Cambridge International A Level Biology Revision Guide

Chapter 2: Biological molecules
The tails of some fatty acids have double bonds between
neighbouring carbon atoms, like this: –C C–. Such fatty
acids are described as unsaturated because they do not
contain the maximum possible amount of hydrogen. They
form unsaturated lipids. Double bonds make fatty acids
and lipids melt more easily – for example, most oils are
unsaturated. If there is more than one double bond, the
fatty acid or lipid is described as polyunsaturated; if there
is only one it is monounsaturated.
Animal lipids are often saturated (no double
bonds) and occur as fats, whereas plant lipids are often
unsaturated and occur as oils, such as olive oil and
sunflower oil.
Alcohols and esters
Alcohols are a series of organic molecules which contain a
hydroxyl group, –OH, attached to a carbon atom. Glycerol
is an alcohol with three hydroxyl groups (Figure 2.12).
The reaction between an acid and an alcohol produces
a chemical known as an ester. The chemical link between
the acid and the alcohol is known as an ester bond or an
ester linkage.
C COOH + HO C C COOC + H 2 O
acid alcohol ester
The –COOH group on the acid reacts with the –OH
group on the alcohol to form the ester bond , –COO− .
This is a condensation reaction because water is formed
as a product. The resulting ester can be converted back to
acid and alcohol by the reverse reaction of adding water, a
reaction known as hydrolysis.
Triglycerides
The most common lipids are triglycerides (Figure 2.13).
These are fats and oils. A glyceride is an ester formed by
a fatty acid combining with the alcohol glycerol. As we
have seen, glycerol has three hydroxyl groups. Each one is
able to undergo a condensation reaction with a fatty acid.
When a triglyceride is made, as shown in Figure 2.12, the
final molecule contains three fatty acids tails and three
ester bonds (‘tri’ means three). The tails can vary in length,
depending on the fatty acids used.
37
H
O
head
H
3 hydrocarbon
tails
O
H
H
C
C
OH
OH
HO
HO
C
O
C
O
condensation
3H 2 O
H C O
H C O
C
O
C
O
H
C
OH
HO
C
H
C
O
C
H
H
glycerol +
3 fatty acid molecules
with hydrocarbon tails
triglyceride
molecule
fatty acid
fatty acid
or, more simply
glycerol
fatty acid
fatty acid
condensation
3H 2 O
glycerol
fatty acid
fatty acid
Key
glycerol
fatty acid
ester bond
Figure 2.12 Formation of a triglyceride from glycerol and three fatty acid molecules.