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Chapter 12 Pathways to biomolecules
E1.
Since the amount of polyunsaturated fats (PUFA) in your diet may affect your health,
it is worthwhile to look closely at the contents of several different foods that you
might use to prepare sandwiches. Use the Internet and food labels to investigate the
PUFA content of butter, several different brands of margarine, and avocados.
a Which have the highest total fat content?
b What proportion of the fat content is unsaturated?
c Does the labelling distinguish between the amount of saturated, monounsaturated and polyunsaturated fats?
d Is information about the source of these fats provided on the label?
EA1.
Answer will depend on student’s research.
Q1.
Which two functional groups are directly involved in the formation of a lipid?
A1.
hydroxy and carboxy
Q2.
a
b
Write an equation showing the formation of the triglyceride glyceryl trioleate
from the reaction of glycerol and oleic acid, CH3(CH2)7CH=CH(CH2)7COOH.
Draw the structure of glyceryl trioleate, showing the ester bonds.
A2.
a
b
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Q3.
A fat present in a vegetable oil has the structure shown in Figure 12.8.
Figure 12.8
A fat found in a vegetable oil.
a
b
c
Circle an ester functional group.
Is this fat saturated or unsaturated? Explain.
The fat is broken down into two different substances in the stomach and small
intestine.
i Write the semi-structural formulas of the reaction products.
ii What chemical, apart from the fat, is a reactant in this process?
iii What name is given to this type of reaction?
A3.
a
b
c
The fat is saturated because the alkyl groups in the molecules (CH3(CH2)14–)
contain only single carbon–carbon bonds. Alkyl groups with the general formula
CnH2n + 1 are saturated.
i HOCH2CHOHCH2OH and CH3(CH2)14COOH
ii water
iii hydrolysis
Q4.
Describe the types of fatty acids that combine with glycerol to form:
a saturated fats
b mono-unsaturated fats
c polyunsaturated fats
A4.
a
b
c
Saturated fats have single bonds between carbon atoms.
Monounsaturated fats have a single carbon to carbon double bond.
Polyunsaturated fats have multiple double bonds.
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Q5.
Sunflower oil is a liquid and can be used to make margarine. How are polyunsaturated
oils, such as sunflower oil, converted to a solid form?
A5.
Some of the double bonds in sunflower oil are converted to single bonds via a
catalysed addition reaction with hydrogen.
Q6.
Glucose is the most abundant monosaccharide. Draw a structural formula for glucose
and use it to explain why it is highly soluble in water.
A6.
The glucose molecule has five polar OH groups which form hydrogen bonds with
water. As a result glucose is highly soluble in water.
Q7.
Glucose is described as a monosaccharide, maltose as a disaccharide and glycogen as
a polysaccharide.
a What is the difference between these three types of carbohydrate?
b What type of reaction is involved in converting glucose to maltose and then to
glycogen?
c Which functional group in the molecules is involved in these reactions?
A7.
a
b
c
Whereas glucose is a monosaccharide, maltose is made up of two
monosaccharide molecules bonded together and starch is a polymer of
monosaccharide molecules.
condensation
hydroxy functional groups
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Q8.
The structure of lactose is shown in Figure 12.21. Lactose undergoes hydrolysis in the
digestion process.
Figure 12.21
The structure of lactose.
a
b
c
d
What is meant by ‘hydrolysis’?
Circle a hydroxy and a glycosidic functional group in the molecule.
Draw the structures of the hydrolysis products.
Lactose is a carbohydrate. Why does 10 g of lactose provide more energy for our
bodies than 10 g of the main carbohydrate found in celery?
A8.
a
b
A hydrolysis reaction is one in which water is a reactant.
c
d
Our digestive system secretes enzymes that can rapidly hydrolyse lactose to
glucose, which is used in respiration. We do not produce enzymes that can break
down cellulose, the main carbohydrate in celery.
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Q9.
a
i
ii
b
i
ii
Write a chemical equation for the condensation reactions between two
glucose molecules to form a maltose molecule.
Draw structural formulas for all molecules involved in this reaction and
circle the glycosidic linkage in maltose.
Write a chemical equation for the condensation reactions between a glucose
molecule and a fructose molecule to form a sucrose molecule.
Draw structural formulas for all molecules involved in this reaction and
circle the glycosidic linkage in sucrose.
