Membrane diffusion lateral

Lipids also undergo rapid lateral motion in membranes. A typical phospholipid can diffuse laterally in a membrane at a linear rate of several microns per second. At that rate, a phospholipid could travel from one end of a bacterial ceil to the other in less than a second or traverse a typical animal ceil in a few minutes. On the other hand, transverse movement of lipids (or proteins) from one face of the bilayer to the other is much slower (and much less likely). For example, it can take as long as several days for half the phospholipids in a bilayer vesicle to flip from one side of the bilayer to the other. 

Edidin, M., Zagyansky, Y., and Lardner, T. (1976). Measurement of membrane protein lateral diffusion in single cells. Science 191, 466—468. 

Internalization The toxins diffuse laterally in the membrane, bind to protein receptors and are internalized by receptor-mediated endocyto-sis. After this stage is completed, the toxins are protected against antibody inactivation. The internalized HC forms aggregates and induces pores in endosomal membrane. The pores serve as gates through which the LC enters a cytoplasm (Ledoux et al., 1994). 

Some local anaesthetics, such as benzocaine, are totally insoluble in water and cannot ionise. Consequently, there is no cation and therefore no Na+ channel block from within the cell. It is suggested that agents, such as benzocaine, which are very lipid-soluble, exert their effect in the phospholipid bilayer of the axon. This is the basis of the membrane expansion theory of local anaesthetic action. It is also possible that they diffuse laterally form the bilayer into the Na+ channel without ever accessing the axoplasm and in effect produce another variety of Na+ channel block. Repetitive depolarisation of a nerve recruits more Na+ channels and maintains them in the open state for a longer period than normal. 

Fig. 4. Schematic representation of transient method employed by Devaux and McConnell9 to measure the rates of lateral diffusion of phospholipids in model membranes. The upper diagram represents a concentrated patch of labels at the beginning of the experiment, time f = 0. At later times f>0, the molecules diffuse laterally, as shown in the lower two drawings. The paramagnetic resonance spectra depend on the spin-label concentration in the plane of the membrane, and an analysis of the time dependence of these spectra yielded the diffusion constant. [Reprinted with permission from P. Devaux and H. M. McConnell, J. Am. Chem. Soc., 94, 4475 (1972). Copyright by American Chemical Society.]...

Individual lipid molecules can move laterally in the plane of the membrane by changing places with neighboring lipid molecules (Fig. 11-16c). A molecule in one mono-layer, or leaflet, of the lipid bilayer—the outer leaflet of the eiythrocyte plasma membrane, for example—can diffuse laterally so fast that it circumnavigates the erythrocyte in seconds. This rapid lateral diffusion within the plane of the bilayer tends to randomize the positions of individual molecules in a few seconds. 

Many membrane proteins seem to be afloat in a sea of lipids. Like membrane lipids, these proteins are free to diffuse laterally in the plane of the bilayer and are in... 

Lipids and proteins can diffuse laterally within the plane of the membrane, but this mobility is limited by interactions of membrane proteins with internal cytoskeletal structures and interactions of lipids with lipid rafts. One class of lipid rafts consists of sphingolipids and cholesterol with a subset of membrane proteins that are GPI-linked or attached to several long-chain fatty acyl moieties. 

The diacylglycerols released by phospholipase C diffuse laterally through the bilayer and, together with the incoming Ca2+, activate protein kinases C. These kinases also require phosphatidylserine for their activity and phosphorylate serine and threonine side chains in a variety of proteins.329 330b They are stimulated by the released unsaturated diacylglycerols. In addition protein kinases C can be activated by phorbol esters, which are the best known tumor promoters (Box 11-D). The diacylglycerol requirement favors a function for these protein kinases in membranes. They also appear to cooperate with calmodulin to activate the Ca2+-dependent contraction of smooth muscle.330... 

The fluid-mosaic model for biological membranes as envisioned by Singer and Nicolson. Integral membrane proteins are embedded in the lipid bilayer peripheral proteins are attached more loosely to protruding regions of the integral proteins. The proteins are free to diffuse laterally or to rotate about an axis perpendicular to the plane of the membrane. For further information, see S. J. Singer and G. L. Nicolson, The fluid mosaic model of the structure of cell membranes, Science 175 720, 1972. 

