PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
CHAPTER 5
ENZYMES
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
5.1 Enzymes Are Biological
Catalysts
In this section, you will learn the following:
• Describe a catalyst as a substance that can
increase the rate of a chemical reaction,
without being chemically changed by the
reaction.
• Describe enzymes as biological catalysts,
which are made of proteins and are
involved in all metabolic reactions.
• Describe why enzymes and reaction rate
are important in sustaining life in living
organisms.
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Biology Student's Book
What are catalysts?
A catalyst is a substance that can increase the rate of a
chemical reaction, without being chemically changed by the
reaction.
Manganese dioxide can be used to speed
up the breakdown of hydrogen peroxide.
What are enzymes?
• Enzymes are biological catalysts that can increase the
rate of a chemical reaction, without being chemically
changed by the reaction.
• Enzymes are proteins.
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Biology Student's Book
What reactions do enzymes catalyse?
1. Reactions in digestion
• Food is broken down by enzymes during digestion.
• Examples of digestive enzymes and their functions:
Enzyme Function
Amylase Digests starch to maltose
Maltase Digests maltose to glucose
Protease Digests proteins to amino acids
Lipase Digests fats to fatty acids and glycerol
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Biology Student's Book
What reactions do enzymes catalyse?
Only small molecules such as glucose and amino acids can
pass through the cell membrane. (Figure 5.2 of Student’s Book)
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Biology Student's Book
What reactions do enzymes catalyse?
2. Reactions that build up or break down complex substances
• Cells build up or synthesise • Cells break down complex
complex substances from substances to simple substances.
simpler ones.
• Example:
• Example:
• Enzymes catalyse practically all the chemical reactions that occur in an organism, but they are produced
only when needed.
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Biology Student's Book
How are enzymes named?
The name of each enzyme
• shows the substance on which the enzyme acts;
• ends in ase.
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Biology Student's Book
5.2 How Does an Enzyme Work?
In this section, you will learn the following:
• Describe enzyme action in terms of the
complementary shape of the active site to
its substrate and product formation.
• Explain enzyme action in terms of the
active site, enzyme-substrate complex,
substrate and product formation.
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Biology Student's Book
How does an enzyme work?
• The substance on which an enzyme acts is called a
substrate.
• Each enzyme has a groove on its surface called the
active site.
• Only a substrate with a complementary shape can fit into
the active site (just like a key fitting into a lock).
• When the substrate binds to the active site of the
enzyme, an enzyme-substrate complex is formed.
Three-dimensional model of enzyme
pepsin with its active site
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
How does an enzyme work?
The mode of action of an enzyme (Figure 5.6 of Student’s Book)
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Biology Student's Book
5.3 Characteristics of Enzymes
In this section, you will learn the following:
• Investigate and describe the effect of temperature
and pH on enzyme activity.
• Explain enzyme-substrate specificity.
• Explain the effect of temperature on enzyme
activity.
• Explain the effect of pH on enzyme activity.
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Biology Student's Book
What are the characteristics of enzymes?
1. Enzymes catalyse reversible reactions. 3. Every enzyme has an optimum pH at which it is most
active.
• Some reactions in living cells can proceed
in a forward or backward direction. • Extreme changes in the acidity or alkalinity of the solutions
denature the enzymes.
• Example:
At pH 4 or above 9,
2. Enzymes are catalysts. the enzyme is completely
Enzymes speed up the rates of chemical denatured. The active site
of the enzyme loses its
reactions that occur in a cell.
original shape.
Effect of pH on the rate of reaction catalysed by amylase
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Biology Student's Book
What are the characteristics of enzymes?
4. Enzymes are required in minute amounts. 6. Every enzyme has an optimum temperature at which it is
most active.
A small amount of enzyme can catalyse a
large number of chemical reactions. • Increasing the temperature from low to the optimum
temperature increases the rate of enzyme reaction.
• Increasing the temperature above the optimum temperature
5. Enzymes are specific in action.
causes a rapid decrease in the rate of reaction.
Only a substrate with a shape
complementary to that of the active site can fit Increasing the
into the enzyme to form the enzyme-substrate temperature increases the
complex. kinetic motion of
molecules, resulting in an
increased rate of effective
collision between the
enzyme and substrate
molecules. This increases
the proportion of effective
collisions.
Effect of temperature on the rate of enzyme reaction
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Biology Student's Book
What causes enzymes to denature?
• Denaturation is the change in the three-dimensional structure of an enzyme or any other soluble protein,
caused by heat or chemicals.
• Denaturation results in the loss or alteration of the enzyme’s active site, such that the substrate can no longer
fit into the enzyme.
At temperatures above 45°C, some enzymes are denatured.
Enzymes lose their active sites when they are denatured.
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Biology Student's Book
• At very low temperatures, amylase is inactive. The activity of amylase increases as the
temperature increases.
• Beyond a certain temperature, the activity of amylase decreases.
• Amylase is denatured at very high temperatures.
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Biology Student's Book
• In test-tube A, the protein in egg white will be digested to a soluble product.
• There will be no change in test-tube C as the contents in the test-tube are alkaline.
• Test-tube B is the control.
• Test-tube D shows that pepsin is inactive in neutral conditions.
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Biology Student's Book
1. “Without digestive enzymes, some food molecules are useless to the body.” Is this
statement valid? Why or why not?
2. The unripe fruit of the papaya tree contains a particular type of digestive enzyme that is
often used to soften meat.
(a) Infer the kind of organic molecule that this enzyme acts on.
(b) Suggest how the enzyme may cause the softening of meat.
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Biology Student's Book
What have you learnt?
Can you draw your own mind map?
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What have you learnt?
Can you draw your own mind map?
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Biology Student's Book
Wrap-up
Let’s play a quiz on your personal learning device to test your prior knowledge from the past lessons!
Launch Quiz
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Biology Student's Book
Acknowledgements
• Slide 1: washing powder on stained clothes © FotoDuets | Shutterstock.com
• Slide 3: manganese (IV) oxide © MCE
• Slide 9: pepsin molecular structure © molekuul | 123rf.com
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