Cell Free diffusion

In systems comprised of cells in culture, there is no formal architecture (such as might be encountered in a whole tissue) that would hinder free diffusion. Such... 

The in vitro system we have been using to study the transepithelial transport is cultured Madin-Darby canine kidney (MDCK) epithelial cells (11). When cultured on microporous polycarbonate filters (Transwell, Costar, Cambridge, MA), MDCK cells will develop into monolayers mimicking the mucosal epithelium (11). When these cells reach confluence, tight junctions will be established between the cells, and free diffusion of solutes across the cell monolayer will be markedly inhibited. Tight junction formation can be monitored by measuring the transepithelial electrical resistance (TEER) across the cell monolayers. In Figure 1, MDCK cells were seeded at 2 X 104 cells per well in Transwells (0.4 p pore size) as described previously. TEER and 14C-sucrose transport were measured daily. To determine 14C-sucrose... 

Poison cell (free boundary method) Diffusion coefficient determination 3... 

We emphasize that any utilization of Eq. (5) already rest upon a number of (often reasonable) assumptions. Equation (5) represents an ordinary deterministic differential equation, based on assumption of homogeneity, free diffusion, and random collision, and neglecting spatial [102] or stochastic effects [103]. While such assumptions are often vindicated for microorganisms, the application of Eq. (5) to other cell types, such as human or plant cells, sometimes mandates careful verification. 

An important function of the secreted H"" ions is to promote HCOs resorption (top right). Hydrogen carbonate, the most important buffering base in the blood, passes into the primary urine quantitatively, like all ions. In the primary urine, HCOa reacts with H"" ions to form water and CO2, which returns by free diffusion to the tubule cells and from there into the blood. In this way, the kidneys also influence the C02/HC03 buffering balance in the plasma. 

I wish to ask about the concept of a unit of length in a biological cell. My concern is with the numerical value to be used for the diffusion coefficient. Most of biological space is heavily organized even in a single cell, and therefore a diffusion constant is not a simple property. To put my question simply, it is easiest, however, to consider enzyme sites that are otherwise identical in two situations (1) with water between them, (2) with a biological membrane between them. Is it not the case that the unit of length is quite different, for the diffusion in (1) is virtually free diffusion whereas in (2) the diffusion is constrained most probably as a series of activated hops ... 

Tight junctions between endothelial cells of blood capillaries in the brain prevent free diffusion of compounds from the blood stream into brain cells and form the blood-brain barrier.140 Tight junctions between neurons and adjacent cells surround the nodes of Ranvier (Chapter 30). 

Fluorescence microphotolysis, or photobleaching, has been widely used to study translational mobility of lipids and proteins in membranes. An attenuated laser beam may be focused down to the diameter of a cell or less. Then the intensity can be suddenly increased by several orders of magnitude, bleaching any fluorescent material present. The return of fluorescent material by free diffusion from a neighboring region (fluorescence recovery after photobleaching) or by diffusion through a membrane into a cell can then be... 

Development in Drosophila and other insects follows a somewhat different pathway, as is indicated in Fig. 32-6B. The egg, which is surrounded by follicle cells and 16 nurse cells, does not divide. However, its nucleus divides repeatedly, about once every nine minutes, to form -6000 nuclei. Only then do separating membranes form to give individual cells.9,285 316 317 During the first two hours the nuclei form a syncytium, in which they are embedded in a common cytoplasm, that allows free diffusion of signaling compounds. At first the nuclei are in the center, but later most migrate to the periphery and form a single layer of cells comparable to the blastoderm of amphibian cells. A few nuclei remain in the central cavity to become yolk cells, and some at the posterior pole become separated into pole cells. 

Cory and Garroway [13] introduced the NMR pulsed gradient stimulated echo method to study compartments which are too small to be observed by conventional NMR imaging. They showed so-called proton displacement profiles of bulk water and dimethyl sulfoxide. The displacements are due to free diffusion and are Gaussian shaped. The profile of water in yeast cells showed restricted diffusion with a characteristic cell width of approximately 5 /xm. 

Hannoun, B. J. M. and G. Stephanopoulos, "Diffusion Coefficients of Glucose and Ethanol in Cell-free and Cell-occupied Calcium Algmate Membranes," Biotech. Bioeng. 28 (1986) 829 - 835. 

The BBBs to macromolecules and most polar solutes are created by the formation of tight junctions between the cerebral endothelial cells, the choroid plexus epithelial cells, and the cells of the arachnoid membrane. These tight junctions are a key feature of the BBB and effectively abolish any aqueous paracellular diffusional pathways between the endothelial cells from the blood plasma, or somatic ECF, to the brain ECF. This removal of the paracellular pathway efficiently prevents the free diffusion of polar solutes from blood to brain [11],... 

It is the presence of these tight junctions that occludes the aqueous paracellular diffusional pathway between the endothelial cells, and blocks the free diffusion of macromolecules, polar solutes, and ions from blood plasma into the ECF of the brain. It is this impediment to the movement of ions that results in the high in vivo electrical resistance of the BBB, of approximately 1800 fl cm2 [17]. This high electrical resistance or low conductance of the potential paracellular pathway emphasizes the extreme effectiveness of the tight junctions in occluding this pathway by effectively reducing the movement of ions. The radius of a sodium... 

Whether the components of the gene carriers actually remain associated during import into the nucleus or enter individually cannot be answered by optical methods as their resolution is limited. A possible technique to study the complexation of DNA within cells is fluorescence correlation spectroscopy (FCS). Clamme et al. studied the intracellular fate of PEI after transfection with polyplexes by two-photon fluorescence FCS [54]. They showed that PEI binds to the inner membrane of endosomes and lysosomes and shows free diffusion in the cytosol as well as the nucleus. However, they did not detect any PEI/DNA complexes inside the nucleus. 

Free diffusion columns are arranged to be sufficiently long for the initial concentrations at the extreme ends of the cell to remain unaltered during the course of the experiment. For a monodispersed system under these conditions the concentration gradient curves (Figure 2.4b) can be shown, by solving Fick s equations, to take the shape of Gaussian distribution curves represented by the expression... 

A model of thyroid hormone action is depicted in Figure 38 1, which shows the free forms of thyroid hormones, T4 and T3, dissociated from thyroid-binding proteins, entering the cell by diffusion or possibly by active transport. Within the cell, T4 is converted to T3 by 5 -deiodinase, and the T3 enters the nucleus, where T3 binds to a specific T3 receptor protein, a member of the c-erb oncogene family, which also includes the steroid hormone receptors and receptors for vitamins A and D. The T3 receptor exists in two forms, c and 3. Differing concentrations of receptor forms in different tissues may account for variations in T3 effect on different tissues. 

To study the function of the chimeric cefE enzymes generated by recombination in S. lividans, the cell-free ring expansion abilities of several recombinants were determined. Three white strains, W21 (6.3-kb plasmid), W25 (8-kb) and W76 (8-kb) as well as the controls Bll and B18 (melanin-positive pJA680) showed activity, producing inhibition zones in the agar diffusion bioassay. Although S. 

The simplest of these functions is that of a permeability barrier that limits free diffusion of solutes between the cytoplasm and external environment. Although such barriers are essential for cellular life to exist, there must also be a mechanism by which selective permeation allows specific solutes to cross the membrane. In contemporary cells, such processes are carried out by transmembrane proteins that act as channels and transporters. Examples include the proteins that facilitate the transport of glucose and amino acids into the cell, channels that allow potassium and sodium ions to permeate the membrane, and active transport of ions by enzymes that use ATP as an energy source. 

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