Lipid bilayer, of the plasma membrane

A family of related, membrane-spanning glycoproteins that catalyze the transport of glucose across a lipid bilayer of the plasma membrane along a concentration gradient. 

Five basic mechanisms of transmembrane signaling are well understood (Figure 2-5). Each uses a different strategy to circumvent the barrier posed by the lipid bilayer of the plasma membrane. 

Another possibility is that certain drugs, of appropriate partition coefficients, would preferentially remain within the lipid bilayer of the plasma membrane, rather than partitioning out into the cell cytoplasm. Such moieties could thus diffuse along the lipid bilayer of the membrane, down the side of the cell (rather than through it), emerging finally at the basolateral surface of the cell. However this scenario is limited by the fact that the lipid membrane constitutes a minute proportion of the available surface area of the cell also cell junctions can act as diffusion barriers within the lipid bilayer of the plasma membrane. 

Boron enters plant roots as undissociated boric acid. Significant differences in boron uptake occur, even when plants are grown under identical conditions. Recent findings indicate that the lipid composition of the plasma membrane can affect total boron uptake by a plant. Evidence also has been obtained indicating that boron enters plant cells in part by passive diffusion through the lipid bilayer of the plasma membrane... 

Many drug carriers are made of hydrophobic materials such as lipids and poly(butyl cyanoacrylate). It will be thermodynamically unstable for submicron particles made of these materials to remain dispersed in an aqueous environment such as blood circulation. Surfactants or block co-polymers are therefore routinely included in these formulations to prevent particle aggregation. Studies showed that a number of these agents, most noticeably the nonionic surfactants such as polysorbates (also known as Tweens) and Tritons and block co-polymers such as poloxamers (also known as Pluronics), may inhibit the ABC transporters [97-99]. As previously discussed, ABC transporters interact with their substrates in the lipid bilayers of the plasma membrane. Surfactants can disrupt the arrangement of the lipid bilayer expressing the transporters and subsequently inhibit their drug efflux activities [97, 100]. It... 

NCB-20 cells. A schematic representation of these proteins within the lipid bilayer of the plasma membrane is shown in Figure 8.27. The arrowed solid lines represent the normal points of interaction between the proteins. Activation of the catalytic subunit E may be effected, in the absence of a specific agonist for surface receptors (R), by stimulation of the G-protein complex with fluoride ions. F ions probably produce their effect by facilitating dissociation of from G py, and G is then free to complex with E. This association is an essential step in the activation of adenylate cyclase. The possibility that desensitization of PGI2 receptors of NCB-20 cells might be... 

With the above-mentioned background, it is informative to describe an application of the N- P equation as applied to the nerve axon, which is surrounded by a plasma membrane. One of the main functions of the plasma membrane is to control the passage of ions and molecules into and out of the cell. For most biomembranes, the intracellular [K+]i greatly exceeds extracellular [K+]o, and the opposite is true for the extracellular [Na+]o and [Cl ]o. These concentration differences are due to the active transport system embedded in the lipid bilayer of the plasma membrane [3]. 

Figure 3.6a shows that the light scattering observed from a solution of 44 in Tris buffer increased significantly at relatively low eoncentrations of 44, confirming the molecule s propensity to self aggregate in solution. This was also the case for 43 (Fig. 3.6a). Analysis of this data showed the CMC of 44 in solution to be 40.1 pM, and that of 43 to be 19.0 pM. These values are comparable to phosphatidylcholine molecules with fatty acid chains of 9-carbon length (CMC of 09 0 PC = 0.029 mM) [9], This behaviour strengthens the argument that 44 and 43 are structurally similar to phospholipids and so would reside comfortably within the lipid bilayer of the plasma membrane.

All phosphoinositides are found in the cytosolic half of the lipid bilayer of the plasma or intracellular compartment membranes (left part). The different kinases acting on phosphoinositides in mammalian cells are shown in solid lines and the phosphoinositide 3-kinases, in bold. The phosphoinositides counterpart pathways catalysed by known phosphatases are represented by dashed lines. The best known phosphatases are PTEN (Phosphatase and tensin homolog deleted on chromosome 10) and SHIP (SH2 domain-containing inositol 5-phosphatase). 

The Cell Surface Significant amounts of carbohydrate are attached to membrane protein and lipids on the external surface of cells. (Colored balls = sugar residues 6PI anchor = a complex lipid molecule that connects many cell surface proteins to the phospholipid bilayer of the plasma membrane)... 

Cholesterol does not form micelles because it is not sufficiently amphiphilic (even though it does have an —OH group) and its flat, rigid, fused-ring structure gives a solid rather than a liquid hydrocarbon phase at physiological temperatures. Such fluidity is required for micelle formation. However, cholesterol can form mixed micelles with amphiphilic lipids, and it enters monolayers as well where it constitutes -25% of the mass of the lipid bilayer in the plasma membranes of mammalian cells. 

Mueller, Rudin, Tien, and Wescott in 1961, at the Symposium of the Plasma Membrane [531] described for the first time how to reconstitute a lipid bilayer... 

The lipid bilayer arrangement of the plasma membrane renders it selectively permeable. Uncharged or nonpolar molecules, such as oxygen, carbon dioxide, and fatty acids, are lipid soluble and may permeate through the membrane quite readily. Charged or polar molecules, such as glucose, proteins, and ions, are water soluble and impermeable, unable to cross the membrane unassisted. These substances require protein channels or carrier molecules to enter or leave the cell. 

Ras is strictly localized to the inner side of the plasma membrane. A lipid anchor covalently attached to the C-terminus of Ras penetrates into the lipid bilayer. This membrane anchorage is essential for the biological activity of Ras. Hence, the inhibition of anchor attachment has become an attractive pharmacological target [ 13]. See Waldmann H, Thutewohl M,Ras-Farnesyltransferase-inhibitors as promising anti-tumor drugs, this volume. 

Several glycoproteins, which are present in the lipid bilayer of the virus, are necessary for infection. One is known as GP120. It binds to the CD4 protein on the surface of the Th lymphocyte (i.e. the CD4-I- ceU). This initiates fusion with the plasma membrane of the CD4-I- cell so that the viral RNA and its proteins enter the cell (i.e. it infects the CD4-t cell). The original infection probably occurs in the peripheral circulation but the lymphocytes will be transported by the blood to the spleen, other lymph nodes and the brain, where the microglia become infected (Figure 17.46). 

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