Biology Reference
In-Depth Information
constituents of biological self-assembling systems in motion, and the behaviour of the tiles is
observed. The random motion of the tiles brings them into collision, and if the surfaces that
meet are magnetically compatible, they may stick. This random meeting and sticking causes
the tiles to form larger and larger aggregates, 'tiling' part of the surface of the water. The
'binding energy' of the magnets is not enough to make binding an irreversible step, and tiles
are sometimes lost from the aggregate. More strikingly, when two small aggregates combine,
their association tends to be unstable if their edges do not fit together well, but stable if there
is a good fit with no gaps. The final result is the spontaneous construction of a large area of
regular tiling.
SELF-ASSEMBLY OF BILAYERED MEMBRANES
A more biologically relevant self-assembling system, but one still capable of producing
structures that can be viewed with a simple light microscope, is provided by phospholipids,
which are the main constituents of cell membranes. Phospholipids are amphipathic; they
have both hydrophilic and hydrophobic characters which are separated into different
domains of the molecule; the 'head' is hydrophilic and the 'tail(s)' hydrophobic (
Figure 3.2
).
The hydrophilic head can form energetically favourable hydrogen bonds with water but the
tail cannot, and the effect of immersing the tail region in water would be to disrupt the
hydrogen bonds between many water molecules, which would be energetically unfavour-
able from the point of view of the complete solute-plus-solvent system. Phospholipids there-
fore combine to form structures in which the net free energy change, including disruption of
water
e
water interactions, is as favourable as possible.
The structure formed by a given type of phospholipid is determined largely by the gross
shape of the momoner, particularly the ratio of the widths of the head region and the tail
regions.
2
Some phospholipids, such as gangliosides, have large head groups, giving the
molecules an approximately conical shape (
Figure 3.3
a). When mixed with water, such
wide-head conical molecules tend to associate together in spherical monolayered shells
FIGURE 3.2
The general structure of a membrane phospholipid.
