Membrane diffusion lipid

Unlike other Eukarya, animal cells lack cell walls, though they tend to be surrounded by a highly developed glycocalyx of up to 140 nm in thickness [108]. This diffuse layer of densely packed oligosaccharides has a heterogeneous composition and is connected to the membrane via lipids or integral proteins. The boundary of the cell usually extends beyond the mere lipid bilayer with its embedded proteins, and the extracellular structures provide initial sites of interaction or are themselves targets for MAPs such as antimicrobial peptides [115]. 

Simple diffusion is another mechanism by which substances cross membranes without the active participation of components in the membranes. Generally, lipid-soluble substances employ this method to enter cells. Both simple diffusion and filtration are dominant factors in most drug absorption, distribution, and elimination. 

It is recognized that filtration is operational, that colloidal-bound PCB congeners are not retained by the filter, and that operational dissolved measurements may be biased positively by colloidal material. Techniques to measure truly dissolved PCBs include gas sparging, differential diffusion into membrane-bound lipids (e.g., semipermeable membrane devices, [230]), and selective adsorption (e.g. non-equilibrium solid phase microextraction [231, 232]). Unfortunately, none of these techniques has sufficient sensitivity to reliably and unambiguously measure truly dissolved PCB congeners at the levels present in the Great Lakes. 

Trauble [193] made an interesting attempt to take into account the influence of the membrane molecular structure on the transmembrane transfer of small molecules. The transmembrane motion of these molecules was considered under the assumption that the thermal motion of the hydrocarbon chains of the membrane lipid leads to the appearance of the mobile structural defects (so called kinks ) in the membrane. The kinks are small free volumes in the hydrocarbon phase of the membrane diffusing in the membrane. The molecules from the aqueous phase may be captured by these kinks and, moving together with them, transferred to the other side of the membrane. Such a model was shown to describe satisfactory the translocation of small neutral molecules such as water. 

In the 1970s, the fluid mosaic concept emerged as the most plausible model to account for the known structure and properties of biological membranes [41]. The fact that membranes exist as two-dimensional fluids (liquid disordered) rather than in a gel state (solid ordered) was clearly demonstrated by Frye and Ededin [42], who showed that the lipid and protein components of two separate membranes diffuse into each other when two different cells were fused. Since that time, numerous studies have measured the diffusion coefficient of lipids and proteins in membranes, and the diffusion rates were found to correspond to those expected of a fluid with the viscosity of olive oil rather than a gel phase resembling wax. 

The rate of diffusion of a topically applied toxicant across the rate-limiting stratum corneum is directly proportional to the concentration gradient across the membrane, the lipid/water partition coefficient of the drug, and the diffusion coefficient for the compound being studied. This can be summarized by Fick s law of diffusion in the equation... 

Passive Transport. Transport by simple diffusion This mode of transport is available for apolar molecules. Permeation is predominantly governed by partitioning of the substrate between the lipid and water. The membrane simply acts as a permeability barrier small molecules pass more easily than large ones. The transport is explained in terms of a simple diffusion model involving three steps passage of the substrate from the exterior into the membrane, diffusion through the membrane, and passage out of the membrane. 

Tocanne J-F, Dupou-Cezanne L, Lopez A. Lateral diffusion of lipids in model and natural membranes. Progr. Lipid Res. 1994 33 203-237. 

Cold sensitivity. Some antibiotics act as carriers that bind an ion on one side of a membrane, diffuse through the membrane, and release the ion on the other side. The conductance of a lipid-bilayer membrane containing a carrier antibiotic decreased abruptly when the temperature was lowered from 40°C to 36°C. In contrast, there was little change in conductance of the same bilayer membrane when it contained a channel-forming antibiotic. Why ... 

Superoxide dismutase and catalase are remarkably efficient, performing their reactions at or near the diffusion-limited rate (Section 8.4.2). Other cellular defenses against oxidative damage include the antioxidant vitamins, vitamins E and C. Because it is lipophilic, vitamin E is especially useful in protechng membranes from lipid peroxidahon. 

Orally administered corticosteroids are effective in the treatment of chronic bronchial asthma. The inhalation route has been widely used in attempts to avoid systemic side-effects, such as adrenal suppression, but evidence suggests that inhaled steroids are absorbed systemically to a significant extent. The respiratory tract epithelium has permeability characteristics similar to those of the classical biological membrane, so lipid-soluble compounds are absorbed more rapidly than lipid-insoluble molecules. Cortisone, hydrocortisone and dexamethasone are absorbed rapidly by a nonsaturable diffusion process from the lung, the half-time of absorption being of the order of 1-1.7 min. Quaternary ammonium compounds, hippurates and mannitol have absorption half-times, in contrast, of between 45 and 70 min. 

In 1972 Singer and Nicolson first proposed he fluid mosaic model of membrane structure (Fig. 3-26). The membrane exists as a two-dimensional fluid of freely diffusing lipids, dotted or embedded with proteins that may function as chaimels, or transporters of solutes across the membrane, as linkages to the cytoskeleton, or as receptors. Some membrane proteins perform two of more of these functions. 

---