Lipid Layer in Cell Membranes

Abstract

MEASUREMENTS of the extent of shrinkage of membranes which results from hydrolysis of phospholipid components by phospholipase C provide a direct indication of the area occupied by lipid in cell membranes. Maximum treatment of erythrocyte ghosts with phospholipase C (Clostridium welchii) liberates approximately 70% of the membrane phospholipid phosphorus and the surface area of the ghosts is decreased by 45–55% as calculated from diameter measurements made on spherical ghosts observed by phase contrast microscopy¹ and more recently from ghost volume measurements by an exclusion technique involving the use of ¹⁴C labelled sucrose. There is strong evidence that the phospholipid molecule as a whole is displaced from the membrane but the fate of the other membrane lipids is not established although it has been demonstrated that a residual lipoprotein structure persists. To indicate the quantitative significance of the observations, however, we have assumed that all lipids are removed in the same proportions as for the phospholipid. The observed shrinkage of the membrane by 50% would then be attributed to the loss of 70% of the membrane lipid. This would indicate that lipid occupies an area corresponding to 100 × 50/70 = 70% of the overall membrane area. Should any lipid be displaced to a lesser extent, then the figure for the area occupied by lipid would be increased to more than 70%.