The effect of membrane‐lipid phase transitions on membrane structure and on the growth of acholeplasma laidlawii B

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Abstract
The physical state of the membrane lipids, as determined by fatty acid composition and environmental temperature, has a marked effect both on the temperature range within which A. laidlawii can grow and on the temperature coefficient of growth within the permissible temperature range. The minimum growth temperature under certain conditions is clearly defined by the lower boundary of the gel–to–liquid‐crystalline phase transition of the membrane lipids. The physical state of the membrane lipids can also influence the optimum and maximum growth temperatures. An a brupt increase in the temperature coefficient of growth is noted at temperatures between the phase transition boundaries. Both the absolute rates and the temperature coefficients of cell growth are similar for cells whose membrane lipids exist entirely or predominantly in the liquid‐crystalline state, but absolute growth rates decline rapidly and temperature coefficients increase when most of the membrane lipids become solidified. Some cell growth, however, can continue at temperatures at which less than 10% of the total lipid remains in the fluid state. Conversion of the membrane lipid from the liquid‐crystalline to the gel state is accompanied by a progressive aggregation of intramembranous protein particles. An appreciable heterogeneity in the physical state of the membrane lipids can apparently be tolerated by this organism without a detectable loss of membrane function.