A9.
a i, ii
b i, ii
Q10.
Starch, glycogen and cellulose are all polymers that contain glucose monomers.
Explain why the human body can completely digest the starch and glycogen but
cannot digest cellulose to any great extent.
A10.
Humans lack the enzyme (cellulase) that catalyses the hydrolysis of cellulose to form
glucose.
Q11.
Write a chemical equation for:
a the hydrolysis of sucrose
b the hydrolysis of tristearin (see Figure 12.3)
A11.
a
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b
E2.
Why must vegetarians ensure that their diet contains complementary proteins?
AE2.
The human body only manufactures some of the required amino acids. The other
amino acids, called essential amino acids are provided in our food. Animal protein
provides all these essential amino acids. Vegetable protein is often deficient in one or
more essential amino acids.
E3.
A healthy diet includes proteins that include essential amino acids. Why should
essential amino acids be included in a healthy diet?
AE3.
Nine out the 20 amino acids required by the body cannot be manufactured in the
body. They must be provided by the food we eat. These are called essential amino
acids.
E4.
Kwashiorkor or protein–energy malnutrition is a disease found in some traditional
African societies. Find out more about this disease and its causes.
AE4.
Lack of protein in the diet of young children causes a form of starvation known as
protein–energy malnutrition. It is the most common form of death of young children
in the developing world.
Q12.
What two functional groups are present in all amino acids?
A12.
amino and carboxy
Q13.
Draw the structural formula of glycine as it is likely to exist:
a in an acidic solution
b in an alkaline solution
c in neutral solution
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A13.
a
b
c
Q14.
With the aid of Table 12.7:
a draw structural formulas of serine and cysteine
b write an equation to show the formation of a dipeptide from these amino acids
c name the type of reaction in part b
A14.
a
b
c
H2NCH(CH2OH)COOH + H2NCH(CH2SH)COOH 
H2NCH(CH2OH)CONHCH(CH2SH)COOH + H2O
condensation
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Q15.
Why is protein more important in your diet as a source of raw materials for building
other molecules, rather than as a source of energy?
A15.
There are no substitutes for the essential amino acids required by your body. The
other nutrients, fats and carbohydrates, can supply your energy needs.
Q16.
Explain what is meant by the primary, secondary and tertiary structures of a protein.
Clearly outline the bonding type involved at each level of protein structure. (Include
labelled diagrams.)
A16.
Primary structure refers to the sequence of amino acids in a protein.
The folding of a section due to, for example, hydrogen bonding between peptide links
is known as the secondary protein structure.
The overall three-dimensional shape of a protein is called the tertiary structure.
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Q17.
Draw the structure of the two amino acids produced when the dipeptide in
Figure 12.30 undergoes hydrolysis.
Figure 12.30
A17.
Q18.
Use the information in Table 12.7 to sketch the structural formula of the tripeptide
that would be represented as ala ser val.
A18.
Q19.
a
b
c
Find the names and describe the functions of two enzymes in the body.
Explain why the action of enzymes justifies the statement ‘Enzymes make life
possible’.
Why is the action of an enzyme often described as operating like a ‘lock and
key’?
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A19.
a
b
c
Enzymes in the body include pepsin, which hydrolyses peptide bonds of certain
amino aids:
DNA polymerase – which replicates and repairs DNA
lactase – which breaks down the sugar lactose in the small intestine
salivary amylase – which breaks down polysaccharides in the mouth.
Almost all the chemical reactions occurring in living creatures are controlled by
enzymes. Enzymes speed up the reactions that are essential for life processes by
as much as 1010 times; reactions that do not contribute to the functioning of a
creature are not catalysed and occur at much slower rates.
The shape and functional groups in the active site of the enzyme allow it to bind
only with certain substrates so that only a specific reaction is catalysed. In a
similar way, a lock will only open using a key of a certain shape.
Q20.
What are the main differences between enzymes and inorganic catalysts?
A20.
Compared to inorganic catalysts, enzymes can produce much faster reaction rates,
operate under milder conditions and are highly selective. Whereas an inorganic
catalyst might increase the rates of many different reactions, enzymes are usually
effective for just one reaction.
Q21.
