Permeability coefficient cell wall

Next, we calculate the permeability coefficient for a solute that has a diffusion coefficient of 2 x 10-10 m2 s-1 for a cell wall. We will assume a representative value of 1 pm for the cell wall thickness. Using Equation 1.9 (Pj = DjKjlAx), we can thus estimate that Pj for the cell wall is... 

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 ... 

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 ... 

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... 

To reach the systemic circulation, a drug must move from the intestinal lumen through an unstirred water layer and mucous coat adjacent to the epithelial cell structure. Movement across the epithelial layers takes place by two independent routes, transcellular flux (i.e., movement across the cells) and paracellular flux or movement between adjacent epithelial cells. The solute molecules then encounter a basement membrane, interstitial space, and mesenteric capillary wall to access the mesenteric circulation. Any and all of these microenvironments can be considered a resistance to solute molecule movement, each with an associated permeability coefficient. Therefore, the overall process consists of a number of resistances (i.e., reciprocal of permeability) in series. Furthermore, the influence of drug structure with permeability in these different domains may be different. For example, permeability in an unstirred water layer is inversely related to solute size, whereas paracellular permeability Is a function of both size and charge. Furthermore, cations exhibit greater permeability than neutral species, which in turn manifest greater permeability than anions. 

High-intensity electric field pulses accelerated osmotic dehydration of carrot [18], A Fickian diffusion coefficient for water and solute increased exponentially with electric field strength. This effect was attributed to increased cell wall permeability, which was also manifested by the softening of product. 

When the network structure of a gel is heterogeneous and the gel consists of small cells with microspace, the diffusion of the solvent is restricted spatially and will not follow Eq. (19). Thus, the observed diffusion coefficient reduces over time. Tanner [7] and Meerwall [18] studied such a limited diffusion using a model with parallel blocking walls with distance a and permeability p. They derived the relationship between the spin echo intensity A t) when the pulsed magnetic gradient with... 

---