Abstract
Abstract Rotation of single swollen thylakoid vesicles (‘blebs’) was induced by means of a rotating electric field of strength 104 V · cm −1 , inducing a membrane voltage of 72 mV peak. Within the range of medium conductives described (40–300 μ S · cm −1 ), measurement of the field frequency (2–100 kHz) giving maximum rotation rate is equivalent to measuring the electrical time constant of the bleb membrane. Hence the membrane capacity (specific capacitance) was determined, and the value found at pH 8.1 (0.93 ± 0.07 μ F · cm −2 ) is in agreement with values deduced from measurements using other techniques. However, the capacity was also found to decreased with pH: a minimum value of 0.77 ± 0.01 μ F · cm −2 was measured at pH 4.4. The present study was extended to measurements of the effects of the lipid-soluble anion of dipicrylamine on the membrane capacity. At pH 7.2 and dipicrylamine concentration of 1.0 μM, a minimum estimate of the apparent membrane capacity was found to be 2.0 ± 0.2 μ F · cm −2 , with 2.6 ± 0.2 μ F · cm −2 being observed at 5.0 μM concentration. In addition, it was found possible to measure the membrane resistivity (specific resistance) in the presence of either gramicidin (1.0 to 10 nM) or valinomycin (1.0 to 10 μM). In the case of gramicidin, it was possible to derive a maximum estimate of the mean channel conductance, and this agrees very well with the values for individual, single channels that may be deduced from artificial bilayer work. Unless the gramicidin channels in blebs are in fact substantially more conductive than in artificial bilayers, this indicates that a high percentage of the added gramicidin forms channels which are open for most of the time. In the case of valinomycin, a much greater amount had to be added to produce the same reduction of membrane resistivity as seen with a given concentration of gramicidin. However, calculations indicate that the majority of this effect is due to the difference in partioning behaviour of the two ionophores.
Published Version
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