Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
Dipoles occur in many different molecules, particularly
wherever there is an –OH, –CO or –NH group. Hydrogen
bonds can form between these groups, because the
negatively charged part of one group is attracted to the
positively charged part of another. These bonds are very
important in the structure and properties of carbohydrates
and proteins.
C
O δ−
δ+
H
Molecules which have groups with dipoles, such as
sugars, are said to be polar. They are attracted to water
molecules, because the water molecules also have dipoles.
Such molecules are said to be hydrophilic (water-loving),
and they tend to be soluble in water. Molecules which
do not have dipoles are said to be non-polar. They are
not attracted to water, and they are hydrophobic (waterhating).
Such properties make possible the formation of
cell membranes (Chapter 4).
N
Lipids
It is difficult to define precisely what we mean by a ‘lipid’
because lipids are a very varied group of chemicals. They
are all organic molecules which are insoluble in water.
The most familiar lipids are fats and oils. Fats are solid at
room temperature and oils are liquid at room temperature
– chemically they are very similar. We could say that true
lipids are esters formed by fatty acids combining with
an alcohol.
Fatty acids
Fatty acids are a series of acids, some of which are found
in fats (lipids). They contain the acidic group –COOH,
known as a carboxyl group. The larger molecules in the
series have long hydrocarbon tails attached to the acid
‘head’ of the molecule (Figure 2.11). As the name suggests,
the hydrocarbon tails consist of a chain of carbon atoms
combined with hydrogen. The chain is often 15 or 17
carbon atoms long.
36
OH
C
O
acid head
OH
C
O
H C H
H C H
H C H
H C H
H C H
hydrophobic
hydrocarbon tail
H C H
H C H
H C H
H C H
H C H
H
C
H
H
C
H
H C H
H C H
H C H
H C H
H C H
H C H
C
H H
H
H C H
H
H
C
C
C
C
C
C
C
H
H
H
H
H
H
H
double bond
causes kink
in tail
C
H
saturated fatty acid
H
H
C
C
H
H
H
unsaturated fatty acid
unsaturated fatty acid
Figure 2.11 Structure of a saturated and an unsaturated fatty acid. Photographs of models are shown to the sides of the
structures. In the models, hydrogen is white, carbon is grey and oxygen is red.
saturated fatty acid