The Biological Membrane
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The Biological Membrane Mark Alper
All living cells, and many of the structures within these cells (mitochondria, nuclei, chloroplasts) are surrounded by biological membranes which serve to separate the cell contents from the surrounding environment. The biological membrane is an extraordinary material. It controls the highly selective transport of molecules into and out of the cell. It senses the environment outside the cell and transmits information about it to the intracellular machinery. It reports information about the cell to the outside world—its identity and its state of function. It transports electrons, converts sunlight to chemical and electrical energy, pumps small molecules against a concentration gradient, and uses that gradient as a source of energy. The membrane is a generally robust structure, and one that can be modified in a controlled manner, making it adaptable for use in nonbiological applications. It has served as a model for sensors and detectors, for surface modification agents, for drug delivery systems, and for information storage and delivery, as well as other optoelectronic functions. Membranes are, in general, composed of individual lipid and protein molecules in a bilayer structure 60-100 A thick (Figure 1). The protein can constitute from 25% to 80% of the complex by weight. What is so remarkable about the membrane is that, in a process known as "selfassembly," its thousands and thousands of individual component molecules spontaneously associate, align, and create its precisely defined structure. The molecules are not covalently bonded to each other, yet they form a compact film that is strong, stable, rigid, impermeable (even to ions as small as H + ), asymmetric (its inside surface is different from its outside surface), and chemically active—and all in a controllable manner. The lipid molecules involved in memMRS BULLETIN/NOVEMBER 1992
brane formation are amphipathic; that is, they are composed of a polar or charged "head group" attached to a variable-length nonpolar "tail". Amphipathic molecules, depending on the nature of their structure and that of the solution, spontaneously form micelles, vesicles, monolayers, or membranes (Figures 1 and 2). In all cases, the driving force is primarily entropic in nature. The so-called "hydrophobic effect" favors these structures because they minimize the contact between the nonpolar regions of the molecules and the surrounding water. Minimizing this contact area minimizes the structural ordering of water molecules around the nonpolar groups. The lipid tails also show van der Waals attractions and the polar head groups show electrostatic and hydrogen bond attraction to the solvent water. Thus, random motion of molecules seeking a thermodynamic energy minimum leads, in a cooperative fashion, to the formation and maintenance of the membrane structure. There is motion within the membrane. Proteins and lipids can translocate laterally in the structure, at rates of up to several microns per minute, giving it a fluid, almost solutionlike nature.
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