. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. MOLECULAR INTERACTIONS ON THE CELL SURFACE 167 q K) in CN q 5™ o. in d 0 45 90 135 180 225 270 315 360 0 (degrees) Figure 2. The relative surface concentration profile [CT(0)/C0T] of IgE-R on RBL cells equilibrated in an applied field of E(, = 15 V/cm. Shown is the average over an ensemble of about 30 cells like those dis- played in Figure 1. [CT(S)/C0T] is displayed as a function of angle 6 with respect to the direction of the applied field E0 (see insert). The standard deviation of the mean values for the data points shown
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. MOLECULAR INTERACTIONS ON THE CELL SURFACE 167 q K) in CN q 5™ o. in d 0 45 90 135 180 225 270 315 360 0 (degrees) Figure 2. The relative surface concentration profile [CT(0)/C0T] of IgE-R on RBL cells equilibrated in an applied field of E(, = 15 V/cm. Shown is the average over an ensemble of about 30 cells like those dis- played in Figure 1. [CT(S)/C0T] is displayed as a function of angle 6 with respect to the direction of the applied field E0 (see insert). The standard deviation of the mean values for the data points shown was typically ~ 10%. The solid line is an attempted fit to Boltzmann statistics includ- ing an immobile fraction. Reprinted from Science (Ryan el 1988). copyright 1988 by the AAAS. D = -XB [XADB + XBDA] = DB + XB(DA - DB) 1-XB (7) The relaxation process is further complicated by the fact that both DA and DB are also likely to be concentra- tion dependent. Saxton (1987) has recently calculated the concentration dependence of tracer diffusion from Monte Carlo simulations of molecular motion on a lat- tice and finds D(c) D(0) + (8) where D(0) is the tracer diffusion in the infinite dilution limit of the concentration c (here c = XB). Neglecting high order terms, we see that in this case the effects of the non-ideal activity coefficient in the extra driving force is roughly compensated for by the slowing of diffusion by crowding. In most cell membranes of high protein concentration (XB ~ ) typical values for DA ~ 1CT* cnr/s and DB < 10"'° cnr/s imply D an order of magnitude larger than DB. This result is apparently confirmed by most avail- able data on cell membranes. The one known exception is the case of IgE-R on RBL cells (Wolf et ai. 1980; Menonf/c;/.. 1986; McCloskey et 1984) where exper- iments show clearly that DPER ~ DFPR. The reason for this exception is not clear. Nevertheless, we conclude that observed differences between DPER and
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