Membrane Lipids Form Bilayers
المؤلف:
Peter J. Kennelly, Kathleen M. Botham, Owen P. McGuinness, Victor W. Rodwell, P. Anthony Weil
المصدر:
Harpers Illustrated Biochemistry
الجزء والصفحة:
32nd edition.p470-471
2025-11-04
52
The amphipathic character of phospholipids suggests that the two regions of the molecule have incompatible solubilities. However, in a solvent such as water, phospholipids spontaneously organize themselves into micelles (Figure 1), an assembly that thermodynamically satisfies the solubility requirements of the two chemically distinct regions of these molecules. Within the micelle the hydrophobic regions of the amphipathic phospholipids are shielded from water, while the hydrophilic polar groups are immersed in the aqueous environment. Micelles are usually relatively small in size (eg, ~200 nm) and consequently are limited in their potential to form membranes. Detergents commonly form micelles.
Fig1. Diagrammatic cross-section of a micelle. The polar head groups are bathed in water, whereas the hydrophobic hydrocarbon tails are surrounded by other hydrocarbons and thereby protected from water. Micelles are relatively small (compared with lipid bilayers) spherical structures.
Phospholipids and similar amphipathic molecules can form another structure, the bimolecular lipid bilayer, which also satisfies the thermodynamic requirements of amphipathic molecules in an aqueous environment. Bilayers are the key structures in biologic membranes. Bilayers exist as sheets wherein the hydrophobic regions of the phospholipids are sequestered from the aqueous environment, while the hydrophilic, charged portions are exposed to water (Figure 2). The ends or edges of the bilayer sheet can be eliminated by folding the sheet back on itself to form an enclosed vesicle with no edges. The closed bilayer provides one of the most essential properties of membranes. The lipid bilayer is impermeable to most water-soluble molecules since such charged molecules would be insoluble in the hydro phobic core of the bilayer. The self-assembly of lipid bilayers is driven by the hydrophobic effect, which describes the tendency of nonpolar molecules to self-associate in an aqueous environment, while in the process excluding H2O. When lipid molecules come together in a bilayer, the entropy of the surrounding solvent molecules increases due to the release of immobilized water.
Fig2. Diagram of a section of a bilayer membrane formed from phospholipids. The unsaturated fatty acid tails are kinked and lead to more spacing between the polar head groups, and hence to more room for movement. This in turn results in increased membrane fluidity.
Two questions arise from consideration of the information described earlier. First, how many biologically important molecules are lipid-soluble and can therefore readily enter the cell? Gases such as oxygen, CO2 , and nitrogen—small molecules with little interaction with solvents—readily diffuse through the hydrophobic regions of the membrane. The permeability coefficients of several ions and a number of other molecules in a lipid bilayer are shown in Figure 3. The electrolytes Na+, K+, and Cl− cross the bilayer much more slowly than water. In general, the permeability coefficients of small molecules in a lipid bilayer correlate with their solubilities in nonpolar solvents. For instance, steroids more readily traverse the lipid bilayer compared with electrolytes. The high permeability coefficient of water itself is surprising, but is partly explained by its small size and relative lack of charge. Many drugs are hydrophobic and can readily cross mem branes and enter cells.
Fig3. Permeability coefficients of water, some ions, and other small molecules in lipid bilayer membranes .The permeability coefficient is a measure of the ability of a molecule to diffuse across a permeability barrier. Molecules that move rapidly through a given membrane are said to have a high permeability coefficient.
The second question concerns non–lipid-soluble molecules. How are the transmembrane concentration gradients for these molecules maintained? The answer is that membranes contain proteins, many of which span the lipid bilayer. These proteins either form channels for the movement of ions and small molecules or serve astransportersfor molecules that otherwise could not readily traverse the lipid bilayer (membrane). The nature, properties, and structures of membrane channels and transporters are described later.
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