Chapter overviewChapter overview
TABLE 2.3.1 MAJOR FUNCTIONS OF CARBOHYDRATES IN THE HUMAN BODY
10 List the major functions of carbohydrates in the human body, giving one example of a carbohydrate that performs each function
Section 2.3 Exercises
Oils and fats are part of a class of molecules called lipids. Fats and oils are triglycerides (see fi gure 2.4.5), while other members including phospholipids and steroids have different structures from the triglycerides. Triglycerides are generally very large, non-polar molecules that are insoluble in water. A
triglyceride is formed by the condensation reaction between three fatty acid molecules (carboxylic acids with very long hydrocarbon chains) and one glycerol molecule. It therefore contains three ester linkages (see fi gure 2.4.5). However, not all lipids are hydrophobic. Phospholipids, together with glycolipids, cholesterol and proteins are the major components of all biological membranes.
A phospholipid molecule has a hydrophilic (polar) head that consists of a phosphate group and hydrophobic (or lipophilic) tail made up of two long hydrocarbon chains. In a membrane, twin layers (a bilayer) of phospholipids form, with the non-polar tails lining up against one another, forming a
membrane with hydrophilic heads on both sides facing the aqueous surroundings. Lecithin (phosphatidylcholine) is a phospholipid that can be isolated from egg yolk. (The Greek word for egg yolk is lekithos.)
Cholesterol is a sterol, an alcohol with a fused ring system.
This substructure is also found in steroid hormones such as testosterone and progesterone (see fi gure 2.6.2, p. 113).
Cholesterol is classifi ed as an alcohol because it has a hydroxyl group (–OH) in position 3 of the ring system. Cholesterol is produced by the liver and is found in all body tissues, where it helps to control the permeability of cell membranes. Cholesterol derivatives in the skin are converted to vitamin D when the skin is exposed to sunlight. Steroids are primarily made up of carbon and hydrogen with a small amount of oxygen.
HO
Figure 2.4.3 Cholesterol is made up of four fused six- and five-membered rings with a hydroxyl group on the first ring and a short hydrocarbon chain on the fourth ring.
CH3(CH2)14 C
Figure 2.4.2 The structure of lecithin.
Since cholesterol is insoluble in blood, it is transported in the circulatory system within lipoproteins. There is a large range of lipoproteins within blood, of which two are low density lipoprotein (LDL) and high density lipoprotein (HDL). There is no difference between the chemical composition of the cholesterol that is carried by the various lipoproteins; however, HDL molecules are smaller and denser than LDL molecules due to the larger proportion of protein (with a higher molecular mass than cholesterol) in HDL.
The LDL molecules contain much more cholesterol than HDL molecules.
2.4 LIPIDS
B.4.1
Compare the composition of the three types of lipids found in the human body.
© IBO 2007
hydrophilic phosphate group
hydrophobic fatty acids
Figure 2.4.1 A phospholipid bilayer such as that found in cell membranes.
B.4.2
Outline the difference between HDL and LDL cholesterol and outline its importance. © IBO 2007
CHAPTER 2 HUMAN BIOCHEMISTRY Higher concentrations of LDL (or inversely, low concentrations of
HDL) are strongly associated with cardiovascular disease. LDL promotes the narrowing of arteries (atherosclerosis) by
accumulating beneath the inner elastic wall of the artery and the smooth muscle surrounding it. This disease process leads to heart attack, stroke and other diseases caused by the blockage of large peripheral arteries. As a result, cholesterol bound up in LDL is known as ‘bad cholesterol’. On the other hand, it is hypothesized that high concentrations of HDL can remove cholesterol from cells and reduce atherosclerosis by removing cholesterol from blockages within arteries and transport it back to the liver for excretion or re-utilization. For this reason HDL-bound cholesterol is sometimes called ‘good cholesterol’.
Triglycerides are formed by the condensation reaction between glycerol, a polyalcohol, and fatty acids, long-chain carboxylic acids.
Note that an ester link forms (fi gure 2.4.5). Triglycerides are usually classifi ed according to the type of fatty acids involved in their formation, although a triglyceride does not have to be composed of three identical fatty acids. We fi nd that most naturally occurring fats contain a mixture of saturated, mono-unsaturated and polymono-unsaturated fatty acids so they are classifi ed according to the predominant type of unsaturation present.
