4 Biological molecules
Focus
In the last chapter, you learned about diffusion, osmosis and active transport
and their importance in moving materials in and out of cells. You will now
understand how living organisms make use of these processes and the problems
that can be faced by cells. In this chapter you will develop a knowledge of the
main nutrients and substances that make up living things, and how you can use
biochemical tests for their presence in food. Which foods do you think are good
sources of carbohydrate, fats and proteins? Will the food tests you carry out
confirm this? Those of you who are studying the supplement of the syllabus will
also be able to develop an understanding of the structure of DNA.
Biological molecules
FOCUS POINT
● What are the chemical elements that make up
carbohydrates, fats and proteins?
● What are larger molecules made up of?
● What are the chemical tests for the presence of starch,
reducing sugars, proteins, fats and oils, and vitamin C?
● What is the structure of a DNA molecule?
Carbon is an element present in all biological molecules.
Carbon atoms can join to form chains or ring structures, so
biological molecules can be very large. They are often built of
repeating sub-units (monomers). Oxygen and hydrogen are
other elements that are always present. Nitrogen is
sometimes present. When molecules are made of long chains
of monomers held together by chemical bonds, they are
called polymers (poly means ‘many’). Examples are
polysaccharides (chains of single sugar units like glucose)
and proteins (chains of amino acids). Molecules built of lots of
small units often have different properties from their subunits. This makes them suitable for specific jobs in living
things. For example, glucose is very soluble and has no
strength, but cellulose (a large molecule made of glucose
units) is insoluble and very tough – ideal for making cell walls
around plant cells.
Cells need chemical substances to make new cytoplasm
and to release energy, so the organism must take in food to
supply the cells with these substances. However, it is not as
simple as this; most cells have special jobs (Chapter 2) and
so have different needs. All cells need water, oxygen, mineral
ions and food substances though, and all cells are made up of
water, proteins, fats, carbohydrates, mineral ions and
vitamins, or forms of them.
Carbohydrates
These may be simple, soluble sugars or complicated
materials like starch and cellulose, but all carbohydrates
contain carbon, hydrogen and oxygen only. A common simple
sugar is glucose, which has the chemical formula C6H12O6.
The glucose molecule is often in the shape of a ring,
shown as:
▲ Figure 4.1 Glucose molecule showing ring structure
Going further
Two molecules of glucose can be joined to make a molecule
of maltose, C12H22O11 (Figure 4.2).
▲ Figure 4.2 Formation of maltose
Sugars with a single carbon ring are called
monosaccharides, e.g. glucose and fructose. Those sugars
with two carbon rings in their molecules are called
disaccharides, e.g. maltose and sucrose. Mono- and
disaccharides are readily soluble in water.
Glycogen (Figure 4.3) is a large molecule that is a food
storage substance in many animal cells.
The starch molecule is made up of hundreds of glucose
molecules joined to make long chains. Starch is an important
storage substance in plant cells.
Cellulose is made of even longer chains of glucose
molecules. The molecules in the chain are grouped together
to make microscopic fibres, which are laid down in layers to
make the cell wall in plant cells (Figures 4.4 and 4.5).
Glycogen, starch and cellulose are not very soluble in
water.
▲ Figure 4.3 Part of a glycogen molecule
▲ Figure 4.4 Cellulose. Plant cell walls are made of long, interconnected
cellulose fibres. These are large enough to be seen with the electron
microscope. Each fibre is made up of many long-chain cellulose molecules
▲ Figure 4.5 Scanning electron micrograph of a plant cell wall (×20 000)
showing the cellulose fibres
Fats
Fats are a solid form of a group of molecules. When fats are
liquid they are known as oils. Fats and oils are made from
carbon, hydrogen and oxygen only. A molecule of fat (or oil) is
made up of three molecules of an organic acid, called a fatty
acid, joined with one molecule of glycerol.
Drawn simply, fat molecules can be shown as in Figure 4.6.
▲ Figure 4.6 Fat molecule
Fats form part of the cell membrane and the internal
membranes of the cell like the nuclear membrane. Droplets
of fat or oil form a source of energy when stored in the
cytoplasm.
Proteins
Some proteins are part of structures in the cell, for example,
the cell membranes, the mitochondria, ribosomes and
chromosomes.
There is another group of proteins called enzymes.
