Section 4 — Biological Molecules and Enzymes 27

Biological Molecules
Biological molecules (molecules found in living organisms) are things like carbohydrates, proteins and fats.
They’re generally long, complex molecules made up from smaller basic units.

Learn the Structure of Carbohydrates, Proteins and Fats

Carbohydrates are Made Up of Simple Sugars
• Carbohydrate molecules contain the elements carbon, hydrogen and oxygen.
• Starch (in plants) and glycogen (in animals) are used as short-term energy stores.
Cellulose is the main component of plant cell walls.
• They are all large, complex carbohydrates, which are made up of many smaller
molecules of glucose (a simple sugar) joined together in a long chain.

starch
glucose

Proteins are Made Up of Long Chains of Amino Acids

• Proteins all contain carbon, nitrogen, amino acids proteins
hydrogen and oxygen atoms.
• Some proteins also contain
sulfur atoms.

Fats and Oils are Made Up of Fatty Acids and Glycerol

• Fats contain carbon, hydrogen fat

and oxygen atoms.
glycerol & fatty acids

Proteins Have Many Different Functions

When a chain of amino acids has been assembled, it folds into a unique shape which allows the
protein to perform the task it’s meant to do. Different sequences of amino acids result in proteins with
different shapes. Here are two examples of proteins and how their shape lets them do their task:
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ENZYMES — act as biological catalysts to speed up chemical reactions in the body (see page 31).
In order to speed up a reaction, a part of the enzyme called the active site has to bind to the
substrate (reacting molecule). The shape of the enzyme affects the shape of its active site,
which determines what substrate it can bind to and what chemical reaction it can catalyse.
ANTIBODIES — are part of the immune system and help the body fight pathogens (see page 70).
A part of the antibody called the binding site binds to molecules on the surface of pathogens called
antigens. The shape of the antibody affects the shape of its binding site, which determines which
antigen it can bind to and which pathogen it can help fight.

Biological molecules are the basic units of Biology

EX A M Since all living organisms are made up of carbohydrates, proteins and fats, you’ll come across all
TIP
of these molecules again during your course. This makes them very likely to come up somewhere
in your exams — so make sure you learn everything you need to here before you move on.

Section 4 — Biological Molecules and Enzymes

28

Testing for Biological Molecules

You need to know how you can test for biological molecules using different chemicals.

You Can Test for Sugars Using Benedict’s Solution

There are lots of different types of sugar molecules. Due to their chemical properties, many sugars
(e.g. glucose) are called reducing sugars. You don’t need to know exactly what reducing sugars are,
but you do need to know how to test for them:

1) Add Benedict’s solution (which is blue) to Benedict’s water bath

a sample and heat it in a water bath that’s solution
set to 75 °C.
2) If the test’s positive it will form a coloured
precipitate (solid particles suspended in
the solution).
3) The higher the concentration of reducing food
sugar, the further the colour change goes sample colour change if reducing
— you can use this to compare the amount sugar present
of reducing sugar in different solutions.
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The colour of the precipitate changes from: There won’t be a
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colour change if no
blue green yellow orange brick red sugars are present.
||

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higher sugar concentration

The Biuret Test is Used for Proteins

If you needed to find out if a substance contained protein you’d use the biuret test.

1) First, add a few drops of Negative Positive |

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|
You could use biuret
| | | | | | | | | | | | | | | | | |

potassium hydroxide solution result result

| | | | | | | | | | | | | | | | | | |
test solution, solution instead,
to make the solution alkaline. potassium hydroxide which is just a
2) Then add some copper(II) sulfate and mixture of potassium
solution (which is bright blue). copper(II) sulfate hydroxide and
solution copper(II) sulfate
3) If there’s no protein, solutions.
the solution will stay blue.
|

| | | | | | | | | | | | | | | | | | |

solution staying blue

4) If protein is present, indicates no protein purple colour
the solution will turn purple. indicates protein

Starch is Tested for with Iodine

Just add iodine solution to the test sample. iodine
colour changes
solution
1) If starch is present, the sample to blue-black if
changes from browny-orange starch is present
to a dark, blue-black colour.

colour remains
2) If there’s no starch,
browny-orange if
it stays browny-orange.
food sample starch isn’t present

Section 4 — Biological Molecules and Enzymes

 29

Testing for Biological Molecules

There are a couple more tests coming up on this page — for fats and oils, and vitamin C.

