Lipid: Structure & Classifications

LIPID
Structure & Classifications

Introduction
 Lipids , a third major class of biomolecules –
molecules that do not form polymers
 Lipids – often defined as water insoluble organic

compounds found in biological systems.
- they have little or no affinity for water
- hydrophobic becausethey consist mostly of

hydrocarbons, which form nonpolar covalent
bonds
 Lipid functions:
Cell membrane structure – creates a
barrier for the cell, controls flow of
materials
Energy storage – fats stored in adipose
tissues
Hormone – communication between cells
Vitamins – assist in the regulation of
biological processes
Important families of lipids
Fats and oils Steroids

(Triglycerides)
Phospholipids
Fats
• Most familiar lipids
• Large molecules
• Not polymers
• Major function – energy storage
• Are made from :
Glycerol + Fatty acids = Fats

Glycerol
• is a three-carbon alcohol with a hydroxyl

group attached to each carbon
Fatty Acids
• consists of a carboxyl group attached to a
long carbon skeleton
• has long carbon – 16-18 carbon atoms in
length (form the hydrocarbon chain of fatty
acids)
Fatty acids
Saturated Fatty acids Unsaturated Fatty acids

-Has straight chain -Have 1 or more double
bonds
-No double bond
between the carbons -Have kink in its shape
atoms wherever a double bond
occur
Fig. 5-12a
Structural
formula of a
saturated fat
molecule
Stearic acid, a
saturated fatty
acid
(a) Saturated fat
Fig. 5-12b
Structural formula
of an unsaturated
fat molecule
Oleic acid, an
unsaturated
fatty acid
cis double
bond causes
(b) Unsaturated fat bending
• Fats made from saturated fatty acids are called
saturated fats - solid at room temperature
- most animal fats are saturated
- example : butter, bacon grease
• Fats made from unsaturated fatty acids are

called unsaturated fats or oils - liquid at room
temperature
- Plant fats and fish fats are usually unsaturated
- example : corn oil, olive oil
 Unsaturated fatty acids have lower melting
points than saturated fatty acids.
 Among the saturated fatty acid, as the

lengths of hydrocarbon tails (C-H-C)
increase, the melting points also increase
(more energy is required to disrupt the
interactions).
• A shorthand notation for identifying fatty acids uses two
numbers separated by a colon;
 First, refers to the number of carbon atoms in the fatty
acid.
 Second, refers to the number of carbon-carbon

double bonds, with their positions indicated as
superscripts following a Greek symbol, Δ.
Examples:
a) Palmitat is written as:
16:0 [ CH3(CH2)14COO- ]
• Essential fatty acids (EFAs)
- Fatty acid that cannot be synthesised in
the body – should be supplied in the diet
– e.g.: Linoleic acid, linolenic acids and
arachidonic acid
– All there fatty acids are necessary for the
formation of phospholipids (important
component for cell membrane)
– Also essential for normal development of the
brain and normal growth.
• Non essential fatty acids
- our body is able to synthesise them
Triacylglycerols
• (historically called triglycerides).
• Formed from condensation of 3 molecules
of fatty acids linked to one molecule of
alcohol (glycerol).
• Involving alcohol and acids, it is known as
esterification
• The bond between glycerol and fatty acids
– ester bonds
Fig. 5-11
Fatty acid
(palmitic acid)
Glycerol
(a) Dehydration reaction in the synthesis of a fat
Ester linkage
(b) Fat molecule (triacylglycerol)

Phospholipids
• Phospholipids have two fatty acids
attached to glycerol and a phosphate
group at the third position.
– The phosphate group carries a negative
charge.
– Additional smaller groups may be attached to
the phosphate group to form a variety of
phospholipids.
Fig. 5-13
Hydrophilic head
Choline
Phosphate
Glycerol
Hydrophobic tails
Fatty acids
Hydrophilic
head
Hydrophobic
tails
(a) Structural formula (b) Space-filling model (c) Phospholipid symbol

• When phospholipids are added to water,
they self-assemble into a bilayer, with the
hydrophobic tails pointing toward the interior
• The structure of phospholipids results in a
bilayer arrangement found in cell
membranes
• Phospholipids are the major component of
all cell membranes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 5-14
Hydrophilic WATER
head
Hydrophobic
tail WATER
Steroids
• are lipids characterized by a carbon skeleton
consisting of four fused rings
• contain four fused rings:
Three 6 carbon rings designated A, B & C.
One 5 carbon D ring.
• have different structures from other lipids
although they are insoluble in water and soluble
in organic solvents.
• have a basic four fuse rings hydrocarbon
structure with different functional side chains.
Steroids
Steroids
• Steroid cholesterol is an important component of

animal plasma membranes and helps to
maintain the fluidity of cell membrane.
• Cholesterol is actually a sterol – it has OH group

at C-3.
• Other steroids include stigmasterol (plant sterol),

ergosterol (fungi & yeast sterol), sodium cholate
(a bile salt) and mammalian steroid hormones.
Cholesterol
• Cholesterol often accumulates in lipid deposits (plaques)

on the walls of blood vessels.
• These plaques have been implicated in cardiovascular

disease, which can precipitate heart attacks or strokes.
• Most of the cholesterol in the blood are complexed with

protein and fatty acids to form low density lipoproteins
(LDL).
• High density lipoproteins (HDL) tends to carry excess

cholesterol to the liver to be processed.
Cholesterol
• When cholesterol exceeds the normal levels in

the body, they are deposited on the inner walls
of arteries.
• Artherosclerosis is a thickening of the
innerwalls of arteries due to deposits of
cholesterol, fibrous tissue and mineral salts
forming artheroma in the walls.
• This causes narrowing of the lumen and
reduced the blood supply to tissues which may
cause stroke or myocardial infarction (heart
attack).

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LIPID

Structure & Classifications

 Lipids – often defined as water insoluble organic

- they have little or no affinity for water

- hydrophobic becausethey consist mostly of

Fats and oils Steroids

Glycerol + Fatty acids = Fats

• is a three-carbon alcohol with a hydroxyl

Saturated Fatty acids Unsaturated Fatty acids

• Fats made from unsaturated fatty acids are

 Among the saturated fatty acid, as the

 Second, refers to the number of carbon-carbon

(b) Fat molecule (triacylglycerol)

(a) Structural formula (b) Space-filling model (c) Phospholipid symbol

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

• Steroid cholesterol is an important component of

• Cholesterol is actually a sterol – it has OH group

• Other steroids include stigmasterol (plant sterol),

• Cholesterol often accumulates in lipid deposits (plaques)

• These plaques have been implicated in cardiovascular

• Most of the cholesterol in the blood are complexed with

• High density lipoproteins (HDL) tends to carry excess

• When cholesterol exceeds the normal levels in

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