The enzyme carbonic anhydrase catalyses the decomposition of carbonic acid
molecules to carbon dioxide and water in the lungs. When heated to more than 60°C,
the enzyme becomes denatured.
a What is meant by the term ‘denatured’?
b Describe the events that usually occur to the structure of an enzyme when it is
denatured.
c Does the primary structure of the carbonic anhydrase enzyme change during this
process?
d Why is the functioning of the enzyme closely related to its tertiary structure?
A21.
a
b
c
d
Denaturation occurs when there is a change to the structure and shape of an
enzyme that prevents it from functioning.
Denaturation usually involves disruption of the bonds that hold the enzyme’s
protein chain in a particular shape, followed by unfolding of the chain. The
unfolded chains will often clump together, in a process called coagulation.
no
The overall three-dimensional shape of an enzyme is known as its tertiary
structure. Since enzyme action depends upon the existence of a suitable hollow or
cavity within the molecule, the tertiary structure is crucial to the enzyme’s
operations.
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Q22.
Explain why LDH is not used as a specific marker for heart attack.
A22.
Increase in the level of the protein marker LDH can be caused by other diseases such
as anaemia or pneumonia.
Q23.
A competitor collapses during a race. A blood test showed a raised level of total CPK
enzyme.
a Does the test indicate that the competitor has had a heart attack?
b What other enzyme tests would confirm a heart attack?
A23.
a
b
CPK levels rise after vigorous exercise.
Troponin and a specific form of CPK, CPK-MB
Q24.
Explain why doctors sometimes rely on information from a number of tests and
observations to make a diagnosis rather than relying solely on the presence of a
protein marker.
A24.
A number of factors other than the disease under investigation may lead to an increase
in the level of a specific protein marker.
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Chapter review
Q25.
Explain the difference between:
a a fatty acid and a fat
b a polyunsaturated fat and a saturated fat
A25.
a
b
A fatty acid molecule consists of a hydrocarbon chain with a carboxy (–COOH)
group at one end. A fat, or triglyceride, is the product of a condensation reaction
between three fatty acid molecules and a glycerol molecule.
The fatty acid groups that make up a saturated fat contain only single carbon–
carbon bonds. A polyunsaturated fat contains fatty acid groups with more than
one carbon–carbon double bond.
Q26.
By referring to Figure 12.3, write reactions that show:
a the hydrolysis of the fat tristearin
b the complete oxidation of the products from part a
A26.
a
b
2C3H8O3(aq) + 6O2(g)  6CO2(g) + 8H2O(l)
Glycerol
C17H35COOH(aq) + 26O2(g)  18CO2(g) + 18H2O(l)
Stearic acid
Q27.
The structure of a fatty acid, linoleic acid, is shown in Figure 12.38.
a Name the chemical needed to convert linoleic acid into a lipid.
b Draw the structure of the lipid made from linoleic acid.
c Name and circle the functional group found in the lipid molecule.
d Classify the lipid as saturated, mono-unsaturated or polyunsaturated.
e Is this fat likely to be a solid or liquid at room temperature? Explain.
Figure 12.38
The structure of linoleic acid.
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A27.
a
b
glycerol
c
d
e
Ester group (circled in part b)
polyunsaturated
The fat is likely to be a liquid at room temperature. Decreasing saturation results
in relatively greater spacing between the hydrocarbon chains in the fatty acids
and, therefore, reduced dispersion forces.
Q28.
a
b
By referring to Figure 12.13, draw structural formulas to show:
i the formation of a disaccharide from two fructose molecules
ii the hydrolysis of maltose
Apart from water and maltose, what other substance is required for the hydrolysis
of maltose?
A28.
a
i, ii
b
enzyme
Q29.
Why is glycogen sometimes called ‘animal starch’?
A29.
Glycogen is the main storage polysaccharide in animals. It therefore fulfils the same
role in animals that starch does in plants.
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Q30.
The human brain relies almost exclusively on glucose as an energy source. Normal
functioning of the brain depends on a constant supply of glucose from the blood.
Explain how your body maintains a fairly constant concentration of glucose in the
blood even though you don’t eat foods that supply glucose constantly throughout the day.
A30.
When the body digests food and absorbs glucose, the glucose is transported by the
blood to the liver where it is converted to glycogen, a storage polysaccharide. The
glycogen is slowly hydrolysed between meals to maintain a fairly constant
concentration of glucose in the blood for use by tissues such as those in the brain.
