Electron spin resonance analysis of suspensions of animal cell plasma membranes consistently reveals four characteristic temperatures for lateral phase separations in the membrane lipids. Similar analysis of an aqueous dispersion of lipids extracted from these membranes reveals only two characteristic temperatures, indicating that some aspect of lipid organization in membranes is destroyed by the extraction procedure. The characteristic temperatures for surface membranes from two different species of homeothermic animals were nearly identical and were approximately 37°, 31°, 21°, and 15°. A treatment of the physical data revealed that these temperatures could identify independent phase transitions for two hydrocarbon compartments of approximately equal size with lower and upper characteristic temperatures of 21° and 37°, and of 15° and 31°. The analysis of the effects of temperature on a number of physiological parameters indicates that 21° and 37° are likely to define the boundaries for lateral phase separations in the inner monolayer and 15° and 31° the boundaries for lateral phase separations in the outer monolayer.

Wisnieski, B. J., Parkes, J. G., Huang, Y. O., & Fox, C. F. (1974). Physical and Physiological Evidence for Two Phase Transitions in Cytoplasmic Membranes of Animal Cells. Proceedings of the National Academy of Sciences, 71(11), 4381–4385.