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Fundamental Chemistry: Theory and Practice
Topic 13 – Biomolecules
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Fundamental Chemistry: Theory and Practice
Topic 13 – Biomolecules
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Contents
Topic 13 – Biomolecules
1
Carbohydrates
1
Proteins
5
Fats and Lipids
9
Answers to SAQs
14
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Fundamental Chemistry: Theory and Practice
Topic 13 – Biomolecules
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Topic 13 – Biomolecules
Biomolecules is the general term for naturally occurring organic molecules. In this
topic we will briefly introduce carbohydrates, proteins and lipids.
Carbohydrates
Carbohydrates are naturally occurring polymers; they may be classified as being either
simple or complex depending on their structure. They are the body’s favoured source
of energy and play a major structural role in plants and some insects.
Carbohydrates
Sugars
Monosaccharides
eg glucose
and fructose
Polysaccrides
Disaccarides
eg sucrose
and lactose
Storage
eg glycogen
and starch
Structural
eg cellulose
and Chitin
Monosaccharides and disaccharides are described as simple sugars they dissolve in
water to give a sweet tasting solution. The simple sugars provide the building blocks
for making the polysaccharide polymers. Polysaccharides are insoluble in water and
do not taste sweet.
Simple sugars are composed of ring structures; these may have 5 carbons (pentoses)
or six carbons (hexoses). They all contain free or potentially free carbonyl groups
(C=O) which account for the reducing properties of some sugars.
Monosaccharides include glucose, which is a six membered ring and fructose a five
membered ring.
Glucose has a ring structure and formula C6H12O6. To identify the formation of bonds
and the way in which bonds are formed the carbon atoms are identified by a number
as shown in the diagram below.
6
5
4
3
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Additionally glucose exists as two isomers α and β glucose as shown below, the
difference is in the position of the OH group on carbon 1.
In water the sugars exist at equilibrium in both cyclical and linear forms. The open
chain representation of a sugar molecule is known as a fischer projection.
The diagram below shows the equilibrium that exists between the linear and cyclical
forms of glucose. The cyclic form is made when the hydroxyl group on carbon 5 reacts
with the carbonyl group on carbon 1.
The diagram at the bottom of the page represents the equilibrium between the ring and
linear forms of D- glucose.
In the same way that glucose can form isomers in the cyclical form it can also form
isomers in the linear form. In this case we talk about D and L configuration. These
descriptions refer to the position of the hydroxyl group on the second last carbon on
the chain. We say our sugar is a member of the D series if the second last hydroxyl
group is located on the right of the open chain structure.
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CHO
CHO
OH
H
H
HO
H
H
HO
H
HO
H
OH
H
OH
OH
H
OH
CH2OH
CH2OH
D-Glucose
L-Glucose
The aldehyde groups may be readily oxidised by fehlings solution (giving a red/brown
precipitate), by tollens reagent (produces a silver mirror) or by benedicts (blue to
orange), as the sugars are oxidised they reduce the fehlings or tollens reagent hence
the term reducing sugar. Other reducing monosaccharides include fructose and
galactose.
Disaccharides are composed of two sugars linked together by a glycosidic bond. This
is an example of a condensation reaction where a molecule of water is released.
The diagram below shows two glucose molecules linking together to form maltose.
This is an α 1-4 glycosidic link as two α-glucose molecules are joining and linking
between carbon 1 on the first molecule and carbon 4 on the second.
Other examples of disaccharides include the milk sugar lactose which is made from
glucose and galactose and sucrose which is composed of glucose and fructose.
Because of the way the sugars are linked in sucrose it is not possible for free carbonyl
groups to form in solution and sucrose is not a reducing sugar.
Polysaccharides are formed when many sugars link together forming large insoluble
molecules. They may be either structural or storage polysaccharides. Examples of
structural polysaccharides include cellulose in plant cell walls and chitin found in the
wings of many insects. Storage polysaccharides provide a means of storing energy
that does not affect the osmotic balance of cells and include cellulose found in plant
cell walls and glycogen found mainly in the muscles and liver cells of animals.
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In addition polysaccharides may be described as being homopolysaccharides that is
composed of the same sugar monomers or heteropolysaccharides which are
composed of more than one type of sugar monomer.
1
1)
Draw the linear and cyclical forms of glucose.
2)
Highlight and name the functional groups.
3)
Explain why glucose is described as being a reducing sugar
4)
Name a chemical that could be used to determine fructose is a reducing sugar.
5)
Is fructose a reducing sugar?
6)
Explain why the disaccharide sucrose is not a reducing sugar.
7)
Explain why polysaccharides are used as an energy store.
