A Critique of Pure (Genetic) Information

88 Chapter 3
little by way of explanation. Highly ordered and specific patterns of
organization, as we’ve seen with respect to the radial or concentric
pattern of compartmentalization in the cell, do pose explanatory challenges
with respect to the preservation of information. We will now
consider evidence as to whether the movement of proteins in the plane
of the plasma membrane—and thus largely orthogonal to the direction
of the differentiation of the membranous strata—also poses such a
challenge.
The principal technique that has been used for examining the lateral
movement of membrane-embedded constituents has been fluorescence
recovery after photobleaching (FRAP). In this method, cell-surface components
are labeled with a fluorescent dye. A laser is focused on a small
(1 to 10mm 2 ) area on the surface of the labeled cell. Fluorescence in this
area is monitored by a photomultiplier. By momentarily increasing the
intensity of the laser by 10 3 to 10 5 -fold the fluorophores in the region
can be photochemically bleached. The laser is then attenuated and
fluorescence once again measured. The time it takes for full recovery of
fluorescence (due to the diffusional replacement of the bleached fluorophores)
is used to calculate the D L of the membrane-embedded cell
surface components. The level of fluorescence immediately after bleaching
is designated f 0. The complete or highest fluorescent level is designated
f µ.". From the times it takes to recover maximum fluorescent
recovery is derived the t 1/2, or half-recovery time. The diffusion coefficient,
DL, is then given by the equation D L = (w 2 /4t 1/2) g, where n is the
radius of the beam, t 1/2 the half-recovery time, and g a parameter which
accounts for the degree of bleaching and beam profile. Under typical conditions
g=1.3 (Cherry 1979).
Simple-model membranes can be constructed in the laboratory in
which nothing is present above or below the plane of the membrane.
Various proteins can be incorporated into these model membranes and
the lipid composition can be altered in order to evaluate the influence of
lipid composition on diffusional coefficients. The model membranes provided
a good opportunity to test the predictions of the Saffman-Delbrück
equation. In a number of FRAP studies using different lipid compositions
and different proteins, D L’s were found to be in the 10 -7 to 10 -8
range, which is in fairly good accordance with the Saffman-Delbrück