Permeability coefficient plasma membrane

Most of the permeability coefficients for small solutes crossing the plasma membrane range from 10-10 to 10-6 m s-1. Hence, a cell wall generally has a higher permeability coefficient than does a membrane, which means that the cell wall is usually more permeable for small solutes than is the plasma membrane. For comparison, let us consider a permeability coefficient appropriate for an unstirred liquid layer adjacent to a cell wall or membrane. Specifically, Dj for a small solute may be 1 x 10-9 m2 s-1 in water, Kj is 1 in the aqueous solution, and let us assume that Ax is 30 pm for the unstirred... 

C. Assuming that the partition coefficient for CO2 is 100 times greater in the cell wall than in the plasma membrane, in which barrier is the permeability coefficient larger, and by how much ... 

Consider a solute having a permeability coefficient of 1CT6 ms-1 for the plasma membrane of a cylindrical Chara cell that is 100 mm long and 1 mm in diameter. Assume that its concentration remains essentially uniform within the cell. 

As discussed previously, the different ionic concentrations on the two sides of a membrane help set up the passive ionic fluxes creating the diffusion potential. However, the actual contribution of a particular ionic species to Em also depends on the ease with which that ion crosses the membrane, namely, on its permeability coefficient. Based on the relative permeabilities and concentrations, the major contribution to the electrical potential difference across the plasma membrane of N. translucens comes from the K+ flux, with Na+ and Cl- fluxes playing secondary roles. If the Cl- terms are omitted from Equation 3.20 (i.e., if Pci is set equal to zero), the calculated membrane potential is -154 mV, compared with -140 mV when Cl- is included. This relatively small difference between the two potentials is a reflection of the relatively low permeability coefficient for chloride crossing the plasma membrane of N. translucens, so the Cl- flux has less effect on Em than does the K+ flux. The relatively high permeability and the high concentration of K+ ensure that it... 

The resistance to diffusion of a molecular species across a barrier equals the reciprocal of its permeability coefficient (Chapter 1, Section 1.4B). In this regard, we will let f COi be the permeability coefficient for CO2 diffusion across barrier j. To express the resistance of a particular mesophyll or chlo-roplast component on a leaf area basis, we must also incorporate Am sIA to allow for the actual area available for diffusion—the large internal leaf area acts like more pathways in parallel and thus reduces the effective resistance (Fig. 8-4). Because the area of the plasma membrane is about the same as that of the cell wall, and the chloroplasts generally occupy a single layer around the periphery of the cytosol (Figs. 1-1 and 8-11), the factor AmesIA applies to all of the diffusion steps of CO2 in mesophyll cells (all five individual resistances in Eq. 8.21). In other words, we are imagining for simplicity that the cell wall, the plasma membrane, the cytosol, and the chloroplasts are all in layers having essentially equal areas (Fig. 8-11). Thus, the resistance of any of the mesophyll or chloroplast components for CO2 diffusion,, is reduced from 1 /P co, by the reciprocal of the same factor, Ames/A ... 

We next examine rj, the resistance of the plasma membrane of mesophyll cells to the diffusion of the various forms of CO2. Although we do not know the actual permeability coefficient of the plasma membrane of mesophyll... 

Based on the relative values for the two permeability coefficients, the resistance to the diffusion of HC03 across the plasma membrane is about 5 x 104 times higher than that for C02, for example, 5 x 106 s m-1. Because of the extremely high resistance for HCO3-, we conclude that bicarbonate does not diffuse across the plasma membrane at a rate necessary to sustain photosynthesis. The plasma membrane resistance calculated for CO2 (100 s m-1) is relatively small (Table 8-4), which suggests that diffusion of CO2 is adequate for moving this substrate of photosynthesis across the plasma membrane. 

