Membrane local concentration

Membranes separate one part of the cell from the other. Proteins and other molecules can be localized in the membrane. Membrane localization concentrates the molecules and makes it easier for them to find each other (two-dimensional diffusion) than it is for two molecules in solution (three-dimensional diffusion). Because most molecules can t pass through the membrane by themselves, the cell machinery can create con-... 

Zi(Air, x) and 7)(N2, x) are spin-lattice relaxation times of nitroxides in samples equilibrated with atmospheric air and nitrogen, respectively. Note that W(x) is normalized to the sample equilibrated with the atmospheric air. W(x) is proportional to the product of the local translational diffusion coefficient D(x) and the local concentration C(x) of oxygen at a depth x in the membrane, which is in equilibrium with the atmospheric air ... 

The two terms in the final expression of Equation 9.11 can compete against one another and with only a small membrane potential the concentration gradient chemical potential can be overcome. Electrochemical energy is thus harnessed to allow nutrients to flow into a cell and increase the local concentration. 

Within seconds of an ischemic insult, normal brain electrical activity ceases, as a result of the activation of membrane K+ channels and widespread neuronal hyperpolarization [1]. The hyperpolarization may be due to opening of K+ channels responding to acute changes in local concentrations of ATP, H+ or Ca2+, or it may reflect altered nonheme metalloprotein association with and regulation of specific K+ channels [2]. This response, presumably protective, however fails to preserve high-energy phosphate levels in tissue as concentrations of phospho-creatine (PCr) and ATP fall within minutes after ischemia... 

The third feature is the recruitment of a molecule, in this case Sos, to the membrane in order to fulfill its function. Both increased local concentrations at the membrane and two-dimensional diffusion of the collision partners, Sos and Ras, lead to enhanced encounter of the two proteins and thus to accelerated nucleotide exchange on Ras. 

Phosphate ions are constituent parts of two universally found biopolymers, DNA and RNA. Phosphate ion is found in membrane lipids (phospholipids) and associated with the metabolism of many small molecules. The binding of dioxygen by hemoglobin is regulated by local concentrations of H+ (known as the Bohr effect), CO2 concentration, and organic phosphates such as diphos-phoglycerate (DPG), whose structure is shown in Figure 5.1. ... 

Differential effects of the isoforms of a particular factor could be the consequence of receptor-mediated signaling. Proliferation leading to selfrenewal could depend on the signaling intensity through the factor/receptor complex (Zandstra et al., 2000). In this context, membrane-bound factors mediating cell-cell interactions may substitute for high local concentrations of a soluble factor resulting in delayed internalization of the factor/receptor complex... 

In the previous section, the role of solvent extraction was limited to preparing the analyte for subsequent analysis. A large majority of procedures that use solvent extraction in chemical analysis are used in this fashion. However, the extraction itself, or rather the distribution ratio characterizing it, may provide an appropriate measured signal for analysis. Examples of this use of solvent extraction are found in spectroscopy, isotope dilution radiometry, and ion-selective electrodes using liquid membranes. In the latter case, electrochemical determinations are possible by controlling the local concentration of specific ions in a solution by extraction. 

First of all, the behavior of the enzymes in the membrane differs markedly from the behavior of the unbound enzymes in solution. It is pertinent to note that the medium in which the enzyme bound to a membrane acts might be determined not only by the composition and structure of the membrane itself, but also by the local concentration distribution of substrate and products. The microenvironment in the membranes is the result of a balance between the flow of matter and enzyme reactions. The substrate and product concentrations in the membrane differ from point to point across the membrane and also from those at the outer solution. By electron microscopy this was experimentally demonstrated beyond doubt with the DAB-peroxidase system by Barbotin and Thomas.16 The effects of these profiles were studied with... 

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

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