TOPIC 4 LIPIDS
4.1 INTRODUCTION
• Lipids are another group of organic substances that play a vital role in organisms.
•
They are an integral part of all cell membranes and are also used as an energy
store.
• Lipids are made of the elements carbon, hydrogen and oxygen, although they
have a much lower proportion of water than other molecules such as
carbohydrates.
• These organic compounds are nonpolar molecules, which are soluble only in
nonpolar solvents and insoluble in water because water is a polar molecule.
• In the human body, these molecules can be synthesized in the liver and are found
in oil, butter, whole milk, cheese, fried foods and also in some red meats.
4.2 PROPERTIES OF LIPIDS
• Lipids are oily or greasy nonpolar molecules, stored in the adipose tissue of the
body.
• Lipids are a heterogeneous group of compounds, mainly composed of
hydrocarbon chains.
• Lipids are energy-rich organic molecules, which provide energy for different life
processes.
• Lipids are a class of compounds characterised by their solubility in nonpolar
solvents and insolubility in water.
• Lipids are significant in biological systems as they form a mechanical barrier
dividing a cell from the external environment known as the cell membrane.
4.3 BIOLOGICAL FUNCTIONS OF LIPIDS
• Lipids perform many functions, such as:
o Energy storage
o Making biological membranes
o Insulation
o Protection - e.g. Protecting plant leaves from drying up
o Buoyancy
o Acting as hormones
4.4 FATS AND OILS
• Fats and oils are important groups of lipids. They are chemically similar but fats
such as butter are solid at room temperature whereas oils such as olive oil are
liquids. Fats come mainly from animal sources while oils are mainly from plant
sources.
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• Like carbohydrates, the chemical elements that all lipid molecules contain are
carbon, hydrogen and oxygen.
• However, lipids contain much lower proportion of oxygen than carbohydrates.
• Fats and oils contain two types of organic chemical substances: fatty acids and
glycerol. These are combined using ester bonds.
4.5 FATTY ACIDS
• They are carboxylic acids with general formula: CH3(CH2)nCOOH, where :
• CH3 represents an alkyl group.
• (CH2) n represents a variable number of hydrocarbon chain, where n varies from 4
to commonly 14 to 24.
• COOH represents a carboxylic group of atoms, which renders the molecule the
acidic characteristic
• Living tissues contain more than 70 different types of fatty acids.
4.5.1
•
4.5.2
The fatty acid configuration
o The fatty acid configuration or structure varies through its hydrocarbon chain.
o They all contain a long hydrocarbon chain- a folded backbone of carbon
atoms with hydrogen atoms attached, and a carboxyl group (-COOH) at one
end.
o The chain usually contains an even number of carbon atoms, with 16 or 18
being the most common number.
o This variation accounts for the chemical properties or identity of a lipid.
o Fatty acids are lipids because of the nonpolar character of their hydrocarbon
‘tails’, which dominate their properties.
Fatty acids vary in two ways:
1. The length of the carbon chain can differ (but is often15-17 carbon
atoms long)
2. The fatty acid may be a saturated fatty acid or an unsaturated fatty
acid.
Saturated fatty acids
o They are saturated as in the case of stearic acid (Figure 4.1) when the
hydrocarbon chain holds a maximum number of hydrogen atoms around the
carbon atom.
o In this case every carbon atom in the hydrocarbon part of the molecule has a
single bond around it, so the molecule is highly reduced or put differently less
oxidised.
o By implication such molecules will have relatively higher energy reserves.
o Contains maximum number of hydrogen atoms and all carbon atoms in the
hydrocarbon chain have single bonds.
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o The saturated fatty acids have higher melting points compared to unsaturated
fatty acids of the corresponding size due to their ability to pack their molecules
together thus leading to a straight rod-like shape.
o Hence they are solids at ordinary temperature and pressure (room temperature).
o Animal fats are a source of saturated fatty acids.
o They pack easily and form rigid structures (e.g., fatty acids are found in
membranes).
Figure 4.1 Stearic acid general formula: CH3
(CH2)16 COOH. Stearic acid is a common example
of saturated fatty acid. It is majorly found as a
component of shea butter and cocoa butter.
