This document provides information about biological molecules and enzymes for an International GCSE Biology exam. It defines carbohydrates, proteins, and lipids as the three main types of biological molecules, and describes their structures and components. It also explains enzyme function using the lock and key theory and discusses how enzyme activity can be affected by changes in temperature and pH. Practical investigations are described for testing food samples for glucose, starch, protein and fat, as well as experiments on how enzyme activity of amylase is influenced by temperature and pH changes.

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INTERNATIONAL
GCSE
Biology (9-1)
© 2020 MR

2. Biological molecules
Learning objectives:
 Be able to describe the structure of carbohydrates, proteins and lipids.
 Be able to investigate food samples for the presence of glucose, starch,
protein and fat.
 Be able to understand the role of enzymes and factors that affect them.
 Be able to investigate how enzyme activity can be affected by changes in
temperature and pH.
Starter:
1. What are the five levels of organisation that make up an organism starting
from the biggest to smallest?
2. Describe the similarities and differences in plant and animal cells?
3. Explain the problems around using stem cells?

3. From the specification
(c) Biological molecules
2.7 identify the chemical elements present in carbohydrates, proteins and lipids (fats
and oils)
2.8 describe the structure of carbohydrates, proteins and lipids as large molecules
made up from smaller basic units: starch and glycogen from simple sugars, protein
from amino acids, and lipid from fatty acids and glycerol
2.9 practical: investigate food samples for the presence of glucose, starch, protein
and fat
2.10 understand the role of enzymes as biological catalysts in metabolic reactions
2.11 understand how temperature changes can affect enzyme function, including
changes to the shape of active site
2.12 practical: investigate how enzyme activity can be affected by changes in
temperature
2.13 understand how enzyme function can be affected by changes in pH altering the
active site
2.14B practical: investigate how enzyme activity can be affected by
changes in pH

4. Biological molecules
There are three main groups of biological molecules: carbohydrates, proteins
and lipids.
These molecules are examples of organic compounds as they contain the
element Carbon. The table below indicates other elements that they share.
Biological
molecule
Carbon Hydrogen Oxygen Nitrogen
Protein ✓ ✓ ✓ ✓
Lipid ✓ ✓ ✓ x
Carbohydrate ✓ ✓ ✓ x

5. Carbohydrates
These molecules provide energy for living organisms.
The basic units of these molecules are simple sugars
which can join together to make complex sugars.
Carbohydrates
Monosaccharides (simple sugar)
Disaccharides
Polysaccharides (complex sugars)

6. Monosaccharides
glucose fructose galactose
They are fast-acting sources of energy.

7. Disaccharides
Formed when two monosaccharides join together.
sucrose lactose maltose

8. Polysaccharides
These molecules are important for the slow release of
energy.
Polysaccharide Role
Found in the plant cell wall, can’t be digested by
humans so acts as a fiber to create bulk to help
muscles of the intestinal wall push food through
the digestive system.
Is how glucose is stored in plants; it can be broken
down to release energy for the plant.
Excess glucose in animals is stored as this molecule
in the liver and can be broken down to release
glucose.
GlycogenCellulose Starch

9. Lipids
These molecules are a store of energy and provide
more energy than both carbohydrates and proteins.
Glycerol
Fatty acid

10. Proteins
These molecules are essential for growth and repair; they
can also be used as a source of energy if lipid and
carbohydrate levels are low.
Amino acid
Each protein is made from
subunits called amino acids.
There are 20 different types
of amino acids; some of
these amino acids are made
by the body (nonessential),
and some need to be
obtained from food
(essential).

11. Exam-style question
The human digestive system can break down complex
carbohydrates.
a) What elements make up a carbohydrate molecule? (1 mark)
…………………………………………………………………………......
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12. Exam-style questions
The human digestive system can break down complex
carbohydrates.
a) What elements make up a carbohydrate molecule? (1 mark)
Carbon, Hydrogen and Oxygen all three needed for 1 mark. Accept
correct symbols.

