LECTURE

LIPIDS
 Introduction
• Lipids are loosely classified as water insoluble
organic compounds.
• Lipids consist of hydrogen, carbon and oxygen
only.
• They are non polar as hydrocarbons have no
tendency to form hydrogen bonds with water
molecules
• Readily soluble in organic solvents such as
hydrocarbons, chloroform, benzene, ethers and
alcohols.
• Lipids includes triglycerides (fats, oils), waxes,
phospholipids, sphinolipids, steroids and
carotenoids.
• Triglycerides are the main form of lipids and
are grouped into fats and oils.
• Fats are solid at room temperature and are a
long term storage of energy in animals
• Oils are liquids at room temperature and are a
long term energy storage for plants
 Triglycerides (Fats and Oils)
• Consist of two molecules; glycerol and fatty acids.
• Glycerol is a compound with 3 hydroxyl groups (-
OH).
• The –OH makes glycerol polar and soluble in
water.
• Fatty acids are long unbranched hydrocarbon
chains ending with a Carboxyl group (-COOH).
• General formula of fatty acid ; CH3(CH2)nCOOH.
• The hydrocarbon chain vary in length (16- 18) and
are non polar (hydrophobic –water hating).
 • The terminal carboxyl group is polar and
renders the molecule acidic.
• Fatty acids may be saturated (single bonds)
or unsaturated (double bonds).
Glycerol Fatty acids
Ester linkage formation
• During dehydration reaction, 3 hydroxyl group
may react with carboxyl group of 3 fatty acids to
form a triglyceride (triacylglycerol) and 3 water
molecules.
Glycerol + 3 Fatty acid = Triglyceride +3 H20
• The bond formed between the glycerol molecule
and fatty acid molecule is called ester bond.
• If 1 hydroxyl group is esterified it is called
monoglyceride, if its 2 its called diglyceride.
Saturated and Unsaturated Lipids
• Unsaturated lipids are triglyceride made of fatty
acid with shorter hydrocarbon and containing one or
more double bonds between carbon atoms e.g oleic
acid .
• They are oils (soft fats) at room temperature.
• Occur mainly in plants.
• Saturated lipids are triglyceride made from fat acids
with longer hydrocarbon chains containing single
bonds between carbon atoms e.g stearic acids. .
• They are solid (fats) at room temperature.
• They mainly occur in animals.
Fatty acids configuration
• Fatty acids configuration or structure varies
through hydrocarbon chains.
• Long chain of carbon and hydrogen atoms forms
the hydrocarbon tail.
• Properties of lipids are determined by these tails.
• The hydrocarbon tails are hydrophobic hence
responsible for insoluble nature of lipids in water.
• Fatty acids can either be saturated or
unsaturated.
 Saturated Fatty acids
• Hydrocarbon chains consist single bonds only.
• Hydrogen atoms attached to the carbon
skeleton is maximised (saturated).
• No more hydrogen atom can be added.
• For example stearic acid:
Formula: CH3 (CH2)16-COOH
• All carbon atoms have single bonds
• Contains maximum number of hydrogen atoms
• The molecules is straight and therefore molecules
can pack closely together.
• Hence resulting fats are solid at room temperature
and high energy reserves (highly reduced).
• Animal fats with stearic and palmitic acids
(common in meat) and butyric acids (common in
butter) are examples of saturated fats.
• Saturated fats increases cholesterol levels
contribute to plaque formation in arteries (heart
complications).
Unsaturated Fatty acids
• Have one or more double bond .
• Each double bond may be a Cis or trans
configuration.
• In a Cis, both hydrogen atom are on the same
side of the hydrocarbon.
• In a trans, hydrogen atom are on the opposite
sides.
• A Cis double bond causes a bend or “Kink”.
• A Kink prevents fatty acids from packing tightly,
because attraction between molecules is weak.
• The Kink keeps the unsaturated fats (oils) liquid
at room temperature.
• Olive oil, corn oil and cod liver oil are examples of
unsaturated fats.
• Unsaturated fats helps to lower blood cholesterol
levels.
• For example Oleic acid;
Formula: CH3 (CH2)7-CH=CH-(CH2)7-COOH
• Contains fewer number of hydrogen atoms.
• Have double bond between carbon 9 and 10.
• The molecule bends at the site of the double
bond and hence can not be packed tightly.
• Resulting fats are liquid at room temperature.
• If there is one double bond in the molecule it
is monounsaturated fats (e.g olive oil) and if it
has more than one double bond it is
polyunsaturated (e.g Canola oils).
• Animals store extra fats in the adipose tissue
under the skin (extensive in mammals living in
cold regions).
• Plants store oils in seeds and fruits e.g
coconuts, groundnuts, soyabeans and
sunflower seed.
• “Partially hydrogenated vegetable oil” is a
term often found on food ingredient labels.
• It means that some of the fatty acids in the
oil were “hydrogenated” (reduced, so that
the double bonds were converted to single
bonds).
• Saturated fat and monounsaturated fat form
solids at room temperature.
• Polyunsaturated fat is (usually) liquid at room
temperature.
• Mixing fatty acids of varying saturation is a
way of regulating consistency of food, and also
of regulating the consistency of biological
systems.
• This is because, in membranes or in bulk lipid,
cis double bonds alter packing density, and
therefore decrease the number of favorable
van der Waals contacts.
• The presence of cis double bonds therefore
results in lower melting temperature because
the double bonds result in less regular and
less stable structures.
• In mammals lipids are mainly in form of
triglyceride (Fats and oils).
• They are reservoirs of stored energy.
• They high proportion of hydrogen atom than
carbohydrates hence;
 Are less oxidised molecules and are strong
reducing agents.