4.5.3
Unsaturated fatty acids
• When unsaturated as in the case of oleic acid (Figure 4.2), the hydrocarbon
part of the molecule has one or more double bonds between carbon atoms.
• The molecule accounts for fewer hydrogen atoms around the carbon atom
than usual.
• The molecule is less reduced and slightly more oxidised. Such molecules
yield relatively less chemical energy.
• A fatty acid with one double bond is called monounsaturated. If it contains
two or more double bonds, it is called polyunsaturated.
• The melting point of a fatty acid is influenced by the number of double bonds
that the molecule contains and by the length of the hydrocarbon tail.
o The more double bonds it contains, the lower the melting point.
o As the length of the tail increases, the melting point increases.
• Plants are the source of unsaturated fatty acids.
Oleic acid formula:
CH3-(CH2)7-CH=CH-(CH2)7-COOH
• Contains fewer number of hydrogen atoms.
• Some carbon atoms in the hydrocarbon chain have double bonds.
• The molecule bends at the site of the double bonds and therefore cannot be
straight (Figure 4.3).
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Figure 4.2 Oleic acid general formula: CH3-(CH2)7CH=CH-(CH2)7-COOH. Oleic acid is a common example
of unsaturated fatty acid.
• Therefore the molecules cannot pack very tightly together because the attraction between
molecules is weak.
• Hence they are liquids at ordinary temperature and pressure
• These molecules are examples of oils
Figure 4.3 Oleic acid showing its fluidness because of
ability to bend at the position of the double bond.
4.6 GLYCEROL
• The formula and molecular configuration for glycerol are given below.
Glycerol formula: C3H8O3
H
H
C
OH
H
C
OH
H
C
OH
H
Figure 4.5 Glycerol configuration
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• Is glycerol a carbohydrate or not?
• Glycerol has 3 carbon atoms, 8 hydrogen atoms, and 3 oxygen atoms.
• It has a molecular weight of 92.09 g/mol and its IUPAC name is 1, 2, 3Propanetriol or 1, 2, 3- Trihydroxypropane.
• Glycerol is seen in biological systems as an intermediate in carbohydrate and lipid
metabolism because surplus carbohydrate can be converted into long chain fatty acids
and esterified with the three hydroxyl groups.
Uses of Glycerol
•
Its use as a commercially important chemical began with its application in the
production of dynamite. Dynamite was necessary in the discovery and extraction
of underground minerals, and in the construction of infrastructure. Therefore, it
propelled industrial development.
• Glycerol is used in the cosmetics industry as a moisture-control reagent and to
enhance the texture of lotions and creams. Glycerol’s ability to retain moisture
and its emollient properties make it an attractive ingredient in many moisturizing
formulations.
•
In food, the utility of glycerol arises from its ability to form inter-molecular
hydrogen bonds, especially with water molecules. This increases the water
content in preserved food, without compromising on shelf life, and also
enhances viscosity and texture.
• Its low toxicity and lack of a disagreeable odor or flavor allow the use of glycerol
as an emulsifier. Glycerol usage in the pharmaceutical industry is to improve
smoothness and taste.
• It is used in the creation of tablets so that they are easy to swallow. The coating
can disintegrate within the body. Cough lozenges often use glycerol to give a
sweet taste.
• Suppositories of glycerol can act as laxatives since they can irritate anal mucosa.
4.7 GLYCERIDES
• The most common lipid classes in nature consist of fatty acids linked by an ester
bond to the trihydric alcohol - glycerol, or to other alcohols such as cholesterol.
• Glycerides are synthesised by a condensation reaction between a molecule of
glycerol and one, two or three molecules of fatty acids.
• When glycerol is esterified (condensed) to one fatty acid, leaving two of the
hydroxyls untreated, the resulting compound is called a monoglyceride.
• If two of the hydroxyl groups are reacted in this way the result is a diglyceride,
the corresponding triester is a triglyceride.
• The long chains of fatty acids, which are the neutral lipids on the basis of their
solubility characteristics, are also exceedingly diverse group of compounds.
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The oily rather than lardy nature of plant glycerides is due to their higher
content of double bonds and hence low melting points; the animal fats, which
are comparatively rich in stearic acid or palmic acids, melt at higher
temperatures.
