2.3.2.2—
Membrane Fluidity

An essential feature of the model in Fig. 2.10 is that the various components of the membrane can move relative to one another. Rather than being fixed points, many of the proteins are best thought of as drifting around in the lipid, the viscosity of which will determine the rate at which they move. Evidence that membrane particles can move in the plane of the membrane comes from freeze

Figure 2.10
A representation of the fluid mosaic model of membrane structure
showing peripheral and integral proteins in a 'sea' of lipid molecules.
Not shown are the polarized water layers which are bound to the polar
regions of the lipids and proteins. The round-headed objects with twin
tails represent phospholipids, the obovoid structure with the single tail
represents sterol and the large irregularly shaped objects are proteins.
(Based on ideas in Singer & Nicholson, 1972 and Capaldi, 1974).

fracture studies of the plasmalemma of Mycoplasma mycoides, in which it was seen that particles became aggregated as the membrane was cooled down but dispersed again when it was warmed up (Rottem et al., 1973). A second line of the evidence comes from studies where two animal cells were induced to fuse with one another (Edidin & Farnbrough, 1973). One of the cells has a specific antigen in its plasma membrane which the other cell lacked. The distribution of the antigen could be visualized by the binding of a fluorescent antibody. The fusion of the cells to form a heterocaryon and the subsequent redistribution of the fluorescent antibody were observed using a fluorescence microscope. At first the fluorescent marker was restricted to one half of the heterocaryon, but quite quickly, and in an orderly progression the fluorescence spread right around the coat indicating that the antigenic protein, known to be integrated into the membrane matrix, had diffused in the plane of the membrane. For it to have done this it must have drifted through the lipid. If the heterocaryon was cooled below the transition temperature of its lipids, no mixing of the antigen occurred.

From what we have said above and from earlier comments (p. 29) it is clear that temperature will have a very important influence on the diffusion of materials through, and in the plane of, the membrane because of its effect on the viscosity of the lipid (Edidin, 1974). If the membrane is cooled to the extent that the lipids freeze then such diffusion will become very restricted; the biological

significance of this is reflected in the fact that the rates of most physiological processes examined over a range of temperatures are found to have a sharp transition at a temperature which is close to the transition of membrane lipids from a liquid crystalline condition to the gelled condition (Simon, 1974).

Most workers agree that there must be some proteins whose position in the membrane relative to others must be maintained, e.g. components of electrontransport chains. Such proteins may require anchoring points either in parts of the membrane which are less fluid than others, or by association with some extra-membrane protein. It is possible that aggregations of sterol molecules might serve to create more viscous patches, and there is evidence, again from synthetic lipid bilayers, that cholesterol can be concentrated in association with certain phospholipids (de Kruyff et al., 1974).