Biology Reference
In-Depth Information
Spontaneous
Seeded
Time
FIGURE 3.7
The kinetics of vesicle formation by sodium oleate. Spontaneous formation of vesicles has an
appreciable lag, whereas vesicle formation takes off at full speed if the solution is seeded with pre-assembled
vesicles: vesicles therefore catalyse their own growth, which is the basis of their 'reproduction'.
As vesicle reproduction proceeds, the ferritin tag becomes shared between old and new ves-
icles, implying that formation of new vesicles occurs by fission of existing ones rather than by
catalysed production of completely separate new ones
10
(this method of analysis, and its
interpretation, is analogous to the classical experiments of Meselson and Stahl that proved
that DNA replicates semi-conservatively). The precise method of vesicle reproduction is
not yet understood, but the ability of a simple molecule to produce replicating vesicles is
nevertheless a striking example of the power of self-assembly, and one that may be of great
relevance to the problem of the origin of life.
Artificial phospholipid bilayers are frequently used as models for the study of real cell
membrane properties, with which they share many properties and, in the case of vesicles,
a shape very common in biology. It is important to note, however, that the process of self-
assembly seen in artificial systems is not used to produce phospholipid bilayers
in vivo
.
Real membranes are stabilized by the same intermolecular forces, and membrane curvature
Micelles
Growth
Vesicle
Over-large vesicle
Fission
FIGURE 3.8
The 'reproductive cycle' of vesicles of sodium oleate. The precise details have not yet been
elucidated, but it appears that vesicles grow by accretion of material in micelles, and once they become large
enough they undergo fission.