Use the Ethanol Emulsion Test for Fats and Oils

To find out if there are any fats or oils in a sample:

1) Shake the test substance with ethanol for about a minute

until it dissolves, then pour the solution into water.
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| | | |
An emulsion is when one

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liquid

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2) If there are any fats or oils present, they will precipitate doesn’t dissolve in another
—
out of the liquid and show up as a milky emulsion. it just forms little droplets.

|
| | | |
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|

3) The more fat there is, the more

noticeable the milky colour will be.

Test substance Shake Add to Milky colour

and ethanol water indicates fat

Use the DCPIP Test for Vitamin C

To find out how much vitamin C is in a food sample you’d use DCPIP solution.

1) Add DCPIP solution drop by drop to a food DCPIP blue colour

sample containing vitamin C (e.g. juice). solution remains if
2) DCPIP solution changes from blue to vitamin C is
colourless when vitamin C is present. not present
3) Keep adding DCPIP until the blue colour no
longer disappears when it’s mixed with the
sample. (Vitamin C reacts with DCPIP solution, blue colour
making it colourless. The colour stops changing disappears if
when all the vitamin C has been used up.) vitamin C
food sample is present
4) The higher the volume of DCPIP solution added
before the blue colour stops disappearing, the containing
more vitamin C the food sample contains. vitamin C

Make sure you think about all of the hazards...

ICAL
PRACT Iodine solution is an irritant to the eyes, and the chemicals used in the biuret test are dangerous,
TIP
so wear safety goggles when carrying out these tests. If you spill any of the chemicals on your
skin, wash them off straight away. Be careful around the water bath in the Benedict’s test, too.

Section 4 — Biological Molecules and Enzymes

 30

Water and DNA

Water and DNA are essential for life. Water is needed by living organisms to stay alive.
DNA is needed to build proteins, which are required for the cells in living organisms to function.

Water is Important as a Solvent

Water is a major component of the body and cells. It is a solvent, which means that some
substances dissolve in it to form a solution. Most chemical reactions take place in solution.
Here are some examples of why water is an important solvent:

TRANSPORT — water allows soluble molecules, such as urea

(a waste product produced from the breakdown of proteins), glucose
and amino acids, to be transported around the body in the blood.
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Water also acts as a solvent

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DIGESTION — digestive enzymes need to be in
for the products of digestion,

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solution to work properly. Water helps the body to which allows them to diffuse
digest food by acting as a solvent for the enzymes. into the bloodstream.
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EXCRETION — your body uses water to transport

metabolic waste products (such as urea and toxins) out
of the body through, e.g. sweating and urination (weeing).

DNA is a Double Helix

1) A DNA molecule has two strands coiled together
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Supplement
in the shape of a double helix (two spirals).
2) Each strand contains chemicals called bases.
There are four different bases (shown in the diagram
as different colours) — A, C, G and T.
3) The two strands are held together by cross-links
that are formed by pairs of bases.
4) The bases always pair up in the same way — it’s always A-T and C-G.

base pairings
A T

C G

Every living organism has DNA

DNA contains all the instructions needed to make living organisms. If you’re doing the Extended course,
one of the things you’ll need to learn is how DNA’s double helix structure is held together by four bases.

Section 4 — Biological Molecules and Enzymes

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4 The chemicals of life
There are no specific definitions to learn in this chapter.

Exercise 4.1 Carbohydrates

This exercise will give you practice in constructing results charts (A03.3) and drawing conclusions (A03.4),
as well as helping you to remember important facts about carbohydrates.

A student carried out tests on two foods. This is what she wrote in her notebook.

Starch test- food A went brown, food B went black

Benedict's reagent - food A went orange-red, food B went blue
a Construct a results table and complete it to show the student's results. Think carefully about the best way of
showing what she did, what she was testing for, what results she obtained and what these results mean.

IGCSE Biology
b Complete this table about carbohydrates.

