Cell Membrane Features Introductory article Philip L Yeagle, Rutgers University, Newark, New Jersey, USA Article Contents . Introduction . Structure . Function of Cell Membranes . Membrane Fusion . Mammalian Cell Membranes . Other Biological Membranes Online posting date: 15th December 2009 The membranes of living cells support much of the func- intracellular transport, as well as support a host of other tionality of biology. From the subcellular level of organ- cellular activities. In eukaryotic cells, all the internal elles to the supercellular level of cell–cell interactions, organelles are defined by membranes. Consistent with this membranes provide the structures necessary for bio- focus of cellular activity on cell membranes, it is estimated that nearly half of all expressed proteins are integral logical function and organization. Many of those struc- membrane proteins, and many more are associated with tural and functional features are common among cell membranes. See also: Cell Structure membranes in bacteria, eukaryotic cells and viruses. The An appreciation of the many facets of cell membrane lipid bilayer provides the fundamental architecture and structure and function can be gained by initially con- some of the properties common to all biological mem- sidering separately the two major components of cell branes. Membrane proteins confer a myriad of specific membrane structure: the lipid bilayer and membrane functions expressed by cell membranes. Plasma mem- proteins. branes determine the boundary of the cell and many of the interactions of the cell with its environment. Intra- cellular membranes compartmentalize cells into different Structure functional units with differing internal compositions. The composition of the compartments is maintained by Lipid bilayer essential transport functions in concert with permeability control. Signal transduction is supported by membranes. The architecture of biological membranes is designed on Membrane fusion provides a mechanism for intracellular the structure of the lipid bilayer. All biological membranes contain the bilayer structure. The bilayer structure is based vesicular transport and enveloped virus entry into cells. on the chemical structure of the lipid constituents and the hydrophobic effect. See also: Lipid Bilayers Some properties of cell membranes are derived directly from lipid bilayers. These properties include a (nearly) two- dimensional structure, ordered fluid properties, limited Introduction permeability, ability to confer charge on membrane sur- faces, ability to regulate function through individual lipid Membranes compartmentalize cellular function, control species in the bilayer and membrane lipid asymmetry. cell–cell recognition, transduce extracellular signals to regulate internal cellular activity, synthesize adenosine Two-dimensional structure of lipid bilayers triphosphate (ATP), the common currency of cellular Most of the lipids of biological membranes have an energy, create pathways for controlled internal transport of amphipathic chemical structure. Most lipids consist of a materials around the cell, enable and regulate all transport polar, hydrophilic headgroup (often with charged con- of material between the inside and the outside of cells, stituents) and hydrophobic hydrocarbon chains. The connect and disconnect membranes during cell entry and hydrophobic effect drives the formation of lipid bilayers in the aqueous environment, whether as pure lipids in an aqueous environment or as lipids in a biological membrane ELS subject area: Cell Biology in living cells. The hydrophobic hydrocarbon chains of the membrane lipids must be sequestered from the aqueous How to cite: environment, leaving the polar headgroups to interact with Yeagle, Philip L (December 2009) Cell Membrane Features. In: Encyclo- the water. This leads to the formation of the lipid bilayer, as pedia of Life Sciences (ELS). John Wiley & Sons, Ltd: Chichester. represented schematically in Figure 1. Lipid bilayers repre- DOI: 10.1002/9780470015902.a0001261.pub2 sent nearly a two-dimensional world. While membranes ENCYCLOPEDIA OF LIFE SCIENCES & 2009, John Wiley & Sons, Ltd. www.els.net 1 Cell Membrane Features Water Limited permeability The lipid bilayer is relatively impermeable to solutes and thus forms an effective barrier to movement of solutes from one side of a cell membrane to another. Since the interior of the lipid bilayer is hydrophobic, polar compounds are largely excluded from the interior and thus the trans- membrane transit by polar molecules is inhibited. Imper- meability, a fundamental characteristic of lipid bilayers, imparts to cell membranes one of the properties crucial to cell survival, the compartmentalization of cell function. Since solutes cannot readily pass