Figure 2.4.5 Triglycerides form by condensation reactions.
ester linkage
Saturated fatty acids, like other saturated hydrocarbons, have a hydrocarbon chain that contains no carbon–carbon double bonds. Mono-unsaturated fatty acids have one carbon–carbon double bond and polyunsaturated fatty acids have more than one carbon–carbon double bond in the hydrocarbon chain.
The formula of a fatty acid can be used to determine the number of carbon–
carbon double bonds. An unsaturated fatty acid will have the general formula CnH2n + 1COOH. For every double bond that is present in a fatty acid (the degree of unsaturation), two hydrogen atoms will be lost from the formula.
Consequently, the general formula of a mono-unsaturated fatty acid will be CnH2n – 1COOH and of a polyunsaturated fatty acid with two carbon–carbon double bonds will be CnH2n – 3COOH. The pattern continues for other polyunsaturated fatty acids.
Unsaturation in fatty acid leads to ‘kinks’
in the chain. The unsaturated molecules therefore do not pack closely together.
This leads to weaker van der Waals’ forces between the chains and a lower melting
Figure 2.4.4 The accumulation of LDL in the artery wall causes arteries to become narrowed, leading to coronary heart disease.
B.4.6
Describe the condensation of glycerol and three fatty acid molecules to make a triglyceride. © IBO 2007
B.4.3
Describe the difference in structure between saturated and unsaturated fatty acids.
© IBO 2007
Figure 2.4.6 The formulas of fatty acids match a pattern.
point. Generally we classify fats as triglycerides that are solid at room
temperature, and oils as those that are liquids at room temperature. Oils are more likely to contain a large number of carbon–carbon double bonds, so their molecules do not pack together as well as those with no carbon–carbon double bonds, thus explaining their lower melting point.
CHEM COMPLEMENT
Margarine and the presence of trans fats Margarine is the man-made equivalent of butter, the saturated fat derived from the cream from cow’s milk. With butter not always being as available as consumers would like, the possibility of a mass produced vegetable-based substitute became attractive. The difference between butter and vegetable oils is the degree of saturation. Butter is made up of unsaturated triglycerides, so has a higher melting point than polyunsaturated vegetable oils. Hydrogenation, first developed in the 1890s by Paul Sabatier and further improved by the German chemist Wilhelm Normann in 1901, was found to be the way to decrease the number of double bonds in a triglyceride and consequently increase its melting point.
The process uses metal catalysts such as nickel and palladium; however, it has been shown that trans fatty acids are formed if the unsaturated fat leaves the surface of the catalyst too quickly. These trans fats were suggested as early as 1988 to be the cause of large increases in coronary artery disease and deaths from heart disease. In recent years health organizations have lobbied governments for the removal of trans fats from foods and currently many countries require that trans fats be included on the labelling of foods so that consumers can avoid them since their presence is just as bad for health as the saturated fats that they have replaced.
Another class of unhealthy fatty acids originate from the industrial
hydrogenation of plant oils to reduce their levels of unsaturation. These fatty acids are known as trans fatty acids because the hydrocarbon chain takes up a trans rather than cis confi guration around the double bonds in the fatty acid chain. With this confi guration, the fatty acid chain becomes straight like that of a saturated fatty acid, rather than kinked as in the case of naturally occurring polyunsaturated fatty acids. This lowers the melting point of the lipid making it more convenient to use, but the consumption of trans fatty acids is quite detrimental to health. It increases the risk of coronary heart disease by raising the levels of LDL cholesterol in the blood and also lowering the levels of HDL cholesterol.
Two important fatty acids are linoleic acid, C18H32O2, and linolenic acid, C18H30O2. These fatty acids are essential fatty acids; they are essential in the diet of all mammals. Linoleic acid is an omega-6 fatty acid, while linolenic acid is an omega-3 fatty acid. Notice that their structures are very similar, only differing by one carbon–carbon double bond and the consequent reduction in number of hydrogen atoms. These two substances work together in the body to promote health. Linoleic acid is used in the biosynthesis of prostaglandins, while linolenic acid is converted into two other omega-3 fatty acids:
eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) both of which have much more use in the body than linolenic acid itself. Omega-3 fatty acids such as linolenic acid, EPA and DHA help reduce infl ammation, while most omega-6 fatty acids including linoleic acid tend to promote infl ammation. An inappropriate balance of these essential fatty acids contributes to the development of disease, while a proper balance helps to maintain and even improve health. A healthy diet should consist of roughly two to four times more
H
cis arrangement about a double bond
CH2
trans arrangement about a double bond CH2
Figure 2.4.7 The difference between the cis and trans arrangement of atoms around a double bond.
