temperature drops too much below this the enzymes and substrates will have less kinetic energy and collide less frequently reacting slower. If the temperature increases too high above this, the enzymes will become denatured and the active site changes shape. This means they can no longer form enzyme-substrate complexes and the reactions stop.

e) Enzymes have a specific pH which is where they form the most enzyme-substrate complexes and have the highest rate of reaction. At the wrong pH, enzymes are denatured and their active site changes shape. This means the active site is no longer a complementary shape to the substrate which can no longer fit into it. Therefore, no more enzyme-substrate complexes can form and the rate of reaction decreases.

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