Check your answer at the end of this booklet.
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Proteins
Proteins are complex molecules made up of monomers called amino acids. There are
about 20 different amino acids that can be arranged in many different ways to give rise
to a huge variety of proteins. Proteins make up a diverse range of compounds which
include muscles, tendons, fingernails, enzymes antibodies and many hormones.
Generally speaking proteins are classified as being
 Fibrous these are the structural proteins like muscle and tendons
 Globular these tend to be the working proteins and include enzymes and antibodies
 Conjugated these proteins are associated with a non protein component for
example haemoglobin which is associated with iron.
All amino acids have the same general structure which is shown in the diagram below.
The only part of the molecule that varies is the R group. This can be as simple as a
hydrogen atom as in glycine or a complex ring structure such as is found in
phenylalanine.
If you look closely at the structure you will see that each amino acid has two functional
groups:
 An amine group (NH2)
 A carboxylic acid group (COOH)
This results in amino acids being bifunctional and exhibiting properties associated with
both amines and acids.
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The nature of the R group varies and the nature of the R group is important in
influencing the properties of the amino acid. The R groups are classified in 5 ways:
 Hydrocarbon, these may be aliphatic or aromatic. As these organic groups are nonpolar the amino acid is hydrophobic
 Acidic Some R groups may have carboxylic acid functional groups which can
dissociate releasing H+ ions leaving negatively charged R groups, examples include
aspartic acid and glutamic acid
 Similarly some amino acids may contain an NH2 group within their R group that can
attract an H+ from water leaving a surplus of OH- and giving rise to positively
charged R groups, examples include asparagines and glutamine
 Some R groups include polar hydroxyl groups which make the amino acid polar
examples include serine and tyrosine
 Some contain sulphydryl groups which again have the effect of making the molecule
polar.
Generally speaking however amino acids are water soluble and have high melting and
boiling points. This indicates that amino acids are charged molecules that contain
highly polar groups. Compounds of this nature would generally be expected to migrate
when placed in an electric field. Amino acids however do not migrate in an electric field
when in a neutral solution. This has led to the proposition that amino acids contain two
ions which effectively cancel each other out. In fact it is often said that amino acids are
‘zwitterions’.
NH3+
H
C
C
O-
O
This property causes the amino acid to behave like a salt, ie water soluble, high
melting and boiling points and electrically neutral.
In acid solutions amino acids are positively charged and in alkaline solutions they are
negatively charged.
R
+
3HN–CH
R
OH– COOH
+
3HN
– CH – COO-
2HN
– CH- COO-
H+
H+
Acid
R
OH-
Neutral
Alkali
The pH at which an amino acid is neutral and does not migrate to either electrode is
termed the isoelectric point.
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Amino acids like carbohydrates are linked together via a condensation reaction with
the elimination of a molecule of water. In this case the bond formed is called a peptide
bond.
+
+ H2 O
Peptide bond
When two amino acids link together as shown above a dipeptide is formed. A peptide
chain with fewer than 10 amino acids is called an oligopeptide and when more than
seventy amino acids are linked in a chain we say a protein has been formed.
Proteins can be hydrolysed (broken down) naturally by the action of enzymes like
pepsin and trypsin or in the laboratory using acid hydrolysis. It is possible to separate
amino acids using electrophoresis and visualise the separated amino acids using
ninhydrin dye.
Now try SAQ 2 on the following page
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2
1) Explain why amino acids do not migrate in a neutral solution but migrate when
placed in an acid solution.
2) Draw the formation of a peptide bond
3) Give one role and an example of
a
a fibrous protein
b
a globular protein
Check your answer at the end of this booklet.
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Fats and Lipids
Lipids play an essential role in maintaining life in all living organisms. They are
naturally occurring molecules that are insoluble in water but soluble in non-polar
liquids; because of this they are said to be lipophilic. The structure of fats varies
greatly depending on their function and lipids have a variety of functions in living
organisms including:
 Providing an energy store. Gram for gram, twice as much energy is released from
the breakdown of a lipid than from breakdown of glucose
 They form a major component of cell membranes
 They form bile salts which play an important role in the digestion of fats.
 Several vitamins and hormones are made of lipids
 Protection and insulation
 Essential oils in plants
 Important in the transmission of a nerve impulse.
Almost all the commercially important fats are simple fats (triglycerides) Triglycerides
are made of one molecule of the trihydric alcohol glycerol joined to 3 fatty acid
molecules. Fatty acids are naturally occurring alkanoic acids they may be saturated or
unsaturated, straight chain or branched. These fatty acids link to the glycerol via a
condensation reaction forming an ester linkage. Animal fats contain saturated fatty
acids and tend to be solid at room temperature while plant fats are unsaturated and
tend to be liquids at room temperature. The more double bonds the lower the melting
point.