B.4.4
Compare the structures of the two essential fatty acids linoleic (omega-6 fatty acid) and linolenic (omega-3 fatty acid) and state their
importance. © IBO 2007
CHAPTER 2 HUMAN BIOCHEMISTRY
HO C
O linoleic acid
HO C
C17H31 O
or
HO C
O linolenic acid
HO C
C17H29 O
or
Figure 2.4.8 The structures of linoleic acid and linolenic acid.
Halogens undergo addition reactions with unsaturated hydrocarbons and therefore also with unsaturated fatty acids. In particular, reactions between bromine, Br2, or iodine, I2, and unsaturated fatty acids are easy to monitor because bromine and iodine are decolourized (become colourless) when they react with unsaturated hydrocarbons. The reaction with iodine is less dangerous than that with bromine, since an aqueous solution of bromine (bromine water) releases fumes of bromine, creating a respiratory hazard.
Aqueous solutions of iodine are also more stable than those of bromine and can be standardized using a sodium thiosulfate solution. The reaction between iodine and an unsaturated fatty acid can then be performed as a titration.
C
C + I I
I I
C C
One mole of iodine reacts with one mole of carbon–carbon double bonds. This means that the number of mole of I2 reacting with one mole of a fat or oil indicates the number of double bonds present in the fat or oil molecule.
The iodine number of a fat or an oil is the mass of iodine that reacts with 100 g of the lipid. The more unsaturated an oil is, the higher its iodine number will be.
Worked example 1
0.010 mol of linoleic acid reacts with 40 cm3 of a 0.50 mol dm–3 of I2. Determine the number of double bonds present in each linoleic acid molecule.
Solution
n(I2) = cV = 0.50 × 0.040 = 0.020 mol
In 0.010 mol of linoleic acid there are 0.020 mol of double bonds
∴ There are 0 020 0 010
.
. = 2 double bonds per molecule.
Iodine numbers
B.4.5
Define the term iodine number and calculate the number of C=C double bonds in an unsaturated fat/oil using addition reactions.
© IBO 2007
Testing for unsaturation with bromine
Worked example 2
The general formula for saturated fatty acids is CnH2n + 1COOH. The molecular formula of arachidonic acid is C20H32O2.
a Determine the number of carbon–carbon double bonds in arachidonic acid.
b Determine the iodine number of arachidonic acid.
Solution
a If arachidonic acid was a saturated fatty acid, by matching to the general formula, we can see that its formula would be C19H39COOH. However its molecular formula is given as C20H32O2 or C19H31COOH.
This formula has 8 less hydrogen atoms in it than a saturated fatty acid would have, indicating that arachidonic acid has 4 carbon–carbon double bonds in each of its molecules.
b The iodine number of a fat or an oil is the mass of iodine that reacts with 100 g of the lipid.
First we must fi nd the number of mole of the arachidonic acid in 100 g.
M(C20H32O2) = 304.52 g mol–1 n = m
M = 100
304 52. = 0.328 mol
Since there are four mole of double bonds per mole of arachidonic acid:
then I2 : C20H32O2 4 : 1
n(I2) = 4 × 0.328
= 1.31 mol m(I2) = n × M(I2)
= 1.31 × 253.8
= 333 g
The iodine number of arachidonic acid is 333.
Like proteins and polysaccharides, triglycerides undergo enzyme-catalysed hydrolysis during digestion. Unfortunately, triglycerides do not dissolve in water, so they are not easily broken down by fat-digesting enzymes (lipase) in the watery content of the gastrointestinal tract. Thus fats tend to take longer to digest than carbohydrates or proteins.