Enzymes are present in the membrane systems, in the
mitochondria, in special vacuoles and in the fluid part of the
cytoplasm. Enzymes control the chemical reactions that keep
the cell alive (see Chapter 5).
Although there are many different types of protein, they all
contain carbon, hydrogen, oxygen and nitrogen, and many
contain sulfur. Their molecules are made up of long chains of
simpler chemicals called amino acids (Figure 4.7).
▲ Figure 4.7 Protein molecule (part of)
There are about 20 different amino acids in animal proteins.
These include alanine, leucine, valine, glutamine, cysteine,
glycine and lysine. A small protein molecule can be made up
from a chain consisting of a hundred or more amino acids, for
example, glycine–valine–valine–cysteine–leucine– glutamine–,
etc. Each type of protein has its amino acids arranged in a
special sequence.
Going further
Synthesis and conversion in cells Cells can build up
(synthesise) or break down their proteins, fats and
carbohydrates, or change one to another. For
example, animal cells synthesise glycogen from
glucose by joining glucose molecules together
(Figure 4.3); plant cells synthesise starch and
cellulose from glucose. All cells can make proteins
from amino acids. They can also build up fats from
glycerol and fatty acids. Animal cells can change
carbohydrates to fats, and fats to carbohydrates;
they can also change proteins to carbohydrates, but
they cannot make proteins unless they are supplied
with amino acids. However, plant cells can make
their own amino acids using sugars and mineral ions.
The cells in the green parts of plants can even make
glucose starting from only carbon dioxide and water
(see ‘Photosynthesis’ in Chapter 6).
▼ Table 4.1 Summary of the main nutrients
Test yourself
1. State which type of nutrient contains the following subunits:
1. amino acids
2. fatty acids
3. glucose
4. glycerol.
Which type of nutrient contains nitrogen atoms?
Explain why there are much larger numbers of different
proteins than polysaccharides.
1. What do the chemical structures of carbohydrates and
fats have in common?
2. How do their chemical structures differ?
3. Suggest why there are many more different proteins
than there are carbohydrates.
Structure of DNA
A DNA molecule is made up of long chains of nucleotides, formed into two
strands. A nucleotide is a 5-carbon sugar molecule joined to a phosphate group
(–PO3) and an organic base (Figure 4.8). In DNA the sugar is deoxyribose and
the organic base is either adenine (A), thymine (T), cytosine (C) or guanine (G).
Note: for exam purposes, it is only necessary to be able state the letters, not
the names of these bases.
The nucleotides are joined by their phosphate groups to make a long chain,
often thousands of nucleotides long. The phosphate and sugar molecules are
the same all the way down the chain. However, the bases can be any one of the
four listed above (Figure 4.9).
The DNA in a chromosome is made of two strands (chains of nucleotides)
held together by chemical bonds between the bases. The size of the molecules
makes sure that A always pairs with T and C pairs with G. The double strand is
twisted to make a helix (like a twisted rope ladder with the base pairs being the
rungs) (Figures 4.10 and 4.11).
▲ Figure 4.8 A nucleotide (adenosine monophosphate)
▲ Figure 4.9 Part of a DNA molecule with four nucleotides
▲ Figure 4.10 Model of the structure of DNA
▲ Figure 4.11 This schematic shows part of a DNA molecule
Test yourself
1. Which of the four sections of a DNA molecule shown in
Figure 4.12 has all the bases bonded correctly?
▲ Figure 4.12 Four sections of a DNA molecule
6 Which units are present in a nucleotide of DNA?
A phosphate, glucose, base
B phosphate, deoxyribose, base
C deoxyribose, base, amino acid
D glucose, fatty acid, base
7 Which statement about a DNA molecule is correct?
A Each strand is made of chains of glucose molecules.
B Base A bonds with base C.
C The number of C bases equals the number of G bases.
D The molecule is made up of straight, parallel strands.
Going further DNA
In 1869, a chemist working on cell chemistry discovered a compound that
contained nitrogen and phosphorus (as well as carbon). This was an unusual
combination. The substance came from nuclei and was first called ‘nuclein’ and
then ‘nucleic acid’. Further studies showed nucleic acid contained the bases
adenine, thymine, cytosine and guanine, as well as a carbohydrate later
identified as deoxyribose. In the early 1900s, scientists identified the structure
of nucleotides (base–sugar– phosphate, Figure 4.9) They also found how they
linked up to make deoxyribonucleic acid (DNA).