H
H
C
OH
H
C
OH
H
C
OH
O
HO - C - (CH2)n CH3
Fatty acid
Glycerol
H
O
H
C
O
C
O
H
C
O
C
O
H
C
O
C
H
Triglyceride
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The diagram above shows a simple triglyceride molecule. In the cell membrane the
triglycerides have one of the fatty acids replaced by a phosphate group giving the
molecule a polar head and a hydrophobic tail.
O
Hydrophobic water
hating tail
Hydrophilic water
loving phosphate
head
Phospholipids have water loving (soluble) heads and water hating (insoluble) tails. As
such they are described as being amphiphatic, that is they contain water loving
(hydrophilic) and water hating (hydrophobic) regions.
Hydrophilic head
Hydrophobic tail
These molecules arrange themselves
within the membrane to form a lipid
bilayer. The lipid bilayer allows passage
of small molecules but prevents passage
of water and water soluble molecules
which need to enter/leave the cell through
protein channels.
The hydrophilic heads
dissolve in the cytoplasm
and the extracellular fluid,
while the fatty acid tails form
a barrier within the centre of
the membrane
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Cholesterol is a soft waxy substance found within the lipids in the body. It is also a
component of the cell membrane and is the origin of steroid hormones and the bile
salts. Within cell membranes in mammals cholesterol is responsible for moderating
the fluidity of the membrane. Bile salts, which are formed from cholesterol in a
complex series of reactions, are important in fat digestion where they are responsible
for emulsifying the fats allowing digestive enzymes to work more efficiently.
The steroid hormones are also derived from cholesterol in a complex series of
reactions where side chains are added and removed to create the functional
hormones; the fat-soluble vitamins such as vitamin D are also derived from cholesterol.
The general formula for cholesterol is shown below.
The functions of some of the steroid hormones are listed below.
 Glucocorticoids raise blood glucose levels and have an anti-inflammatory effect on
the body eg cortisol
 Mineralocorticoidsxfor example aldosterone which plays an important role in Na+
metabolism
 Testosterone develops and maintains male secondary sexual characteristics.
 Oestrogens maintains female secondary sexual characteristics and repair the
uterine wall following menstruation
 Progesterone stimulates thickening of uterine wall and growth of breast tissue and
maintains endometrium during early stages of pregnancy.
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3
1 The table below shows some fatty acids which have the same number of carbon
atoms (18) but different numbers of double bonds, describe the trend in the melting
points of these molecules.
Fatty acid
No of double bonds
Stearate
Oleate
Linoleate
Linolenate
0
1
2
3
2 Name the molecule that forms the backbone of the triglyceride molecule and name
its functional group.
3 Draw a diagram to show the formation of the bond between the above molecule and
fatty acids and name the linkage.
4 Give three functions of fats in the body.
Check your answer at the end of this booklet.
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Biomolecules Checklist
Tick the boxes only if you:
Topic
Know the basic structure of simple sugars and how they join to
form complex carbohydrates
Understand why some sugars are termed reducing sugars and
the reagents that can be used to test for this
Understand the roles of carbohydrates in living organisms
Know the basic units of proteins and how they link to form
proteins
Understand why amino acids are described as zwitterions
Understand the role of proteins in living organisms
Know the basic structure of fats
Understand how triglycerides are formed
Understand the roles of fats within the body.
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Understand (?)
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Answers to SAQs
Answer to SAQ 1
1 and 2
Carbonyl group
Hydroxyl group
Hydroxyl group
3) Due to the presence of a free carbonyl group when the sugar is at equilibrium in
solution.
4 Fehlings produces a red/brown precipitate
Benedicte blue to brick red
Tollens forms a silver mirror
5 Yes
6 The way in which the glucose and fructose sugars join in sucrose leaves no free
carbonyl group.
7 Polysaccharides are insoluble and do not affect the osmotic balance of the cell.
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Answer to SAQ 2
1 Amino acids form a zwitterion which has both positive and negative areas and is
electronically neutral. In acid solutions the amino acid takes on a positive charge
and will migrate when placed in an electric current.
2
+
3 a
b
Fibrous proteins: Muscles, ligaments, tendons, hair, nails
Enzymes, antibodies
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Answer to SAQ 3
1 As the number of double bonds increases the melting point decreases
2 Glycerol which has three hydroxyl functional groups
3
H
H
H
Example of an ester linkage.
4 Component of the cell membrane, vitamins, hormones, insulation, protection,
transmission of the nerve impulse
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