In mammals, lipids are mostly in the form of triglycerides. These molecules
are located in various organs and particularly in adipose tissue in which a
large proportion of the cytoplasm of highly specialized connective tissue cells
appears to be replaced by lipid droplets.
These regions form reservoirs of stored fatty energy.
Triglycerides have a higher proportion of hydrogen than either carbohydrates
or protein.
It is for this reason triglycerides;
o are less oxidised than carbohydrate molecules (more reduced),
o are strong reducing agents (can readily give up protons),
o yield 38 kJ of energy per gram, which is two times more the yield per
gram of carbohydrates,
o yield a corresponding large amount of water as a byproduct of
oxidation (Table 1).
Table 1. Table to show that lipids have a higher yield of energy per unit weight of food than
carbohydrates and consume a correspondingly higher amount of oxygen with a higher water
output as a byproduct of metabolism.
Substrate
Carbohydrate
Lipid
Metabolic
Metabolic
Oxygen
Energy output
(kJ g-1)
Water produced
in grams g-1 food
Consumed
Dm-3 food
17.2
38.9
0.56
1.07
0.83
2.02
4.7.1 Synthesis of triglycerides (Fats or oils)
• When one molecule of fatty acid reacts with a glycerol molecule, a hydroxyl
group of glycerol reacts with the hydroxyl group of the carboxyl group of a fatty
acid.
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A water molecule is removed from the reacting substances by combining the
hydroxyl group from glycerol and hydrogen from the fatty acid molecule.
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The reaction leads to the formation of an ester bond between the molecules of
glycerol and fatty acid.
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The new bond that is formed is a covalent bond called an ester bond.
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An ester bond forms when an alcohol and an acid react, with the elimination of
water to form an organic compound. The resulting molecule is called a
monoglyceride.
If the monoglyceride reacted with a second fatty acid using its hydroxyl group
on the second carbon of the monoglyceride, a resulting molecule would be
referred to as a diglyceride.
Indeed, if the last hydroxyl group on the glycerol molecule of the diglyceride
reacted with a third fatty acid, the resulting molecule would be called a
triglyceride (Figure 4.6).
Triglycerides are the most common lipids in cells.
Just as polysaccharides can be hydrolysed into their constituent sugar units, so
fats can be hydrolysed into glycerol and fatty acids. Water is necessary, of
course as well as an enzyme called lipase.
The hydrolysis of lipids is an important step in the digestive process
4.8 PHOPHOLIPIDS
•
Phospholipids are like lipids but have a phosphate group in place of one of the fatty
acid chains (figure 4.7), making the molecule hydrophilic. They get their name
from the fact that the phosphate ion (PO4 3- ) is one of the components used in their
formation.
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Attachment of a phosphate group makes phospholipids polar molecules, with the
negatively charged phosphate end becoming hydrophilic.
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Therefore, in water phospholipids form spheres with phosphate ‘heads’ interacting
with water and the fatty acid ‘tails’ buried on the inside.
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This is an example of an amphipathic molecule.
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An amphipathic molecule is a molecule that has both polar and nonpolar regions.
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Amphipathic molecules are extremely important in the human body because they
function as transporters of hydrophobic substances in the hydrophilic environment
of an organism.
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A simple example is that lipids (fat) that circulates in the body along with blood
because it is bonded to an amphipathic molecule or else lipids would not be able to
move in the blood vessels and would be sources of obstruction.
•
This property of phospholipids makes them ideal for cell membrane structure
(figure 4.8)
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Figure 4.6 Condensation reactions leading to the synthesis of a saturated triglyceride.
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Figure 4.7 Chemical structure of a phospholipid.
4.8.1 Properties of Phospholipids
• They are signal mediators.
• They are amphipathic molecules.
• They anchor proteins within the cell membranes.
• They are the major constituents of cell membranes.
• They are the components of bile and lipoproteins.
4.8.2 Functions of Phospholipids
• It regulates the permeability of the membrane.
• It is also involved in the absorption of fat from the intestine.
• It helps in Electron Transport Chain in the mitochondria.
• Because they are amphipathic, phospholipids are effective as emulsifying agents,
compounds that make or stabilize emulsions.
• Phospholipids help by preventing the accumulation of fats in the liver.
