Worked solutions to textbook questions 20
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, mono-unsaturated 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 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.
b Draw the structure of glyceryl trioleate, showing the ester bonds.
A2.
a
b
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 Circle an ester functional group.
b Is this fat saturated or unsaturated? Explain.
c 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 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.
c 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 Saturated fats have single bonds between carbon atoms.
b Monounsaturated fats have a single carbon to carbon double bond.
c Polyunsaturated fats have multiple double bonds.
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 Whereas glucose is a monosaccharide, maltose is made up of two monosaccharide molecules bonded together and starch is a polymer of monosaccharide molecules.
b condensation
c hydroxy functional groups
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 What is meant by ‘hydrolysis’?
b Circle a hydroxy and a glycosidic functional group in the molecule.
c Draw the structures of the hydrolysis products.
d 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 A hydrolysis reaction is one in which water is a reactant.
b
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.
Q9.
a i Write a chemical equation for the condensation reactions between two glucose molecules to form a maltose molecule.
ii Draw structural formulas for all molecules involved in this reaction and circle the glycosidic linkage in maltose.
b i Write a chemical equation for the condensation reactions between a glucose molecule and a fructose molecule to form a sucrose molecule.
ii 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
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
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 H2NCH(CH2OH)COOH + H2NCH(CH2SH)COOH ®
H2NCH(CH2OH)CONHCH(CH2SH)COOH + H2O
c condensation
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.
Q17.
Draw the structure of the two amino acids produced when the dipeptide in Figure12.30 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 Find the names and describe the functions of two enzymes in the body.
b Explain why the action of enzymes justifies the statement ‘Enzymes make life possible’.
c Why is the action of an enzyme often described as operating like a ‘lock and key’?
A19.
a 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.
b 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.
c 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 Denaturation occurs when there is a change to the structure and shape of an enzyme that prevents it from functioning.
b 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.
c no
d 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.
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 CPK levels rise after vigorous exercise.
b 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.
Chapter review
Q25.
Explain the difference between:
a a fatty acid and a fat
b a polyunsaturated fat and a saturated fat
A25.
a 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.
b 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.
A27.
a glycerol
b
c Ester group (circled in part b)
d polyunsaturated
e 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 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
b 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.
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.