Membrane formation

Some micelles at concentrations well above the CMC form extended parallel sheets, called lamellar micelles, two molecules thick. The individual molecules lie perpendicular to the sheets, with hydrophilic groups on the outside in aqueous solution and on the inside in nonpolar media. Such lamellar micelles show a close resemblance to biological membranes, and are often a useful model on which to base investigations of biological structures. Although lamellar micelles are convenient models of cell membranes, actual membranes are highly sophisticated structures. The basic structural element of a membrane is a phospholipid, such as phosphatidyl choline (10), which contains long hydrocarbon chains (typically in the range C₁₄–C₂₄) and a variety of polar groups, such as -CH₂CH₂N(CH₃)₃ + in (10). The hydrophobic chains stack together to form an extensive bilayer about 5 nm across. The lipid molecules form layers instead of micelles because the hydrocarbon chains are too bulky to allow packing into nearly spherical clusters. The bilayer is a highly mobile structure, as shown by EPR studies with spin-labelled phospholipids (Impact I15.2). Not only are the hydrocarbon chains ceaselessly twisting and turning in the region between the polar groups, but the phospholipid and cholesterol molecules migrate over the surface. It is better to think of the membrane as a viscous fluid rather than a permanent structure, with a viscosity about 100 times that of water. In common with diffusional behaviour in general (see Section 21.*), the average distance a phospholipid molecule diffuses is proportional to the square-root of the time; more precisely, for a molecule confined to a two-dimensional plane, the average distance travelled in a time t is equal to (4Dt)1/2. Typically, a phospholipid molecule migrates through about 1 µm (the diameter of a cell) in about 1 min. All lipid bilayers undergo a transition from a state of high to low chain mobility at a temperature that depends on the structure of the lipid. To visualize the transition, we consider what happens to a membrane as we lower its temperature (Fig. 19.42). There is sufficient energy available at normal temperatures for limited bond rotation to occur and the flexible chains writhe. However, the membrane is still highly organized in the sense that the bilayer structure does not come apart and the system is best described as a liquid crystal (Fig. 19.42a). At lower temperatures, the amplitudes of the writhing motion decrease until a specific temperature is reached at which motion is largely frozen. The membrane is said to exist as a gel (Fig. 19.42b). Biological mem branes exist as liquid crystals at physiological temperatures. Phase transitions in membranes are often observed as ‘melting’ from gel to liquid crystal by differential scanning calorimetry (Impact I2.1). The data show relations between the structure of the lipid and the melting temperature. For example, the melting temperature increases with the length of the hydrophobic chain of the lipid. This correlation is reasonable, as we expect longer chains to be held together more strongly by hydrophobic interactions than shorter chains. It follows that stabilization of the gel phase in membranes of lipids with long chains results in relatively high melting temperatures. On the other hand, any structural elements that prevent alignment of the hydrophobic chains in the gel phaselead to low melting temperatures. Indeed, lipids containing unsaturated chains, those containing some C=C bonds, form membranes with lower melting temperatures than those formed from lipids with fully saturated chains, those consisting of C-C bonds only. Interspersed among the phospholipids of biological membranes are sterols, such as cholesterol (11), which is largely hydrophobic but does contain a hydrophilic -OH group. Sterols, which are present in different proportions in different types of cells, prevent the hydrophobic chains of lipids from ‘freezing’ into a gel and, by disrupting the packing of the chains, spread the melting point of the membrane over a range of temperatures.

Peripheral proteins are proteins attached to the bilayer. Integral proteins are proteins immersed in the mobile but viscous bilayer. These proteins may span the depth of the bilayer and consist of tightly packed α helices or, in some cases, β sheets containing hydrophobic residues that sit comfortably within the hydrocarbon region of the bilayer. There are two views of the motion of integral proteins in the bilayer. In the fluid mosaic model shown in Fig. 19.43 the proteins are mobile, but their diffusion coefficients are much smaller than those of the lipids. In the lipid raft model, a number of lipid and cholesterol molecules form ordered structures, or ‘rafts’, that envelop proteins and help carry them to specific parts of the cell. The mobility of the bilayer enables it to flow round a molecule close to the outer surface, to engulf it, and incorporate it into the cell by the process of endocytosis. Alternatively, material from the cell interior wrapped in cell membrane may coalesce with the cell membrane itself, which then withdraws and ejects the material in the process of exocytosis. The function of the proteins embedded in the bilayer, though, is to act as devices for transporting matter into and out of the cell in a more subtle manner. By providing hydrophilic channels through an otherwise alien hydrophobic environment, some proteins act as ion channels and ion pumps (Impact I21.2).

Fig. 19.43 In the fluid mosaic model of a biological cell membrane, integral proteins diffuse through the lipid bilayer. In the alternative lipid raft model, a number of lipid and cholesterol molecules envelop and transport the protein around the membrane.

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مواضيع ذات صلة

Lattices and unit cells

Surface pressure

Micelle formation

The electrical double layer

Classification and preparation

The stability of proteins and nucleic acids

The structure of nucleic acids

Helices and sheets

Conformational energy

The Corey–Pauling rules

The different levels of structure

Viscosity