13. Exam-style questions
Lipase is an enzyme that can be used to break down fats.
a) What two substance are made when fats are broken
down?
Glucose
Fatty acids
Amino acids
Glycerol

14. Exam-style questions
Lipase is an enzyme that can be used to break down fats.
a) What two substance are made when fats are broken
down?
Glucose
Fatty acids
Amino acids
Glycerol
X
X

15. Investigate food samples for the presence of
glucose, starch, protein and fat
Testing for glucose (Benedicts test)
Method:
1) Place your different food samples into separate test tubes.
2) Add 2-5 cm3 of Benedict’s solution into your food samples and mix.
3) Place the test tubes in a water bath at 85OC for 5 minutes.
4) Record the colour changes.
Blue Green Orange Brick red
None Trace Low High
Increasing concentration of
glucose
Corepractical1

16. Investigate food samples for the presence of
glucose, starch, protein and fat
Testing for starch (iodine test)
Method:
1) Place your different food samples into separate test tubes.
2) Add 2-3 drops of iodine onto your food samples.
3) Record the colour changes.
Corepractical1
Orange – No starch present Blue/Black – Starch present
Food sample

17. Investigate food samples for the presence of
glucose, starch, protein and fat
Testing for proteins (Biuret test)
Method:
1) Place your different food samples into separate test tubes.
2) Add 2-5 cm3 of biuret solution into your food samples and mix.
3) Record the colour changes.
Corepractical1
No protein Protein present

18. Investigate food samples for the presence of
glucose, starch, protein and fat
Testing for lipids (Emulsion test)
Method
1) Place your different food samples into separate test tubes.
2) Add 2-3 cm3 of ethanol into your food samples and shake vigorously.
3) Add 2-3 cm3 of water to the test tube
4) Record the colour changes.
Corepractical1
Cloudy white substance will appear if
lipids are present.

19. Exam-style question
Describe how you could test for the presence of glucose
in a unknown food sample. (3 marks)
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20. Exam-style question
Describe how you could test for the presence of glucose
in a unknown food sample. (3 marks)
- Use Benedicts solution
- Heat with a Bunsen burner or place in a water bath
- Record any colour changes. Blue shows there is no glucose, green
shows there are traces of glucose, Orange shows low concentration of
glucose and red shows high concentration of glucose.

21. Exam-style question
Give two safety precautions you would take when
carrying out the test mentioned in the previous
question. (2 marks)
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22. Exam-style question
Give two safety precautions you would take when
carrying out the test mentioned in the previous
question. (2 marks)
- Safety goggles or eye protection
- Safety gloves

23. Enzymes
These are biological catalysts made from proteins that
speed up chemical reactions inside of living organisms.
The lock and key theory describes how an enzyme
works. Each enzyme has an active site where molecules
called substrates bind to. Each enzyme will only bind to
one particular substrate which makes the
enzyme specific.

24. Lock and key theory
E E
E
S S
P
P
1) Substrate will collide with
the active site of the enzyme
Active sites have a complementary shape to the substrate
Substrate
2) Enzymes aid the formation
of new bonds or help break
bonds in the substrate.
3) Products are released, the
enzyme is unchanged and can
catalyse more reactions.
This stage is referred to
as the enzyme substrate
complex. It shows that the fit
between the enzyme and
substrate is like a lock and key.

25. Enzymes - effects of temperature
As the temperature increases of an enzyme-controlled
reaction, the enzymes and substrates gain more kinetic
energy causing more collisions so the reaction occurs
faster.
Temperature OC
Rateofreaction
1
2
3
1) As the temperature increases, so does
the rate of reaction due to more collisions
between the enzyme and substrate. This is due
to them gaining more kinetic energy.
2) Once the temperature reaches the optimum
temperature, the reaction is happening at its
fastest.
3) Once past the optimum temperature, the
enzyme denatures. Denaturing permanently
changes the shape of the active site. This
change in shape prevents the substrate binding
to the active site on the enzyme.

26. E E
S S
Enzymes - effects of temperature
S
E E
S
The shape of the enzyme is unchanged up to the optimum temperature.
Past the optimum temperature, the enzymes active sites begin to change,
preventing the substrate from binding and so the reaction eventually stops.

27. Enzymes- effects of pH
As pH deviates from the optimum pH, this can affect the
bonding in the active site causing the enzyme to
denature.
PH
Rateofreaction
1
2
3
1) If the pH is lower than the optimum, this
can cause the active site to change shape.
2) At the optimum pH, the reaction is
happening at its fastest.
3) If the pH is higher than the optimum, this
can cause the active site to change shape.

28. E E
S S
Enzymes- effects of pH
S
E E
S
The shape of the enzyme is unchanged at the optimum pH
If the pH is higher or lower than the optimum pH, the enzymes active sites
begin to change, preventing the substrate from binding and so the reaction
eventually stops.