• It plays a major role in the transportation and removal of cholesterol from the
cells.
• It forms the structural components of the cell membrane with the association of
proteins.
• They act as surfactants in the respiratory system and are also involved in the
coagulation of blood cells.
• It helps in the synthesis of different lipoproteins, prostacyclins, and
prostaglandins.
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Figure 4.8 Phospholipids are constituents of cell membranes. They give the
cell membrane its fluidity.
4.9 GLYCOLIPIDS
• Glycolipids are lipids that contain a sugar residue. The sugar can be a
monosaccharide, oligosaccharide, or polysaccharide.
• These molecules are widely distributed in tissue, brain and also in nerve cells.
• The basic structure of a glycolipid consists of a mono- or oligosaccharide
group attached to a sphingolipid or a glycerol group (can be acetylated or
alkylated) with one or two fatty acids. These make up the classes of
glycosphingolipids and glycoglycerolipids, respectively (figure 4.9).
• Glycolipids interact and bind to the lipid-bilayer through the hydrophobic
nature of the lipid tail which anchors it to the surface of the plasma membrane
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Figure 4.9 basic
4.10
structure of a glycolipid
WAXES
• Waxes are “esters” formed from long-alcohols and long-chain carboxylic
acids.
• Waxes are found almost everywhere. The fruits and leaves of many plants
possess waxy coatings that can safeguard them from small predators and
dehydration.
• Fur of a few animals and the feathers of birds possess the same coatings
serving as water repellants.
• Carnauba wax is known for its water resistance and toughness (significant for
car wax).
• They are solid at ambient temperature and liquid when melted.
• Waxes are plastic in nature and bend under pressure in the absence of heat.
Waxes are generally insoluble in water.
4.10.1
Animal Wax
• Beeswax a good example of animal wax. It is a tough wax formed
from a mixture of several compounds.
• An approximate chemical formula for beeswax is
C15H31COOC30H61.
• Worker bees (Apis mellifera) produce the wax. It is secreted by
eight wax-producing glands on the inner sides of abdominal
segments.
• Honey bees use the wax to build honeycomb cells in which their
young are raised with honey and pollen cells being capped for
storage.
• Chemically, beeswax is composed mainly of esters of fatty acids
and various long-chain alcohols.
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Small amounts of beeswax have food and flavoring applications,
and are edible in the sense of having similar toxicity to indigestible
plant waxes.
However, the wax monoesters in beeswax are poorly hydrolysed in
the guts of humans and mammals, so they are not considered of
any significant nutritional value.
Some birds, such as honeyguides, can digest beeswax.
Purified and bleached beeswax is used in the production of food
(coating for cheese; by sealing out the air, protection is given
against spoilage (mold growth), cosmetics (eye shadow, blush, and
eye liner, moustache wax and hair pomades, which make hair look
sleek and shiny), Use of beeswax in skin care and cosmetics has
been increasing and pharmaceuticals.
Candle-making has long involved the use of beeswax, which is
highly flammable.
When cold beewax is brittle. At ordinary temperatures it is firm.
4.10.2 Vegetable wax
• Carnauba wax, also called Brazil wax or palm wax is a good
example of vegetable or plant wax.
• It is obtained from leaves of a palm plant (Copernicia prunifera
(Synonym: Copernicia cerifera)) native to and grown only in
Brazil. It is also called the "queen of waxes".
• In hot, dry weather, the plant secretes wax to protect the leaves
from damage. These plants have a thick coating of wax, which can
be harvested from the dried leaves.
• In its pure form the wax is hard and yellow-brown in colour. It
consists of compounds predominantly acids and alcohols in the
C26-C30 range.
• Carnauba wax is the hardest of waxes with high melting point (7885°C. It is also not readily soluble in water. Therefore, it can make
durable surfaces of products able to endure more heat and cannot
be easily broken by water.
• When mixed with beewax it makes various kinds of polishes
(automobile, shoes, floors, furniture, etc.) and cosmetics (lipsticks,
creams, etc.).
• In the food industry it is important for making glazes, candies,
gums, fruit coatings etc. It is also used for paper coating in the
paper industry.
