Biomedical Engineering Reference
In-Depth Information
Figure 5.5
Diffusionofproteinswithinthemembranesurface(a)beforean
external E field is applied, (b) after an external E field is applied and there
is a net negative charge on the surface and (c) after an external E field is
applied where a dipolarcharge isinduced within proteins.
by Poo. A final run was made at weak exposure levels to model
the experimental exposure of Mougeotia protoplasts conducted by
Whiteet al. (1990).
Electric fields are known to cause proteins to diffuse within
plasma membranes. Darnell et al. (1986) discuss freeze fracture
micrographs that clearly show diffusion of intracellular proteins
withintheplasmamembraneofmitochondriaduetoappliedvoltage
gradients resulting from strong E field exposure. Figure 5.5a-c
illustrates how asymmetric and symmetric protein concentrations
develop across a cell as diffusion proceeds in time. Jaffe and
Nuccitelli(1977)andlaterPoo(1981)investigatedproteindiffusion
by means of a chamber to which immobilised cells adhere,
enabling in situ electrophoresis of membrane components. Agar-
filled salt bridges are used to pass current through the saline-filled
chamber. Fluorescent labelling, micro-iontophoresis of membrane
components involved in ion transport across the membrane and
the freeze fracture method are used to experimentally determine
the migration of proteins and other components. Exposure levels in
these experiments were between 100 and 1000 V m
−
1
(Poo, 1981).
Protein diffusion within membranes is involved in the growth
and development of cells via processes that are initiated and
controlled by biogenic E fields. In one such investigation, spherical
Mougeotia
protoplasts, originally cylindrical, were exposed to static
E fields of around 20 V m
−
1
(White et al., 1990). The protoplasts
regenerated, displaying a normal sequence of microtubule reorgan-