29. Investigate how enzyme activity can be
affected by changes in temperature
Method
1) Set up five water baths at ranging temperature from 0OC to 800C.
2) Add amylase to five test tubes. Label these test tubes as A1-5.
3) Add starch to five different test tubes. Label these test tubes as S1-5.
4) Place A1 and S1 together into your first water bath for five minutes. Do this for all your amylase and
starch test tubes.
5) Take a spotting tile and place one drop of iodine into each dimple.
6) Mix A1 and S1 together place back into the water bath and start your timer.
7) Every 60 seconds remove a sample using a pipette and add to the different dimples in the spotting
tile.
8) Repeat step 7 until iodine stops turning black.
9) Repeat steps 6-8 for each temperature.
10) Record results in a suitable table
11) Draw a graph to show the time taken for starch to be digested at different temperatures.
Corepractical2
A1
A2
A3
A4
A5
S1
S2
S3
S4
S5
Paper2only

30. Investigate how enzyme activity can be
affected by changes in temperature
Model results
Temperature Time (seconds) Rate (S-1 )
0 500 0.0020
20 420 0.0023
30 305 0.0032
40 150 0.0067
50 300 0.0033
60 400 0.0025
70 X (Did not change colour) 0
80 X (Did not change colour) 0
To calculate the rate of reaction use the equation Rate = 1
time
Corepractical2
Paper2only

31. Investigate how enzyme activity can be
affected by changes in pH
Method
1) Set up one water bath at 35OC.
2) Add amylase to four test tubes. Label these test tubes as A1-4.
3) Add starch to four different test tubes. Label these test tubes as S1-4.
4) Place Amylase and Starch into your water bath. Do this for all your amylase and starch test tubes. Add
your different pH buffers to all the amylase test tubes leave for five minutes.
5) Take a spotting tile and place one drop of iodine into each dimple.
6) Mix A1 and S1 together place back into the water bath and start your timer.
7) Every 60 seconds remove a sample using a pipette and add to the different dimples in the spotting tile.
8) Repeat step 7 until iodine stops turning black.
9) Repeat steps 6-8 for each temperature.
10) Record results in a suitable table
11) Draw a graph to show the time taken for starch to be digested at different pH’s.
Corepractical3
Paper2only
A1
+pH5
A2
+pH6
A3
+PH7
A4
+pH8
S1
S2
S3
S4

32. Corepractical3
Paper2only
Investigate how enzyme activity can be
affected by changes in temperature
Model results
pH Time (seconds) Rate (S-1 )
5 245 0.0040
6 121 0.0082
7 65 0.0153
8 140 0.00714
To calculate the rate of reaction use the equation Rate = 1
time

33. Exam-style question
Pupils set up an investigation:
Five test tubes contained 5 cm3 of fat and 8 cm3 of
lipase. Each test tube was kept at different temperature
for 5 hours.
A) Give one control variable?
B) What is the independent variable? (2 marks)
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34. Exam-style question
Pupils set up an investigation:
Five test tubes contained 5 cm3 of fat and 8 cm3 of
lipase. Each test tube was kept at different temperature
for 5 hours.
A) Give one control variable?
B) What is the independent variable? (2 marks)
A) (same) amount or 1cm3 fat
(same) amount or 10cm3 lipase or enzyme
(kept for) 5 hours or (same length of) time
B) Temperature

35. Exam-style question
The pH of the solution containing lipase and fat in different test tubes was tested at the beginning
of the investigation and after 5 hours.
Below are the results.
Tube Temperature OC pH at the start pH after 5 hours
1 0 Neutral Neutral
2 20 Neutral Weak acid
3 30 Neutral Strong acid
4 40 Neutral Weak acid
5 50 Neutral Neutral
What more could be done to confirm the optimum temperature for this reaction? (2 marks)
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36. Exam-style question
The pH of the solution containing lipase and fat in different test tubes was tested at the beginning
of the investigation and after 5 hours.
Below are the results.
Tube Temperature OC pH at the start pH after 5 hours
1 0 Neutral Neutral
2 20 Neutral Weak acid
3 30 Neutral Strong acid
4 40 Neutral Weak acid
5 50 Neutral Neutral
What more could be done to confirm the optimum temperature for this reaction? (2 marks)
-Use more temperatures
-between 20-40 OC