There are many assumptions and parameter choices involved in the calculation of rg. For instance, we let Ames/A be 20, whereas many leaves have values from 30 to 40 the latter ratios would reduce rj c to 70 to 90 sm 1. The cell walls of some mesophyll cells are only 0.07 pm thick, which would decrease rg to less than 10sm-1. Permeability coefficients of the plasma membrane of mesophyll cells for CO2 have not been adequately measured. In this regard, Pj is equal to DjK/Ax (Eq. 1.9), where the diffusion coefficients of H20 and CO2 in the plasma membrane are probably about the same (within a factor of 2 of each other), the partition coefficient for CO2 is most likely at least 10 times higher than A 0, and Axpm is the same for H20 and C02- Because Pg] can be 10-4 m s-1 (see Chapter 1, Section 1.4B), our assumed value of 5 x 10-4 m s-1 for Pg may be to° low—we noted in Chapter 1 (Section 1.4B) that Pj for another small molecule, O2, crossing erythrocyte membranes can have an extremely high value of 0.3 ms"1. A higher value for Pg will decrease our estimate for rg and thus for rgcy... 

Experimentally, it has been observed that many substances are transported across plasma membranes by more complicated mechanisms. Although no energy is expended by the cell and the net flux is still determined by the electrochemical potential, some substances are transported at a rate faster than predicted by their permeability coefficients. The transport of these substances is characterized by a saturable kinetic mechanism the rate of transport is not linearly proportional to the concentration gradient. A facilitated mechanism has been proposed for these systems. Substances interact and bind with cellular proteins, which facilitate transport across the membrane by forming a channel or carrier. The two basic models of facilitated diffusion, a charmel or a carrier, can be experimentally distinguished (1,2). 

Stancell et. al. ( 0) reported the possible use of ultrathin films deposited onto relatively permeable substrates as permselective membranes. Ultrathin and highly crosslinked coatings effectively distinguish between molecules of different sizes and increase the permselectivity of the substrate film. Chang et. al. ( ) demonstrated that the permeability coefficient of silicone rubber to oxygen decreased noticeably after depositing a plasma-polymerized ethylene film on the surface. Colter, et. al. (92.93) found similar effects of plasma polymerized films as diffusion barriers in controlled-released drug delivery systems. 

With respect to transcellular permeability, the relationship of solute structure with permeability depends on the mechanism. Historically, a passive diffusion pathway is assumed for most solutes. Nevertheless, a great number of solutes are identified as being associated with active absorption and secretary processes in intestinal epithelial cells. Additionally, although active transport involves specific interactions between a solute and transporter, passive diffusion is dependent on solute partitioning into the cellular plasma membrane and the diffusion coefficient within the membrane. 

In the case of our plasma-modified membrane where amine groups were only grafted on the surface, CO2 facihtated transport cannot occur through the membranes. Instead, the high interfacial concentration in CO2 served as entrance concentration for the classical diffusion through the umnodified Pebax 1657 polymer, leading to an overall improvement of the CO2 permeability coefficient. 

During CAVH/CVVH, drug removal primarily occurs via con-vection/ultraflltration (the passive transport of drug molecules at the concentration at which they exist in plasma water into the ultrafiltrate). The clearance of a drug by either of these methods is thus a function of the membrane permeability for the drug, which is called the sieving coefficient (SC) and the rate of ultrafiltrate formation (UFR). The SC can be calculated as ... 

In the case of the PDMS gas, the membrane permeability of CO2 decreased, but the selectivity of CO2 over CH4 was found to be remarkably improved irrespective of the plasma gas used (NH3, Ar, Nj, O2). The nitrogen plasma treatment seemed to give better selectivity than the ammonia plasma (Matsuyama et al. 1995). The NH3 and N2 plasma treatment of the dense PE (Nakata and Kumazawa 2006) and PP (Teramae and Kumazawa 2007) membranes increased both the permeation coefficient for CO2 and the ideal separation factor for CO2 relative to N2. The effects of both plasma gases are very similar. 

Polybutadiene/polycarbonate membranes with a pp-ethylenediamine layer had an increased gas permeability (in comparison with the unmodified one) due to surface etching. Their selectivity was closely connected with the chemical composition of the top layer. A high nitrogen content was required for high O2 selectivity (Ruaan et al. 1998). The presence of the amine groups on the membrane surface also enhanced the capacity for CO2/CH4 separation. The plasma-polymerized diisopropylamine on the surface of the composite membrane—porous polyimide (support)/ silicone (skin)— made the separation coefficient as high as 17 for a permeation rate of 4.5 X cmVcm sec cmHg (Matsuyama et al. 1994). 

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