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4.11
STEROIDS
• Steroids are hydrophobic in nature (Figure 4.10). Therefore, they can pass
through cell membranes freely.
• Many are involved in the regulation of metabolism.
• Examples of steroids are the male and female sex hormones called
testosterone and estrogen respectively. Others are cholesterol and vitamin D.
• Cholesterol is a structural component of cell membranes and helps to maintain
its structural integrity. It is converted by UV radiation into Vitamin D in our
skin cells.
Figure 4.10 A
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molecule of cholesterol
Cholesterol has a molecular formula C27H45OH. It is a compound that can be
conveniently separated into three parts and theses are; a hydrocarbon tail, a
body of four ring structures and a hydroxyl group.
The hydroxyl group makes cholesterol an alcohol and is responsible for its
solubility in water.
The set of four rings in the molecule forms the identity of all steroid hormones
such as testosterone and estrogen, because all steroids are made from
cholesterol.
The ring parts of the molecule together with its hydrocarbon chain are
nonpolar.
Cholesterol is an example of an amphipathic molecule because it has both a
water soluble and fat soluble component.
Nonetheless, cholesterol is sparingly water soluble to dissolve in blood.
Therefore, together with fats and fat soluble nutrients, cholesterol travels in
the blood through lipoproteins such as low density lipoproteins (LDL) and
high density lipoproteins (HDL).
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4.12
Role of lipids
• Energy and water source
Lipids include a variety of molecules that can serve as energy storage molecules.
Therefore they are important biological fuels, because they form the most important
source of energy in an ordinary diet, and they serve to store energy in the body. The high
proportion of hydrogen in a fat molecule is significant in two ways. First it tells us that
the molecule is much highly reduced than a carbohydrate molecule. This means that a
given unit weight of fat stores much more energy than the same unit weight of
carbohydrate. Therefore, fats provide the most concentrated energy reserve available to
an organism, because C-C and C-H bonds contain more energy than the C-O bonds
common in carbohydrates. Therefore, triglycerides have more bond energy than
carbohydrates.
The energy locked up in fats is released when the molecule is oxidised in the process of
respiration. Oxidation of the large amount of hydrogen in the molecule results in the
production of a correspondingly large amount of water. It used to be thought that fat
deposits were a deep storage form of energy, put down only when surplus food was
ingested and used only in times of partial starvation. The facts now are that deposits in
the body are constantly being formed and broken down again. There is of course a net
accumulation of stored fat during times of plenty and a net loss during times of
starvation. But even with no net gain or loss, there is a constant exchange of fats between
the storage depots and the rest of the body.
Many mammals and birds that live in arid regions consume a high proportion of fats in
their diet. The water produced during the oxidation of these fats satisfies a substantial
part of the water needs of the animal. In fact Kangaroo rats normally do not drink water
even when it is available to them. In another example, the water needs of the unhatched
chick are also met to a large extent by the oxidation of fats.
• Membranes
Lipids serve as structural compounds of cell membranes.
• Insulation
Lipids are heat insulators because fat is a bad conductor of heat; mammals tend to
increase their adipose tissue in winter to reduce heat loss and thereby staying warm.With
mammals, much of the body lipid is located under the skin, preventing excessive heat
loss to the environment. The lipid in plant seeds and fruits presumably provides thermal
insulation against the environment and also prevents moisture loss.
• Diet
Many of the vitamins, which are essential dietary supplements with a wide range of
functions, are also lipid
• Shock absorbers
Lipids act as shock absorbers. They protect delicate mammalian organs such as the
kidneys, which are surrounded by thick layers of fat to cushion them from knocks and
bumps.
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Mixtures of triglycerides aggregate by hydrophobic bonding to form globules of
macromolecular dimensions. Such is the nature of the ordinary neutral fats found in
adipose cells one might trim before boiling. Adipose tissue also surrounds most of our
critical internal organs. Its soft and spongy texture serves to absorb the shock of sudden
blows and cushions vital fragile delicate internal organs from mechanical damage.
Buoyancy
Aquatic single celled organisms produce an oil droplet to help them to float in water.
Hormones
Some lipids are important hormones. The lipid hormones allow a gland in one part of the
body to control the metabolic activity of a tissue in another part, so are basic structural
units of biological membranes.
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