LIPIDS
To be able to
• Identify the elements which make up
lipids
• Explain that glycerol and fatty acids
combine by condensation to produce
triglycerides.
• Demonstrate why the R group of a fatty
acid may be saturated or unsaturated.
• Explain how phospholipids are formed
LIPIDS
•
•
•
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What are lipids?
Why are they useful?
What is their biological use?
What elements make up lipids?
TRIGLYCERIDES
• The most common type of lipids are the
triglycerides also known as fats and
oils, or TRIGLYCERIDES
• Triglycerides are made from a
combination of three fatty acid
molecules and one glycerol molecule
by a condensation reaction.
Fatty Acid
Fatty Acid
Fatty Acid
Glycerol
The glycerol molecule in any triglyceride is
always the same. It is the fatty acids that vary
Glycerol is a
small, 3-carbon
molecule with
three alcohol
groups.
H
H
H
H
C
C
C
OH
OH
OH
H
Fatty acids are long molecules with a
polar, hydrophilic end and a nonpolar, hydrophobic "tail".
H
H H
H H
O
H C
H
Methyl
Group
C
C
H H
C
C
H H
Hydrocarbon Chain
(14-22 carbon atoms)
C
OH
Carboxyl
Group
The hydrocarbon chain can be
from 14 to 22 CH2 units long, but it
is always an even number
because of the way fatty acids are
made..
The hydrocarbon chain is
sometimes called an R group, so
the formula of a fatty acid can be
written as R-COO-
General formula
CH3 – (CH2)n - COOH
Saturated fatty acids
• Saturated fatty acids only have C-C
bonds
• Each carbon in the chain is joined by a
single bond
COO
• Each carbon is SATURATED with
hydrogen
• These fatty acids form straight chains,
and have a high melting points.
Unsaturated fatty acids
• Unsaturated fatty acids only at least
one C=C double bond
• Polyunsaturates have more than one
double bond
H H
H
H
H
H
H
H
H C C
C
C
C
C
C
C
H H
H
H
H
H
COO-
• These fatty acids form bent chains, and
have a low melting point.
Formation of triglycerides
• One molecule of glycerol joins together
with three fatty acid molecules to form
a triglyceride molecule, in another
condensation polymerisation reaction:
H H
H
H
H
H
H C C
C
C
C
C C
H H
H
H
H
H
H H
H
H
H
H
H C C
C
C
C
C C
H H
H
H
H
H
H H
H
H
H
H
H C C
C
C
C
C C
H H
H
H
H
H
O
OH
O
OH
O
OH
H2O
H
HO C
H
H2O
HO C
H
H2O
HO C H
H
H
CH3 – (CH2)n - COOH
HO C
CH3 – (CH2)n - COOH
HO C H
CH3 – (CH2)n - COOH
HO C
H
=
O
CH3 – (CH2)n - C
H
3H2O
H
O C
H
ESTER BONDS
=
=
O
CH3 – (CH2)n - C
O
CH3 – (CH2)n - C
H
O C H
O C
H
H
Phospholipids
•Phospholipids have a similar
structure to triglycerides, but with a
phosphate group in place of one
fatty acid chain.
•There may also be other groups
attached to the phosphate.
Phospholipids have a polar hydrophilic
"head" (the negatively-charged phosphate
group) and two non-polar hydrophobic
"tails" (the fatty acid chains).
O-
H
H
C
O
=
O
CH3 – (CH2)n - C O C H
=
O
CH3 – (CH2)n - C O C
H
H
P= O
O-
This mixture of properties is
fundamental to biology, for
phospholipids are the main
components of cell membranes.
water
Steroids
Steroids are small hydrophobic molecules
found mainly in animals. They include:
•Cholesterol, which is found in animals cell
membranes to increase stiffness
•Bile salts, which help to emulsify dietary
fats
•Steroid hormones such as testosterone,
oestrogen, progesterone and cortisol
•Vitamin D, which aids Ca2+ uptake by
bones.
• Why is it wrong to call lipids ‘fats’?
• Because saturated lipids are mainly animal fats but
unsaturated lipids are called oils
• What is the name given to the type of bond that joins
fatty acids and glycerol in a lipid molecule?
• Ester bond
• Why are lipids insoluble in water?
• Because they have a hydrophobic tail that repels
water and this causes the molecules to clump
together to reduce the surface area in contact with
water.
• Describe the difference between a triglyceride and a
phospholipid. How do these differences affect the
properties of the molecule?
• A phosphate group replaces one of the fatty acid
molecules
• The phosphate group is ionised which makes it
attract water molecules. This end is therefore
hydrophilic as opposed to the fatty acid group which
is hydrophobic