TRANSPORT AND DYNAMICS OF MOLECULES DISSOLVED IN MAIZE ROOT CORTEX MEMBRANES

Translational diffusion of a fluorescent sterol probe was measured in the plasma membranes of protoplasts isolated from cortical cells of th e primary root of maize seedlings. The apparent lateral diffusion coef ficient was typically observed to be nearly insensitive to temperature , while the mobile fraction increased with increasing temperature. The se fluorescence photobleaching recovery (FPR) measurements were compar ed with the electron paramagnetic resonance (EPR) spectra of the methy l ester of 13-doxyl palmitic acid in membranes of corn root tissue in situ. The complex spectra observed with this probe were analyzed as we ighted sums of simpler spectra of various order parameters and rotatio nal correlation times. The reconstituted spectra calculated from the m odel show that EPR also detects a mobile (less ordered, fluid) fractio n, distinguished by the order parameter S = 0.1 to 0.2, which becomes more abundant as temperature increases and is qualitatively comparable to the mobile fraction determined by the FPR method. The observed res ults on the mobile fractions and the diffusion rates for translational (FPR) as well as rotational (EPR) motions are interpreted in terms of membrane organization, thus providing information on the population a nd structural patterns of the coexisting domains with a special emphas is on the response of the membrane to temperature changes.