Biochemistry - Chemical Reactions in Living Cells, Volume 1 - D. Metzler 1980

Membranes and Cell Walls
Membrane Structure
Lipid Bilayer Membrane Model

Membranes consist primarily of Proteins and Lipids¹ [10], with a weight ratio ranging from approximately 1:4 in myelin to 3:1 in bacterial membranes. However, a weight ratio of 1:1 can be considered the most typical for these components. CARBOHYDRATES (less than 5%) and traces of RNA (less than 0.1%) are also sometimes present in membranes in minor amounts. The presence of lipid components confers such membrane properties as high electrical resistance, impermeability to ions and other polar compounds, and permeability to nonpolar substances. For instance, most anesthetic drugs exhibit high lipid solubility, which enables them to penetrate nerve Cell membranes.

According to Electron Microscopy, the thickness of most membranes is approximately 7 nm; small-angle X-ray scattering data indicate a value close to 11 nm. Thus, membranes are extremely thin—their thickness is comparable to the dimensions of large molecules.

In 1926, Gorter and Grendel calculated that the lipid content in erythrocyte ghost membranes was sufficient to form a lipid layer 3.0–4.0 nm thick around The Cell². These calculations, combined with the finding that lipids can aggregate In aqueous solutions to form "micelles"—in which the hydrocarbon "tails" of the lipid molecules are clustered together while their polar "heads" face the surrounding solution—led J. Danielli in 1930 to propose the subsequently famous lipid bilayer model of membranes [12a]. The Main Features of this model are illustrated in Fig. 5-1. Driven by hydrophobic interactions, the hydrocarbon chains of the lipid molecules are held together in an extended conformation, whereas the polar groups of the phospholipid molecules interact with protein proteins located on both sides of The Lipid Bilayer.

Subsequently, A large number of other models were proposed, yet all of them retained METABOLISM/2.html">THE CONCEPT OF the lipid bilayer as their foundational premise [1]. Direct and reliable confirmation of the existence of the lipid bilayer was provided by electron microscopy studies. When most membranes are stained with osmium tetroxide or potassium permanganate, a characteristic trilaminar Structure can be observed (Fig. 5-1), consisting of two dark bands ~2–2.5 nm thick separated by a light band ~2.5–3.5 nm thick.

¹ An exception is found in the gas vacuole membranes of certain Bacteria and blue-green Algae. These thin membranes (approximately 2 nm thick) consist almost entirely of proteins [10a].

² Gorter and Grendel were able to arrive at the correct Conclusion only due to the mutual cancellation of errors in their surface pressure measurement experiments.

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FIG. 5-1. A. Bimolecular lipid layers and membranes

In both myelin and the outer segments of retinal rods, electron micrographs reveal closely apposed pairs of such membranes with a total thickness of 18 nm. Similar structures can be observed using an Electron microscope after staining and fixing Phospholipids in the lamellar "liquid-crystalline" phase. To interpret these results correctly, several additional questions must be addressed. Why, for instance, does OsO₄ stain only the outer protein layer, given that it is known to react with the double bonds of lipid hydrocarbon chains to form osmic acid esters, which are subsequently readily reduced to diols and osmium? Why do membranes from which most of the lipids have been extracted retain their ability to be stained by osmium tetroxide and maintain their trilaminar structure?

FIG. 5-1. B. Freeze-fracture electron micrograph of erythrocyte ghost membranes. The upper portion of the figure shows the membrane face adjacent to the Cytoplasm. The smooth area consists of lipids, with numerous particles visible. The lower portion displays the outer membrane face, which contains a smaller number of particles. The space between the membranes is filled with residual ice. Cytology/cytology/93.html">ELECTRON MICROGRAPHS OF Introduction/36.html">Biological Membranes are also shown in Figs. 1-2; 1-4 and Supplement 1-C. (Courtesy of K. Fisher.)

The mere detection of a trilaminar structure probably reveals very little. Even accurately determining membrane thickness is a formidable task, let alone fully deciphering The structure of an object only 6–10 nm thick.