In the 1940s, a chemist called Chargaff studied a sample of DNA. He found that
the number of adenines (A) are always the same as the number of thymines
(T). In the same way, the amounts of cytosine (C) and guanine (G) are always
equal. Crick and Watson used this information to work out the structure of DNA.
The British physicist, Francis Crick, and American biologist, James Watson,
worked together in the Cavendish Laboratory at Cambridge in the 1950s. They
did not do chemical analyses or experiments. Instead, they used the data from
X-ray crystallography and the chemistry of nucleotides to try out different
models for the structure of DNA.
The regular pattern of atoms in a crystal scatters a beam of X-rays. This allows
the structure of the molecules in the crystal to be found (Figure 4.13(a)). The
scattered X-rays are directed onto a photographic plate. When developed the
plate shows images like the one in Figure 4.13(b).
▲ Figure 4.13 X-ray crystallography
The scientists took careful measurements of the spots on the photograph.
Using some complicated mathematics, they found the molecular structure of
many compounds.
A crystalline form DNA was treated in the same way. Most of the necessary Xray crystallography was carried out by Maurice Wilkins and Rosalind Franklin at
King’s College, London.
Crick and Watson made models on a trial-and-error basis. They judged each
model on how well it matched the X-ray measurements and the chemical
properties of the parts of the molecule.
The evidence suggested a helical structure (like a spiral staircase). At first, they
tried models with a core of three or four nucleotide chains twisted around each
other. The bases were attached to the outside.
However, these models did not fit the X-ray data or the chemical structures of
the nucleotides. Watson tried a two-chain helical model with the bases pointing
inwards. First, he paired adenine (A) with adenine (A), cytosine (C) with
cytosine (C), etc. But thymine (T) and cytosine (C) were smaller molecules than
adenine (A) and guanine (G). He found that this pairing would alter the shape of
the double helix.
This is where Chargaff’s work helped. If there were equal numbers of adenine
(A) and thymine (T), and equal numbers of cytosine (C) and guanine (G), this
pairing of bases, large plus small, would fit inside the sugar–phosphate double
helix without altering its shape.
The X-ray data confirmed that the diameter of the helix would allow this
pairing. Also, the chemistry of the bases would allow them to hold together. The
outcome is the model of DNA shown in Figures 4.9, 4.10 and 4.11.
Crick, Watson and Wilkins were awarded the Nobel Prize for medicine and
physiology in 1962. Unfortunately, Rosalind Franklin died in 1958, so she did
not receive an award for the work she did.
▲ Figure 4.14 Crick (right) and Watson with their model of the DNA
molecule
Practical work
For safe experiments/demonstrations which are related to
this chapter, please refer to the Biology Practical Skills
Workbook that is also part of this series.
Safety
● Eye protection must be worn.
● Take care using iodine solution – it can stain skin and
clothes.
● Take care handling hot water.
Food tests
1 Test for starch
● Shake a small amount of starch powder in a test tube with
some warm water. This will make a starch suspension.
● Add 3 or 4 drops of iodine solution. A blue-black colour
should be produced. Note: it is also possible to use iodine
solution to test for starch in leaves, but a different method
is used (see Chapter 6).
2 Test for reducing sugar
● Heat 2 cm3 depth of glucose solution with an equal volume
of Benedict’s solution in a test tube. Place the test tube
in a beaker of boiling water to heat it (see Figure 4.15), or
warm it in a water-bath. The solution will change from clear
blue to cloudy green, then yellow and finally to a red
precipitate (deposit) of copper(I) oxide, because glucose is
a reducing sugar.
3 Test for protein (biuret test)
● Place 2 cm3 depth of 1% albumen solution (the protein of
egg white) in a test tube. Add 2 cm3 dilute sodium
hydroxide (CARE: this solution is an irritant), followed by 2
cm3 1% copper sulfate solution. This is the biuret test. A
purple colour indicates protein. If you run the copper
sulfate into the test tube without mixing, you will see a
violet halo where the two liquids touch each other.
4 Test for fat
● Shake two drops of cooking oil with about 5 cm3 ethanol in
a dry test tube until the fat dissolves.
