Chemistry Reference
In-Depth Information
function as ion channels (e.g. the Na
channel of nerve membranes), trans-
membrane receptors (like the b adrenoceptor) or transport proteins (as in
the electron transport chain of mitochondria). Human cell membranes also
contain high concentrations of the steroid cholesterol, particularly in nerve
tissue. Chemically, cholesterol is a cyclopentanoperhydrophenanthrene
derivative, but it is much simpler to use the trivial name and call this impor-
tant group of compounds 'steroids.' The structure of cholesterol and a
general structure of membrane phospholipids are shown in Figure 2.4.
Protein
Polar head
Hydrocarbon chains
Phospholipid
bilayer
Figure 2.3
A fluid mosaic model of a cell membrane.
O
H
3
C
CH
3
CH
3
CH
2
OC
1
H
O
CH
3
CH
3
C
2
CO
H
O
HH
HO
P
3
CH
2
O
H
OH
Figure 2.4
The structures of cholesterol and phospholipids. R
1
and R
2
palmityl,
stearyl or oleyl. R
3
ethanolamine, choline, serine, inositol or glycerol.
Cholesterol gets a bad press nowadays. Tabloid newspapers and tele-
vision programmes seem to have latched onto cholesterol as the villain of a
healthy lifestyle. It is true that high levels of cholesterol in the diet, coupled
with high salt intake and lack of exercise, are blamed for causing coronary
heart disease and strokes. In cell membranes, however, cholesterol increases
membrane rigidity and is essential for maintaining the integrity of the