Digestion of fats occurs in the small intestine. In the duodenum (the upper part of the small intestine), bile, produced in the liver but stored in the gallbladder, enters via the bile duct. Bile emulsifi es fats, dispersing them into small droplets which then become suspended in the alkaline contents of the digestive tract. This process of emulsifi cation allows the enzyme lipase, which enters the duodenum from the pancreas, to gain easier access to the fat molecules and thus accelerates their breakdown and digestion. Lipase catalyses the hydrolysis of the triglycerides into glycerol and fatty acids.
The walls of the small intestine are covered by millions of fi nger-like
projections called villi. Inside each villus is a series of lymph vessels and blood Lipids in the diet
B.4.7
Describe the enzyme-catalysed hydrolysis of triglycerides during digestion. © IBO 2007
CHAPTER 2 HUMAN BIOCHEMISTRY The fatty acids are transported in the bloodstream to adipose cells,
where they are stored, or to muscle cells, where they are oxidized for energy. The majority of digested fat is stored as body fat in the adipose cells. The glycerol is transported to the liver where it may be converted into glucose or may be used to help break down glucose to release energy.
In section 2.1 the higher energy value of fats, when compared with carbohydrates, was discussed. The enthalpy of combustion of fat is 37 kJ g–1 while that of carbohydrate is 17 kJ g–1. Although this higher energy value is offset somewhat by the diffi culty in digesting fats, fats have a higher energy value than carbohydrates.
When the formulas of fatty acids and carbohydrates are compared, it is found that there is a greater proportion of oxygen atoms in the carbohydrates than in fatty acids—their degree of oxidation is greater. Therefore the fatty acids have greater potential for oxidation and the subsequent release of energy. The combustion reactions of a fatty acid and a carbohydrate of similar molar mass can be compared to illustrate this difference.
Let us compare a trisaccharide, made up of three glucose units with a molecular formula of C18H32O16 to linoleic acid, C18H32O2.
The combustion reactions are:
C18H32O16+ 18O2→ 18CO2+ 16H2O C18H32O2+ 25O2→ 18CO2+ 16H2O
The standard enthalpy of reaction can be calculated using average bond enthalpy values.
Enthalpy of reaction = ΣD(bonds broken) – ΣD(bonds formed)
where Σ represents the sum of the terms and D represents the bond enthalpy per mole of bonds.
ΔH = [15DC–C+ 21DC–H+ 21DC–O+11DO–H+ 18DO=O] – [36DC=O+ 32DH–O]
The enthalpy of combustion for the trisaccharide C18H32O16 is –6111 kJ mol–1 and the energy content is 12.1 kJ g–1. Figure 2.4.9 A fluorescent light micrograph showing the villi, finger-like projections that increase the surface area of the small intestine.
B.4.8
Explain the higher energy value of fats as compared to
carbohydrates. © IBO 2007
Enthalpy of reaction = ΣD(bonds broken) – ΣD(bonds formed) ΔH = [15DC–C+ 2DC=C+ 31DC–H+ DC=O+DO–H+ DC–O + 25DO=O] – [36DC=O+ 32DH–O]
= [(15 × 348) + (2 × 612) + (31 × 412) + 743 + 463 + 360 + (25 × 496)] – [(36 × 743) + (32 × 463)]
= 33 182 – 41 564
= –8382 kJ M(C18H32O2) = 280.5
Energy content per g = 8382
280 5. = 30.0 kJ g–1
The enthalpy of combustion for the fatty acid C18H32O2 is –8382 kJ mol–1 and the energy content is 30.0 kJ g–1.
From the balanced equations (above) for the combustion of linoleic acid (a fatty acid) and the trisaccharide, the ratio of oxygen to fatty acid is 25 : 1 whereas oxygen to trisaccharide is 18 : 1. A great deal more oxygen is required to oxidize the fatty acid than the trisaccharide. As in the comparison between the complete and incomplete oxidation of hydrocarbons, the reaction that requires a greater proportion of oxygen yields a greater amount of energy.
In summary, we can explain the higher energy content of fats than in carbohydrates as being the result of more energy being released in bond making in products than is required for bond breaking in reactants. This can be traced to the smaller number of hydroxyl groups in a fatty acid than in a carbohydrate and to the larger amount of oxygen required to oxidize a fatty acid than a carbohydrate.
Lipids lead a ‘double life’ with respect to our health. There are many uses in the human body for lipids, but they can easily lead to ill health when they form too great a part of our diet.