The Fluid Mosaic Model of the Structure of Cell Membranes

Abstract

A fluid mosaic model is presented for the gross organization and structure of the proteins and lipids of biological membranes. The model is consistent with the restrictions imposed by thermodynamics. In this model, the proteins that are integral to the membrane are a heterogeneous set of globular molecules, each arranged in an amphipathic structure, that is, with the ionic and highly polar groups protruding from the membrane into the aqueous phase, and the nonpolar groups largely buried in the hydrophobic interior of the membrane. These globular molecules are partially embedded in a matrix of phospholipid. The bulk of the phospholipid is organized as a discontinuous, fluid bilayer, although a small fraction of the lipid may interact specifically with the membrane proteins. The fluid mosaic structure is therefore formally analogous to a two-dimensional oriented solution of integral proteins (or lipoproteins) in the viscous phospholipid bilayer solvent. Recent experiments with a wide variety of techniqes and several different membrane systems are described, all of which abet consistent with, and add much detail to, the fluid mosaic model. It therefore seems appropriate to suggest possible mechanisms for various membrane functions and membrane-mediated phenomena in the light of the model. As examples, experimentally testable mechanisms are suggested for cell surface changes in malignant transformation, and for cooperative effects exhibited in the interactions of membranes with some specific ligands. Note added in proof: Since this article was written, we have obtained electron microscopic evidence (69) that the concanavalin A binding sites on the membranes of SV40 virus-transformed mouse fibroblasts (3T3 cells) are more clustered than the sites on the membranes of normal cells, as predicted by the hypothesis represented in Fig. 7B. T-here has also appeared a study by Taylor et al. (70) showing the remarkable effects produced on lymphocytes by the addition of antibodies directed to their surface immunoglobulin molecules. The antibodies induce a redistribution and pinocytosis of these surface immunoglobulins, so that within about 30 minutes at 37 degrees C the surface immunoglobulins are completely swept out of the membrane. These effects do not occur, however, if the bivalent antibodies are replaced by their univalent Fab fragments or if the antibody experiments are carried out at 0 degrees C instead of 37 degrees C. These and related results strongly indicate that the bivalent antibodies produce an aggregation of the surface immunoglobulin molecules in the plane of the membrane, which can occur only if the immunoglobulin molecules are free to diffuse in the membrane. This aggregation then appears to trigger off the pinocytosis of the membrane components by some unknown mechanism. Such membrane transformations may be of crucial importance in the induction of an antibody response to an antigen, as well as iv other processes of cell differentiation.

Asymmetric Hybrid Polymer–Lipid Giant Vesicles as Cell Membrane Mimics

“…Therefore, as for GHUV, an intermediate diffusion coefficient value is found, and shows that despite the asymmetric character of the membrane, the lipid lateral diffusion coefficient is lowered by the copolymer chains, suggesting some interdigitation between lipids and copolymer chains in the membrane. The lipid lateral diffusion has been also evaluated in aGHUV at 37 °C and is found to be 2.3 ± 0.7 µm 2 s −1 confirming the impact of temperature on membrane's fluidity,30, 71, 72 and the relevance of our systems, with a lateral diffusion coefficient close to those reported in cells' membrane.…”

“…Some key properties have been reproduced, such as compartmentalization,3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 cytoskeleton mimics,15, 16, 17, 18, 19 or simple (bio)chemical reactions 20, 21, 22, 23, 24, 25, 26, 27, 28, 29. The cell membrane structure is often simplified using lipids, or lipid mix with addition of cholesterol, which is really far from reality, not only considering the chemical structure but also the biophysical properties of the membrane 30. Another important feature that is often neglected concerns the asymmetric composition of the membrane.…”

The Role of Sphingolipids and Ceramide in Pulmonary Inflammation in Cystic Fibrosis

“…Ceramide is composed of D-erythro-sphingosine and a fatty acid containing 2-28 carbon atoms in the acyl chain that is connected via an amide ester bond [30]. Based on biophysical experiments determining the melting temperatures of lipids, Singer and Nicolson introduced the classical fluid mosaic model in 1972 predicting free movement of proteins in the lipid bilayer [31]. However, this concept was revised in the last 10-15 years and present concepts indicate the existence of distinct membrane domains [32, 33].…”

Thermotropic Phase Transition in Soluble Nanoscale Lipid Bilayers

“…The specific activity of each batch of radioactive lipids was determined by quantitation of phosphate 41 and scintillation counting using standard procedures. For calibration we used standard phosphorous solutions (Sigma, P3869) and [1,[2][3] H] hexadecanol (Moravec Biochemicals, Bria, CA). Concentrations of scaffold proteins were determined using molar absorption coefficients calculated for the known amino acid sequences according to published methods 42 .…”