 Can yield two times more energy than
carbohydrates.
 However fats are not easily oxidised as
compared to carbohydrates
 Phospholipids
• Are lipids containing a phosphate group.
• The common one is when one of the three –OH
groups combines phosphate group instead of a
fatty acid.
• Contains a phosphate head and two hydrocarbon
tails from two fatty acids.
• A phosphate group carries a charge and the
phospholipid head is polar or soluble in water.
• It phosphate is modified by a polar chemical group
(Choline or Seline)
• In other word it is hydrophilic (water loving).
• The two fatty acid tails are insoluble in water.
• Hence the phosphate head of phospholipid is
soluble and fatty acid end is insoluble.
• Phospholipid is an amphipathic molecules (has
both hydrophilic and hydrophilic parts).
• This property makes them ideal component for
cell membrane (phospholipid bilayer).
• Lecithin is a phospholipid found in all cell
membrane.
• Lecithin polarity is due to the attachment of a
choline compound (CH2CH2-N + (CH3)3) to the
phosphate group.
• In water or aqueous solution, the phospholids
arrange themselves such that only the polar
head interact with water and the fatty acid tails
are buried inside away from water.
• The phospholipid becomes a bilayer (double
layer) in which the hydrophilic head project
outward and hydrophobic tail project inward
• The plasma membrane that surrounds cells
consist of primarily of phospholid bilayer.
• The fluidity of plasma membrane is due to the
presence of Kink in the phospholipid tail.
• A drop of phospholipids in water spontaneously
forms a structure known as micelle.
• Soaps and detergents form micelles as they
contain long non polar hydrocarbon chains which
terminate into polar charged groups.
• This leads to formation of little sphere with
charged group on the surface and hydrocarbon
tail on the interior.
• A micelle helps to dissolve a droplet of fats and
help it to be washed away into the water
Steroids
• They are also forms of lipids.
• Unlike fats and phospholipids have four fused
carbon ring and chains are usually short.
• They are hydrophobic hence insoluble in water.
• Steroid differ by types of functional group
attached.
• Many contain a hydroxyl group (sterols).
• Cholesterol is the most common steroid.
• Synthesised in the Liver but also found in meat,
poultry and diary products.
• Other four fused ring steroids include terpene,
androstane, gonane , pregnane
• Cholesterol is an organic molecule that
humans can not live without.
• It is a structural component in the membrane
of every single cell in the body, helping to
maintains fluidity and stability.
• Used in the formation of steroid hormones
(testosterone and estrogen).
• Used in the formation of vitamin D.
• Used in the formation of bile salts.
• Because cholesterol is fat soluble, it can not
travel through the bloodstream on its own.
• It's like an oil droplet in a glass of water, it
simply won't mix.
• For this reason, cholesterol is carried around
the bloodstream in lipoproteins, which
function as carrier vehicles.
• However, cholesterol can contribute
circulatory disorder by encouraging
accumulation of fatty material inside the lining
of blood vessels which result in high blood
pressure and heart complications
 Glycolipids
• Are lipids attached to carbohydrate by a
glycosidic bond.
• In glycolipids the polar head is a carbohydrate
instead of a phosphate group (e.g mono and
digalactosyldiglyceride).
• They are amphipathics.
• They are also components of cell membrane.
• Used as cellular recognition site for specific
chemicals.
Lipoprotein
• When a polypeptide may conjugated with a
lipid to form a lipoprotein.
• Lipoproteins are classified according to density.
• High protein to phospholipid ratio = high
density.
• Low protein to phospholipid ratio = low density.
• Lipoproteins also carry other molecules, such
as. triglycerides (fats), phospholipids,
choresterol and fat-soluble vitamins
• The major function of LDL is to transport
cholesterol and phospholipids from the liver
to the cells, where they are incorporated into
membranes or, in the case of cholesterol,
transformed into other steroids hormones.
• HDL particles remove fats and cholesterol
from cells, including within artery wall
atheroma, and transport it back to
the liver for excretion or re-utilization.
• LDL and HDL are commonly referred to as the
"bad" and "good" cholesterol respectively.
• High LDL levels in the blood are warning signs
of atherosclerosis (the buildup of cholesterol-
containing deposits in arteries).
• The amount of cholesterol contained in
lipoprotein is a good measure of risk of
cardiovascular disease (CVD).
• Having a lot of cholesterol within LDL
lipoproteins is associated with heart disease,
while having it carried by HDL lipoproteins is
associated with reduced risk.
Waxes
• Consist fatty acids bonded to long chains of
alcohol.
• Similar to triglyceride but don’t contain
glycerol.
• Flexible materials and become hard when
cold.
• Because of their hydrophobic nature they
form protective waterproof coating (cuticle)
in plants.
• In animals wax is used in skin and fur
maintenance.
 Roles of Lipids
• Energy and water storage; Biological fuels
(contains more energy in C-H bonds). Oxidation
of more hydrogen in molecules produces large
amount of energy and water.
• Membranes; Lipids serve as structural
components of cell membrane.
• Insulation; Lipids (adipose) insulate the body
against heat loss.
• Diet; Micelles help in the absorption of fat
soluble vitamins (Vitamin A, D, E and K)
• Shock absorbers; Act as fat cushion to protect
delicate organs such as heart, kidney. Adipose
tissue also acts a shock absorber.
• Bouyancy; Aquatic organisms produce oil
droplets to help the float on water.
• Hormones; Help in formation of hormone which
control metabolic reaction.
Note: Animals including humans can convert
excess carbohydrates to fats. Hence people can
gain weight by eating food high in
carbohydrates.
END OF LECTURE!
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