Q31.
Explain why body builders and weight-lifters eat a large amount of protein.
A31.
Body builders and weight-lifters wish to develop muscle tissue. Muscle tissue
comprises a substantial amount of protein; to build muscle tissue the diet must include
a rich source of amino acids—protein.
Q32.
When alanine and glycine react, two different dipeptides can be formed.
a Write the formula for each dipeptide.
b How many tripeptides can be formed from three different amino acids?
c Proteins are formed using 20 different amino acids and can be hundreds of amino
acid units in length. On the basis of your answers to parts a and b, what can you
say about the number of different proteins possible?
A32.
a
b
c
H2NCHCH3CONHCH2COOH
H2NCH2CONHCHCH3COOH
6 (using each amino acid once in each peptide)
very large numbers
Q33.
Aspartic acid is one of the non-essential amino acids (Figure 12.39). It exists as a
zwitterion at pH 2.8.
Figure 12.39
The structure of aspartic acid.
a
b
Name:
i the acidic functional group
ii the basic functional group
Draw the structure of the molecule as it is most likely to exist in a solution at:
i pH much more than 2.8
ii pH much less than 2.8
iii pH 2.8
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A33.
a
i
ii
carboxy
amino
b
Either or both carboxy protons may be lost.
Q34.
a
b
c
Circle the peptide linkages in this part of the protein chain.
Write the formulas of the amino acids that made up this part of the chain.
Where in the body does hydrolysis of proteins to form amino acids take place?
Figure 12.40
A section of a protein molecule.
A34.
a
b
c
H2NCHCH3COOH; H2NCH(CH2SH)COOH
H2NCH2COOH
H2NCH(CH2OH)COOH
stomach and small intestine
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Q35.
Children need proportionately more protein in their diets than adults. Suggest why.
A35.
Humans and other animals use protein to build bones, tendons, ligaments and muscle.
Since these processes occur more rapidly during childhood and adolescence, children
and teenagers need proportionately more protein in their diets to provide the amino
acids required for the formation of protein.
Q36.
Draw a structure for an amino acid that could not be classified as a 2-amino acid
(-amino acid).
A36.
(i.e. –NH2 and –COOH are not joined to the same carbon)
Q37.
Scientists have spent many years trying to develop a form of the protein insulin that
can be given to diabetics orally rather than by injection. Suggest an aspect of the
chemistry of proteins that has so far prevented insulin from being taken orally.
A37.
Proteins are readily denatured by acids. Stomach acid has prevented the ingestion of
the protein insulin. The protein is hydrolysed into its component amino acids during
digestion.
Q38.
Proteins are often described in terms of their primary, secondary and tertiary
structures. What is the difference between these levels of structure and how does each
contribute to the role of the protein?
A38.
Primary structure: the sequence of the amino acids that make up the polymer chain.
This determines the entire shape of the protein.
Secondary structure: the coiling, folding or pleating of the protein chain caused by
hydrogen-bonding interactions of adjacent amino acids. These regular structures
extend along one dimension like coils in a spring and are important in fibres such as
hair and wool.
Tertiary structure: the bending and folding of the protein chain to form a compact
three-dimensional shape. These globular shapes with a low surface to volume ratio are
important for proteins in solution such as enzymes in blood.
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Q39.
Mutations can cause the wrong amino acid to be inserted into a protein chain when it
is being assembled. In some instances such a mistake has no effect on the function of
the protein, but in others it disables the protein. Explain how a single change in a
protein’s primary structure could cause such dramatically different results.
A39.
The activity of an enzyme, which is a type of protein, is dependent on its threedimensional shape. The tertiary structure results from a combination of hydrogen
bonding, electrostatic attraction between polar groups and disulfide linkages. If the
wrong amino acid is introduced into the chain, these bonds may not form and the
tertiary structure may be distorted. In some instances the enzyme becomes inactive,
depending upon how the tertiary structure is altered.
Q40.
Enzymes allow some reactions to proceed far more rapidly than they would if
inorganic catalysts were used. Find out why enzymes are not used widely in industrial
processes.
A40.