● Pour this solution into a test tube containing a few cm3
water. A milky white emulsion will form. This shows that the
solution contained some fat or oil.
Safety
● Take care handling DCPIP solution – it can stain skin and
clothes.
● Take care handling hot water or a water-bath.
5 Test for vitamin C
● Suck 2 cm3 fresh lemon juice into a plastic syringe.
● Place 2 cm3 of a 0.1% solution of DCPIP (a blue dye) in a
test tube. Add the lemon juice drop by drop. The DCPIP will
become colourless quite suddenly as the juice is added.
Make a note of the amount of juice added from the syringe.
● Repeat the experiment using orange juice in the syringe. If
it takes more orange juice than lemon juice to decolourise
the DCPIP, the orange juice must contain less vitamin C.
Application of the food tests
The tests can be used on samples of food like milk, potato, cassava, raisins,
onion, beans, egg-yolk or peanuts to find out what food materials are present.
Crush the solid samples in a mortar and shake with warm water to get a solution
or suspension. Pour small amounts of the watery mixture into several test tubes.
Test the samples for starch, glucose or protein as described above. To test for
fats, the food must first be crushed in ethanol, not water, and then filtered. Pour
the clear filtrate into water. A milky colour indicates the presence of fats.
▲ Figure 4.15 Experiment to test foods for different nutrients
Practical work questions
1 A test on a food gives the following results:
2 It turns blue-black with iodine.
3 It turns milky with ethanol.
What nutrients does the food contain? Explain your
answer.
2 Describe what safety precautions you would take when carrying out a food
test for
2 protein
3 reducing sugar.
Explain your answers.
Revision checklist After studying Chapter 4 you
should know and understand the following:
Living matter is made up of several important types of
molecules, including proteins, fats and carbohydrates.
All three types of molecule contain carbon, hydrogen
and oxygen atoms. Proteins also contain nitrogen and
sometimes phosphorus or sulfur.
Carbohydrates are made from simple sugar units, often
glucose.
Carbohydrates are used as an energy source; glycogen
and starch make good storage molecules. Cellulose
gives plant cell walls their strength.
Fats are made from fatty acids and glycerol.
Proteins are made of chains of amino acids.
Membranes outside and inside the cell are made of
proteins and fats.
Food tests are used to identify the main biological
molecules.
✔ DNA is another important biological molecule. It has a very
distinctive shape, made up of nucleotides containing
bases, which always pair up in the same way: A with T, and
C with G.
Exam-style questions
1.
1. State what chemical structures carbohydrates and
fats have in common.
[2]
2. Describe how their chemical structures differ.
[3]
2. Protein and carbohydrate molecules have some features
in common and some features which make them
different.
1. State one feature which they both have.
[1]
3. 1.
State
the
name
of
the
group
of
nutrients
to
which
2. State two features which make them different.
[2]
cellulose, glycogen and starch all belong.
[1]
2. Complete the table by stating one function of
each of the molecules and where they are found in a
cell.
nutrient
molecule
[6]
function of
nutrient molecule
where the nutrient
molecule is found in a cell
cellulose
glycogen
starch
4. Three types of fruit juice, A, B and C, were tested with
DCPIP. The results are shown in the table.
The results are shown in the table.
fruit
juice
number of drops of fruit juice needed to decolourise
DCPIP
A
10
B
18
C
7
1. State the colour of DCPIP before adding fruit juice.
2. Explain what the results show about the fruit
[1]
3. juices.
i) The fruit juices tasted sweet. Describe how you would test
[3]
them to see if they contained reducing sugar.
[3]
ii
) State the precautions you would take when carrying out this food
test.
[3]
ii
i) State the colour change you would observe if the fruit juice did
contain reducing sugar.
[1]
5 a Outline the structure of DNA.
b Describe how the bases are arranged in a DNA molecule.
6 Complete the table to compare carbohydrates, fats and proteins.
[3]
[3]
[6]
nutrient
elements present
sub-unit(s) present
carbohydrate
fat
protein
7 The following is a list of nutrient molecules and their sub-units: amino acid
fatty acid
glycogen
State which molecules
cellulose
glucose
protein
fat
glycerol
starch
State which molecules
a are polymers
b are carbohydrates
c are sub-units of larger molecules.
[4]
[4]
[4]