1 Dietary Fat Sources with Varying Degree of Un-saturation Modify Osmotic Fragility of Plasma Membrane of Red Blood Cells K. C. Weerakoon 1 , R.P.V.J. Rajapakse 1 , W.M.T. Madujith 2 , S. J. Weihena 3 H.M.T.U. Herath 4 and L.J.P.A.P. Jayasooriya 3 Department of Pathobiology, Faculty of Veterinary Medicine and Animal Science, University of Peradeniya 1 , Department of Food Science and Technology, Faculty of Agriculture, University of Peradeniya 2 , Department of Basic Veterinary Sciences, Faculty of Veterinary Medicine and Animal Science, University of Peradeniya 3 Department of Medical Laboratory Sciences, Faculty of Allied Health Sciences, University of Peradeniya 4 Introduction Plasma membranes (PM) are dynamic structures and maintain basic architectural features that govern their function and behavior (Mcmurchie, E.J., 1988). The membrane fluidity is the nature, arrangement and interactions of different molecular species of lipids present in the membrane. The PM of the red blood cell (RBC), similar to the other PM mainly consists of a lipid bilayer and proteins. Fluidity of the PM is determined by the proportion of unsaturated fatty acids in the tail part of the phopholipid molecules of the lipid bilayer. Thus, with an increase in degree of unsaturation, the fluidity of the membrane increases and then the fragility is also increased (Octavio T et al., 1990). Membrane fluidity is maintained by its fatty acid and cholesterol content. Long fatty acid chains are with ability to form stronger intermolecular interactions which restrict fluidity. Bends and kinks in the fatty acid chains are formed as a result of unsaturated cis double bonds that may interfere with intermolecular interactions which promote fluidity. Membrane fluidity can therefore be controlled by varying the number of double bonds and the length of fatty acid chains. Since phospholipids are important components of cell membranes, the changes in their fatty acid composition might be expected to have an impact on various membrane properties such as membrane fluidity (Guffy, M.M et al., 1982). Previous studies have shown that the relationship between the fatty acid composition of phospholipids and membrane fluidity (Burns, C.P et al 1979, King, M.E et al., 1978). It is also found that there is an increase in the percentage of polyenoic fatty acids with a resultant increase in the mean number of double bonds per fatty acid molecule that exerts a membrane fluidizing effect (Guffy, M.M et al.,