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TOPIC 8 LIPIDS
8.1 INTRODUCTION
Lipids are a group of naturally occurring esters of fatty acids and alcohols.
They include fats, waxes, fat -soluble vitamins (such as vitamins A, D, E, and K),
glycerides, phospholipids, and steroids ( special type of lipi ds with a ring structure ).
They are water -insoluble and can be extracted from cells using organic solvents such
as ether, chloroform and benzene.
The main biological functions of lipids include (i) energy storage (ii) cell signalling
(iii) structural sup port of cell membranes (iv) heat and electrical insulation (v)
buoyancy (vi) water proofing (vii) production of metabolic water and (viii) protection
of internal organs.
8.2 FATTY ACIDS
A f atty acid is made of a hydrocarbon chain that terminates with a c arboxylic acid
group [CH 3(CH 2)xCOOH ].
This makes the fatty acid molecule amphipathic meaning that it has a polar,
(hydrophilic) head and a nonpolar (hydrophobic) tail.
8.2.1 SATURATED FATTY ACIDS
Fatty acids without any carbon -carbon double bonds are s aid to be saturated meaning
each carbon atom has the maximum number of hydrogen atoms.
They have only carbon -carbon single bonds (C -C).
The absence of double bonds makes saturated fatty acids rigid ( not flexible ).
This rigidity makes fatty acids waxy so lid s at room temperature.
An example of a saturated fatty acid is stearic acid which has 18 carbon s, with a
chemical formula CH 3(CH 2)16 COOH .
Stearic acid is found in butter, cheese, coconut oil, meat, margarine and fried foods.
It is mainly used in the production of detergents, soaps, shampoos and shaving
cream s. 2
Figure 8.1. The structure of stearic acid an un saturated fatty acid
8.2.2 UNSATURATED FATTY ACIDS
Fatty acids with one or more carbon -carbon double bonds (C=C) are said to be
unsaturated meaning that some of the carbon atoms have less than the maximum
number of hydrogen atom s.
Monounsaturated fatty acids have one carbon -carbon double bond while
polyunsaturated fatty acids have two or more.
The presence of a double bonds creates a bend (kink) and this makes unsaturated fatty
acids flexible (fluid) and able to melt at much lower temperature than sa tu rated fatty
acids.
Oleic acid is an example of a mono unsaturated 18 carbon molecule with a chemical
formula CH 3(CH 2)7CH=CH(CH 2)7COOH .
It is an oily liquid at room temperature, found in olive oil, avocado, soya bean oil,
canola oil, sunflower oil and wal nuts.
Figure 8.2. Structure of oleic acid a saturated fatty acid 3
8.2.2.1 CIS AND TRANS UNSATURATED FATTY ACIDS
Cis unsaturated fatty acids have hydrogen bonds on the same side of the double bond
making them relatively more flexible than t rans unsatur ated fatty acids which have
hydrogen atoms on opposite sides of the double bond.
Figure 8.3 . Structure of a cis and a trans carbon -carbon double bond.
8.2.3 GLYCEROL
Glycerol (or glycerine ) is a simple three -carbon poly alcohol which combines with fatt y acids
in the synthesis glyceride lipids .
It is colourless, viscous and sweet -tasting.
The three hydroxyl groups make it water soluble and adsorbent (hygroscopic) .
Its chemical formula is C 3H8O3.
Figure 8.4. Structure of glycerol (propane 1,2,3 triol or 1,2,3 propanetriol).
It is produced from the lipids through saponification (reaction between sodium and a
glycerides) .
It is u sed as a solvent, sweetner , preservative or as an additive in food, pharmaceutical and
cosmetic products. 4
8.2.4 GLYCERIDES
Glycerides are ester compounds of glycerol and fatty acids.
Each of the three hydroxyl (-COH ) groups each of glycerol can react with a carboxyl
group ( -CO OH ) of a fatty acid to form an ester ( -CO OC -) bond.
This is a condensation reaction and water is released in the process.
A m onoglyceride ha s one fatty acid chain; a diglyceride ha s two while a triglyceride
ha s three.
Figure 8.4. Formation of a triglyceride (lipid) from three fatty acids and glycerol by a
condensation reaction
The long hydrocarbon chains of the fatty acids form hydrophobic tails which make
lipids insoluble in water.
Triglycerides are the commonest l ipids in nature and are classified as fats (if they are
solid at 20C) and as oils (if they are liquid at 20C).
8.2.5 PHOSPHOLIPIDS
A phospholipids is a modified triglyceride in which one fatty acid has been replaced
by a phosphate group linked to a nit rogen -containing molecule.
One of the two fatty acid chains of a phospholipid is saturated while the other chain is
unsaturated and has a bend or kink.
Phospholipids form ball -shape d micelles or liposomes in water because of the
hydrophobic interactions b etween the fatty acid chains (tails) which tend to cluster
inwards away from water while the phosphate groups (heads) stick out into the
surrounding water. 5
(a) (b)
Figure 8.6. Formation of a micelle (a) and a liposome (b) in water by phospholipids
Phospholipids are found in the cell membrane where they form a phospholipid bilayer
which is important for the function of the cell membrane (Solomon and Berg, 1995
pages 70 -71).
Figure 8.7. Structure of a phospholipid and the cell membrane bilayer
8.3.6 WAXES
Waxes are mainly esters of fatty acids with long -chain alcohols.
They are used as waterproofing m aterial by plants and animals.
In plants they form an additional protective layer on the cuticle of the epidermis of
leaves, fruits and seeds. 6
In vertebrate animals they are found in skin, fur and feathers while in insects they are
part of the exoskeleto n (chitin) .
Figure 8.5. Formation of a wax ester from a fatty acid and an alcohol.
Beeswax is a special type of wax found in the honeycombs.
The bees use it to store honey and to protect the larvae and pupae.
It is use d a s a glazing agent for fruits, a coating agent in cheese and chewing gum, in
lip balm and lip stick , etc
Carnauba wax also called Brazil wax or palm wax is a wax of the leaves of the
palm tree. It is used in automobile waxes, shoe polishes , instrument polishes ,
floor and furniture waxes and polishes , dental floss and in food products such
as sweets ,
8.3.7 STEROIDS
These are special lipid compounds with four rings which are found in plants, animals
and fungi.
Steroid Cholesterol
Figure 8.8. Chemical structure of a steroid and of cholesterol 7
The large number of CH groups makes steroids non -polar.
The most common type of animal steroid is cholesterol w hich is an important part of
the animal cell membrane structure and function.
Cholesterol is also a precursor to fat -soluble vitamins and steroid hormones ((e.g. sex
hormones (Solomon and Berg, 1995 pages 70 -71))
Figure 8.9. The interaction between cholesterol and phospholipids in the cell membrane .