Biology Reference
In-Depth Information
(a)
Large head
Hydrophobic
core
Conical shape
Micelle
(b)
Aqueous pore in core
Small head
Conical shape
Broad tails
H
ii
(c)
Hydrophobic
core
Cylindrical
shape
Bilayer
FIGURE 3.3
Phospholipids and glycolipids with different geometries self-assemble into different structures
when suspended in water. The bilayer in part c extends upwards and downwards from the short section illustrated.
called 'micelles', in which all of the heads face outwards and can associate with water and the
tails all face in towards the water-free core. Some lipids, such as the monogalactosyldiglycer-
ides found in chloroplasts, are conical the other way round and have a small head and broad
tails. For these molecules, a micellar aggregate would be unstable because of the difficulty of
packing the thick tails in the cramped interior of the micelle and the shape of the molecules
forces them to pack the other way round. The most common resulting structure, called the H
ii
phase, consists of a hexagonal array with heads orientated inwards to form a narrow aqueous
channel and the tails outwards
3
(
Figure 3.3
b).
Phospholipids whose heads and tails are of approximately equal effective diameter,
and which therefore have cylindrical rather than conical shapes, associate most stably
in double-layered sheets in which the hydrophobic tails of one layer face the tails of
the other layer and the hydrophilic heads face the water (
Figure 3.3
c). An elegant
demonstration of the importance of phospholipid geometry to the shape of the supramo-
lecular structure has been provided by mixtures of different molecules. The phospho-
lipid lysolecithin is conical, narrowing towards the tail, and it tends to form micelles.
If it is mixed with cholesterol, the cholesterol associates with the tail region and fills
in 'space' to make the dimer more cylindrical; under these conditions flat sheets form
in preference to micelles.
4
A simple flat sheet of such a bilayer would have the problem
of unstable edges, at which the radius of curvature of the sheets would have to be very