Scientists are investigating the use of enzymes as catalysts in industrial reactions. At
present, enzymes are not used widely in industry because they tend to be present in
biological systems in small quantities, so supplies for industrial use are limited (in
future, enzymes may be generated in larger quantities by using genetic engineering
techniques); they function only within a narrow range of temperatures, pressures and
pH, and outside these ranges they are easily broken down; and their catalytic action
and structure are often poorly understood by chemists.
Since enzymes function under mild conditions, their use in industry could lead to
enormous cost savings. One area of current research involves the synthesis of
molecules with shapes similar to natural enzymes and the capacity to catalyse specific
reactions.
Q41.
Jellied pineapple dessert cannot be made using gelatine and fresh pineapple because,
instead of setting, an enzyme in the pineapple causes molecules in the gelatine to
break down. Suggest how jellied pineapple might be prepared.
A41.
The catalytic property of the enzyme can be destroyed by changing its shape—in a
process called denaturation. This can be done by heating the pineapple. Alternatively,
canned pineapple can be used to make this dessert since the fruit is heated during the
canning process.
Q42.
Digestion of proteins occurs in both the stomach and the small intestine. Trypsin is an
enzyme that catalyses the hydrolysis of proteins. Trypsin is secreted into the small
intestine, where the pH is approximately 8. Would you expect this same enzyme to be
secreted into the stomach, where the pH is approximately 2? Explain your reasoning.
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A42.
The functioning of enzymes is specific. A different enzyme catalyses the hydrolysis of
protein in the stomach.
Q43.
a
b
The steps in the action of an enzyme involve, in particular, an active site and a
substrate. With the use of a diagram describe in detail the action of an enzyme.
The forces of attraction that enable a substrate to bind to an active site can vary.
Describe four such forces.
A43.
a
b
hydrogen bonds, ion-dipole interaction, dipole-dipole interactions, ionic
interactions.
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Q44.
Read the magazine article and answer the following questions.
a This article reflects current research in the development of enzymes for
commercial use. What are the advantages of enzymes compared to inorganic
catalysts?
b What tests would you require the Novo-Nordisk company to perform on the
enzyme Lipolase before it was released commercially?
c Why might governments be cautious about licensing the use of genetically
engineered organisms to produce enzymes? Discuss the issues involved in this
question with other class members.
Engineered enzyme washes whiter than white
Until now, no one has developed a commercially available enzyme that will
break down fats.
In its hunt for a suitable enzyme, Novo-Nordisk investigated a range of
enzymes called lipases. Molecular biologists at the company identified a fungus
which will produce a specific type of lipase that is good at breaking down the
fats found in human foods.
Although the fungi produce these useful lipases naturally, they do not produce
enough to be financially viable. So the geneticists identified the sequence of
nitrogen bases which make up the DNA of the fungus and cloned that sequence.
They then spliced the section of the DNA which holds the gene that codes for
the lipase into a new host organism, a fungus. [They] found that it will produce
the lipase in sufficient quantities.
Novo complains bitterly at the length of time it has had to wait for a licence to
produce Lipolase in Denmark … The same process took only four weeks in
Japan.
A44.
a
b
c
Enzymes are much faster, operate under milder conditions and are more selective
than inorganic catalysts.
Tests could include a study of the effect of the enzyme on human health and on
the environment.
Issues to consider when answering this question include the potential hazards
associated with the development of new organisms and enzymes.
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Q45.
What functional groups are required to react to form each of the following, and with
which biological polymers are each of these linkages associated?
a an ester linkage
b a glycosidic linkage
c a peptide linkage
A45.
a
b
c
carboxy and hydroxy groups; lipids
two hydroxy groups; carbohydrates
carboxy and amino groups; proteins
Q46.
a
b
c
Explain the difference between condensation reactions and hydrolysis reactions.
What is the importance of these reactions in the human body?
Write equations for condensation reactions between:
i two glucose molecules
ii a glycerol molecule and three palmitic acid molecules (see Table 12.1)
iii two alanine molecules
A46.
a
b
c
Condensation reactions involve the linking together of two small molecules and
the elimination of a small molecule, usually water. Hydrolysis reactions consume
water and they can often be regarded as the reverse of condensation reactions.
Vital biochemical processes include condensation reactions (e.g. synthesis of
lipids, proteins and polysaccharides) and hydrolysis reactions (e.g. digestion).
i
ii
iii
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