Ethanol and the fluidity of the yeast plasma membrane.

Evidence is presented for an exponential increase in yeast plasma membrane fluidity (as measured by passive permeability to acetic acid) with ethanol concentration. The role of adaptation of yeast cells to ethanol can be seen in the existence of a threshold concentration before the onset of an observed fluidizing effect. The physiological state of the yeast cells is also demonstrated to influence the sensitivity of the membrane to fluidization by ethanol. On the basis of these results, the concept that increased fluidity is an adaptive response conferring ethanol tolerance is disputed. An alternative hypothesis, namely that the observed increase in fluidity is the net result of a number of more fundamental changes, is presented to explain the observed effects.