Phospholipid molecules in the plasma membrane diffuse rapidly enough to go from one end of an average-sized animal cell to the other in a few minutes. In a bacterial cell, such a trip would take only a few seconds. Integral membrane proteins move more slowly than phospholipids, as we expect in view of their greater mass. Diffusion of membrane proteins plays essential roles in many biochemical processes, including the cellular uptake of lipoproteins (chapter 18), responses of cells to hormones (chapter 24), immunological reactions (supplement 3), vision (supplement 2), and the transport of nutrients and ions. As we see in a later section, however, some membrane proteins cannot move about rapidly because they are attached to cytoskeletal scaffolds. 

Although phospholipids diffuse laterally in the plane of the bilayer and rotate more or less freely about an axis perpendicular to this plane, movements from one side of the bilayer to the other are a different matter. Diffusion across the membrane, a transverse, or flip-flop, motion, requires getting the polar head-group of the phospholipid through the... 

In the bilayer membrane model of the 1980s, cell membranes were based largely on a fluid lipid bilayer in which proteins were embedded [149,150]. The bilayer was highly dynamic lipids and proteins could flex, rotate, and diffuse laterally in a two-dimensional fluid. Based on this, the enhancing mechanisms of absorption enhancers on transcellular routes have been clarified. In summary, most of the mechanisms are strongly associated with membrane fluidity. The fluidity is likely to be changed by the following factors. 

FIGURE 6.2. (a) An IgG antibody approaches a phospholipid membrane containing fluid ligands, (b) The species first binds to one ligand and (c) then diffuses laterally to bind to a second ligand. 

If the model proposed by Andersson and Anderson [109] of total separation of PS I and PS II in the granal chloroplasts were to be accepted, electron transport from the PS II acceptors to P-700 would require a mobile electron carrier(s) which should diffuse laterally in the membrane fast enough to account for the observed electron transport rate. Plastoquinone [112] and plastocyanin are the candidates of choice for this role. The former has been shown to be present at approximately the same activity in the partitions and in the stroma-exposed membranes [43], while PC is known to be located in the intrathylakoid space [113],... 

Cell membranes are two-dimensional fluids that exhibit a wide range of dynamic behaviors. Recent technical advances have enabled unprecedented views of membrane dynamics in living cells. In this technical review, we provide a brief overview of three well-studied examples of membrane dynamics lateral diffusion of proteins and lipids in the plane of the membrane, vesicular trafficking between intracellular compartments, and exchange of proteins on and off membranes. We then discuss experimental approaches to monitor membrane protein and lipid dynamics, and we place a special emphasis on the use of genetically encoded fluorescent probes and live cell-imaging techniques. 

Membranes are two-dimensional flnids whose protein and lipid components continnonsly exchange positions because of Brownian motion, a process commoifly referred to as lateral diffusion. Lateral diffnsion enables proteins and lipids to explore their environment, which enconrages interactions between molecules. Thus, the speed of lateral diffusion is one of the limiting factors... 

On the basis of the dynamic properties of proteins in membranes, S. Jonathan Singer and Garth Nicolson proposed the concept of a fluid mosaic model for the overall organization of biological membranes in 1972 (Figure 1230). The essence of their model is that membranes are two-dimensional solutions of oriented lipids and globular proteins. The lipid bilayer has a dual role it is both a solvent for integral membrane proteins and a permeability barrier. Membrane proteins are free to diffuse laterally in the lipid matrix unless restricted by special interactions. 

The respiratory complexes diffuse laterally in the membrane with diffusion coefficients of 8 X 10 °-2 X 10 cm s [81-83]. Cytochrome c and ubiquinone have been quoted to diffuse at a velocity (10 cm s ) comparable to that of phospholipid molecules [84,85]. However, the bulky isoprenoid side chain of Q may slow down its mobility [86] in chromatophores, which may be compared with mitochondria, a mobility of 10 cm s has been estimated [87]. Overfield and... 

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