through the lipid bilayer, solutes must get into and out of a cell through transport functions catalysed and regulated by membrane proteins. Water Thus, solute movements between cellular compartments Figure 1 Schematic representation of a lipid bilayer. The circles represent and into and out of the cell can be tightly controlled by the polar headgroups of the lipids, and the lines connected to the circle membrane proteins in cell membranes. Charge and represent the hydrophobic hydrocarbon chains of the lipids. These enzymatic regulation by the lipid bilayer. amphipathic molecules are dual nature: one end is hydrophilic and the other end is hydrophobic. They organize so as to limit the exposure of the The lipid compositions of biological membranes are hydrophobic portions to the aqueous phase that is found on both sides of the complex. Many different hydrocarbon chains can be used membrane. and many different headgroups can be used to construct membrane lipids. Consequently, several thousand indi- vidual species of lipids are known to exist in nature. This complexity is thought to regulate the function of mem- extend in two dimensions, the third dimension is limited, brane proteins and other membrane properties. Specific defined by the length of two lipid molecules. Therefore the lipid–protein interactions in which lipids with specific lipid bilayer has some characteristics of a huge but very thin structures bind to particular membrane enzymes modulate macromolecule with many ‘subunits’ (lipids) because it is enzyme function. Lipids with charged headgroups confer an extended structure that can cover a whole cell, yet the local charge on the membrane surface. The structure of the ‘subunits’ are not covalently connected. See also: Hydro- lipid bilayer stabilizes membrane proteins against phobic Effect; Lipids; Membrane Lipid Biosynthesis; denaturation. Water: Structure and Properties Cholesterol is an essential lipid in mammalian cells that exhibits many of these functions. Cholesterol is found Ordered fluid predominantly in the plasma membrane of mammalian cells. Yeast have ergosterol as their essential sterol in their The lipid bilayer of cellular membranes allows lateral plasma membrane. Plant cells have yet different sterols, movement, or diffusion, of lipids in the plane of the mem- such as sitosterol. The specificity of sterol is likely due to the brane. However, movement in and out of the plane of the interaction between specific sterol structures and particular membrane is limited by the hydrophobic effect and the membrane proteins in these cells to regulate crucial cellular chemical structure of the membrane lipids (i.e. the lipids are functions. Cholesterol also orders (see section on Ordered not soluble in water). This limitation imposes some order fluid) the lipid hydrocarbon chains in the bilayer and ren- on the system. Additional order at the submolecular level is ders bilayers even less permeable to solutes than in the imposed by the side-by-side arrangement of the lipids in the absence of cholesterol. Some bacteria have sterols and bilayer. The lipid hydrocarbon chains exhibit some intra- others do not require this lipid for growth. Enveloped molecular order that is manifest in limitations on the viruses (viruses with a membrane around the nucleocapsid) rotation around carbon–carbon bonds in the hydrocarbon will have sterols characteristic of the cell in which they chains of the lipids. There is greater conformational flexi- propagate and some require cholesterol for infection. bility of the hydrocarbon chain in the centre of the bilayer Although much remains to be understood about how these than in the region near the lipid headgroups. effects are achieved, lipids clearly play both fundamental Lateral diffusion of the proteins also occurs and can be and specific roles in biological membrane structure and important to membrane function, allowing some proteins function. See also: Bacterial Cytoplasmic Membrane; to properly associate for expression of their activity. In Cholesterol, Steroid and Isoprenoid Biosynthesis; Viral some cases, the lateral movement of membrane com- Capsids and Envelopes: Structure and Function ponents is restricted. This can lead to patches of different composition in the plane of the membrane (rafts). Extreme constriction occurs when membrane proteins are anchored Membrane proteins to a membrane skeleton, a part of the cell cytoskeleton lying immediately under the plasma membrane and con- Membrane proteins and lipid bilayers are the major com- nected to the latter. See also: Membrane Dynamics ponents of cell membranes. Membrane proteins impart 2 ENCYCLOPEDIA OF LIFE SCIENCES & 2009, John Wiley & Sons, Ltd. www.els.net Cell Membrane Features many functions to biological membranes and thus