Membrane continued bilayer

A continuous lipidic cubic phase is obtained by mixing a long-chain lipid such as monoolein with a small amount of water. The result is a highly viscous state where the lipids are packed in curved continuous bilayers extending in three dimensions and which are interpenetrated by communicating aqueous channels. Crystallization of incorporated proteins starts inside the lipid phase and growth is achieved by lateral diffusion of the protein molecules to the nucleation sites. This system has recently been used to obtain three-dimensional crystals 20 x 20 x 8 pm in size of the membrane protein bacteriorhodopsin, which diffracted to 2 A resolution using a microfocus beam at the European Synchrotron Radiation Facility. 

All biological membranes are constructed according to a standard pattern. They consist of a continuous bilayer of amphipathic lipids approximately 5 nm thick, into which proteins are embedded, in addition, some membranes also carry carbohydrates (mono- and oligosaccharides) on their exterior, which are bound to lipids and proteins. The proportions of lipids, proteins, and carbohydrates differ markedly depending on the type of cell and membrane (see p. 216). 

Figure 18-5 Schematic representation of (a) a membrane lipid, (b) a bilayer structure formed by lipid molecules in polar media the interior of the bilayer is nonpolar, and (c) a continuous bilayer structure (liposome) with polar interior and exterior...

There is a close resemblance between fatty-acid salts and phospholipids (p. 790) in that both possess long hydrocarbon tails and a polar head. Phospholipids also aggregate in a polar medium to form micelles and continuous bilayer structures such as shown in Figure 18-5. The bilayer lipid structure is very important to the self-sealing function of membranes and their impermeability to very polar molecules. 

In contrast, the transmembrane helices observed in the reaction center are embedded in a hydrophobic surrounding and are built up from continuous regions of predominantly hydrophobic amino acids. To span the lipid bilayer, a minimum of about 20 amino acids are required. In the photosynthetic reaction center these a helices each comprise about 25 to 30 residues, some of which extend outside the hydrophobic part of the membrane. From the amino acid sequences of the polypeptide chains, the regions that comprise the transmembrane helices can be predicted with reasonable confidence. 

Cell membrane The cell membrane is composed of about 45% lipid and 55% protein. The lipids form a bilayer that is a continuous nonpolar hydrophobic phase in which the proteins are embedded. The cell membrane is a highly selective permeability barrier that controls the entry of most substances into the cell. Important enzymes in the generation of cellular energy are located in the membrane. 

FIG. 14 A model for the uptake of weakly basic compounds into lipid bilayer membrane (inside acidic) in response to the pH difference. For compounds with appropriate pki values, a neutral outside pH results in a mixture of both the protonated form AH (membrane impermeable) and unprotonated form A (membrane permeable) of the compound. The unprotonated form diffuse across the membrane until the inside and outside concentrations are equal. Inside the membrane an acidic interior results in protonation of the neutral unprotonated form, thereby driving continued uptake of the compound. Depending on the quantity of the outside weak base and the buffering capacity of the inside compartment, essentially complete uptake can usually be accomplished. The ratio between inside and outside concentrations of the weakly basic compound at equilibrum should equal the residual pH gradient. 

We have encountered examples of simple lipid bilayers earlier. These bilayers are composed largely of amphipathic molecules. In water, they have their hydrophobic parts occupying the center of the bilayer and their hydrophilic parts occupying the bilayer surface. Such bilayers form a continuous and essential structural feature of virtually all biological membranes. We need to distinguish between that layer which faces out from the cell and is in contact with the external environment, the exoplasmic leaflet, and that which faces in and is in contact with the cellular contents, the cytoplasmic leaflet. As we shall see, these two aspects of the lipid bilayer are quite distinct. 

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