Section E - Membranes

Membranes form boundaries around the cell and around distinct subcellular compartments. They act as selectively permeable barriers and are involved in signaling processes. All membranes contain varying amounts of lipid and protein and some contain small amounts of carbohydrate. 

The fatty acid chains of glycerophospholipids and sphingolipids consist of long chains of carbon atoms which are usually unbranched and have an even number of carbon atoms (e.g. palmitate C16, stearate C18). The chains are either fully saturated with hydrogen atoms or have one or more unsaturated double bonds that are in the cis configuration (e.g. oleate Cl 8 1 with one double bond). 

Instant Notes in Biochemistry 2nd Edition, B.D. Hames N.M. Hooper, (c) 2000 BIOS Scientific Publishers Ltd, Oxford. 

The fluid mosaic model is now known to be correct for the structure of biological membranes, in which the membranes are considered as two-dimensional solutions of oriented lipids and globular proteins. 

Related topics Prokaryotes (Al) Eukaryotes (A2) Membrane protein and carbohydrate (E2) 

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Let us consider, for example, a flat symmetrical bilayer of which the area is large, so that end-effects can be ignored. Finite size effects are important, and will be discussed in the following section. The membrane is freely floating in solution, i.e. it is not supported by a frame. Combination of the first and second laws of thermodynamics gives for the difference of internal energy dl/of a bulk system with membranes with area A ... 

A schematic of a typical fuel-cell catalyst layer is shown in Figure 9, where the electrochemical reactions occur at the two-phase interface between the electrocatalyst (in the electronically conducting phase) and the electrolyte (i.e., membrane). Although a three-phase interface between gas, electrolyte, and electrocatalyst has been proposed as the reaction site, it is now not believed to be as plausible as the two-phase interface, with the gas species dissolved in the electrolyte. This idea is backed up by various experimental evidence, such as microscopy, and a detailed description is beyond the scope of this review. Experimental evidence also supports the picture in Figure 9 of an agglomerate-type structure where the electrocatalyst is supported on a carbon clump and is covered by a thin layer of membrane. Sometimes a layer of liquid water is assumed to exist on top of the membrane layer, and this is discussed in section 4.4.6. Figure 9 is an idealized picture, and... 

Both surfaces of the conventional membrane are quite similar in appearance, and the cross section is only slightly anisotropic with little difference in pore and cell size from one surface to the other (Figure 10). in contrast, a considerable difference is apparent between the pore sizes at opposite surfaces of the Tyrann-M/E membrane (Figure 11). The structure is highly anisotropic with an approximately five fold difference between the size of the pores at the two surfaces. Approximately... 

Figure 11. SEM photomicrographs of a 0.45fM Tyrann-M/E membrane (a) surface at 1 (b) surface at 2 (c) cross-section with boundary between coarse and fine...

Figure 12, SEM photomicrographs of cross-sections of Tyrann-M/E membranes (a) (b) 0.2 M (c) 0A5pM (d) 0,8fM.

A lipoprotein present in the periplasmic space of E. coli is anchored to the outer bacterial membrane by a triacylated modified N-terminal cysteine containing a glyceryl group in thioether linkage as shown in the following structure (see also Section E,l). 

Synthesis of ATP by mitochondria is inhibited by oligomycin, which binds to the OSCP subunit of ATP synthase. On the other hand, there are processes that require energy from electron transport and that are not inhibited by oligomycin. These energy-linked processes include the transport of many ions across the mitochondrial membrane (Section E) and reverse electron flow from succinate to NAD+ (Section C,2). Dinitrophenol and many other uncouplers block the reactions, but oligomycin has no effect. This fact can be rationalized by the Mitchell hypothesis if we assume that Ap can drive these processes. 

In membranes, the motional anisotropies in the lateral plane of the membrane are sufficiently different from diffusion in the transverse plane that the two are separately measured and reported [4b, 20d,e]. Membrane ffip-ffop and transmembrane diffusion of molecules and ions across the bilayer were considered in a previous section. The lateral motion of surfactants and additives inserted into the lipid bilayer can be characterized by the two-dimensional diffusion coefficient (/)/). Lateral diffusion of molecules in the bilayer membrane is often an obligatory step in membrane electron-transfer reactions, e.g., when both reactants are adsorbed at the interface, that can be rate-limiting [41]. Values of D/ have been determined for surfactant monomers and probe molecules dissolved in the membrane bilayer typical values are given in Table 2. In general, lateral diffusion coefficients of molecules in vesicle... 

Fig. 10.11 Illustration of the possible scale phenomena as they pertain to membrane transport (a) Cross section of a corrosion free membrane, (b) membrane with porous corrosion scale, (c) membrane with cracked corrosion scale, (d) membrane with spalling corrosion scale, (e) membrane with continuous, dense corrosion scale...

In this section we only discuss the ultimate gating event, i.e., membrane effects in forming and breaking the conductive GA channel. Our focus here is the elastic influences arising from perturbation of the membrane s thickness in contact with the insertion. 

Case 1 would represent, for example, the terminating sections (e.g., rectifying) of an entire membrane column configuration. Case 2, alternatively, would be a general MCS placed anywhere within the membrane column or cascade. 

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