Type of
Example Role in living organisms
carbohydrate

glucose
sugar
the form in which carbohydrates are transported in plants

the form in which plants store energy

polysaccharide cellulose

glycogen

4 The chemic.als of life

Exercise 4.2 Proteins
This exercise will help you to learn about the importance of proteins in the body. It is also very good practice
in finding and sorting information (A02) and cutting a large amount of information down to just a very short
summary of the most important points. Not easy!

There are hundreds of different proteins in the human body, each with its own particular role to play.
Use the internet or other resources to find out about each of the proteins listed below. Make notes, and then shorten
them so that you eventually write down just two or three sentences about each protein, each sentence packed with
information about it.

a haemoglobin

b keratin

C collagen

d antibodies

IGCSE Biology
Exercise 4.3 Testing a hypothesis
This exercise will help you to remember the biuret test for proteins. It will also help you to Improve your skills
in planning experiments (A03.2). Your teacher may allow you to carry out your experiment- if so, you are
almost certain to find you want to make some changes to it. That is good - it is what most scientists do.

The biuret test is used to test foods for proteins. The intensity of the colour obtained depends on the concentration of
protein in the sample being tested.
Plan an investigation to test this hypothesis:

Milk from cows contains a higher concentration of protein than milk from goats.

a First, describe how you would do the biuret test.

b Now think about how you could use this test to test the hypothesis.
i What variable would you change in your experiment?

ii What would you keep the same? Try to think of at least three variables you would keep the same.

iii What would you measure in your experiment?

iv How would you measure it?

V If the hypothesis is correct, what results would you expect to obtain?

4 The chemicals of life

0 Exercise 4.4 DNA
In this exercise, you will think about the structure of DNA, and where it is found in a cell.

The diagram shows part of a DNA molecule.

c T T

r
A G G A A

IG ) A

T c T c T

a What are the parts of the molecule labelled A, C, G or T? Draw a circle around the correct term.
amino acids bases genes proteins

b Complete the diagram by writing A, C, G or T in each of the three spaces.

C Which two of the following words could correctly complete this sentence?

DNA is found in the of a cell.

Draw a circle around each correct word.

chromosomes cytoplasm nucleus membrane ribosomes

IGCSE Biology
 4 Biological molecules
Core
1 List the chemical elements present in:

a carbohydrates

b fats

c all proteins.

2 On the diagrams of food molecules (a–c):

i name each of the nutrients shown
ii state one use of the nutrient in the body
iii add labels to the diagrams, using words from the list below. You can use the words once or
more than once.

amino acid fatty acid chemical bond glucose glycerol

a ........................................... i
ii

...........................................

b ...................................................... i
ii

................................... ...................................

c i
ii
................................

...........................................

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4 BIOLOGICAL MOLECULES

3 Draw lines to match the statements to the food tests and the results.

Food tested Chemical reagent used Colour for a positive result

vitamin C iodine solution blue–black

reducing sugar Benedict’s solution purple

fat ethanol brick red

protein DCPIP white emulsion

biuret solution (sodium

starch hydroxide + copper sulfate colourless
solution)

Supplement G

T
4 The diagram shows two strands of a section of DNA,
with the bases on the left strand identified. C

a Complete the diagram by writing the letters of the bases that would A
be present on the other strand. A

C
b What name is given to describe the two strands coiled together?

c 28% of the bases in a DNA molecule are base A. Calculate the percentage of C bases. Show your
working at each stage of the calculation.

16 Photocopying prohibited Cambridge IGCSE™ Biology Workbook Third Edition

 4 Biological molecules

Exam-style questions
Core
1 a Describe how you would carry out a food test on a piece of fish to find out if it
contained oil.

[3]

b State what result would indicate a positive test for oil.

[1]

[Total: 4]
Supplement
2 Which two statements about DNA are correct?
1 Each strand contains chemicals called fatty acids.
2 T always pairs with A.
3 A gene is a length of DNA, coding for a protein.
4 A red blood cell contains DNA in its nucleus.
A 1 and 3
B 1 and 4
C 2 and 3
D 2 and 4 [1]
3 Outline the structure of DNA.

[4]

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