Transport through a membrane

The voltammetry for ion transfer at an interface of two immiscible electrolyte solutions, VITIES, which is a powerful method for identifying the transferring ion and for determining the amount of ion transferred, must be helpful for the elucidation of the oscillation process [17 19]. The VITIES was also demonstrated to be useful for ion transport through a membrane, considering that the membrane transport of ions is composed of the ion transfers at two aqueous-membrane interfaces and the mass transfers and/or chemical reactions in three phases [2,20,21]. 

As demonstrated in work on the ion transport through a membrane [20,21], the relation among AFwi-w2 in curve 1, AFwi/lm in curve 2, and AFlm/w2 in curve 3 at a definite current value can be approximated by Eq. (2), when Wl, W2, and LM contain sufficient concentrations of ions as in the case of Eq. (3). This equation suggests that the membrane potential in the presence of sufficient electrolytes in Wl, W2, and LM is... 

A famous example of the same category in irreversible coupling phenomena is "active transportation" [44], in which K+ ions are transported through a membrane from a diluted side to the other concentrated side against entropy increase, with the expense of another entropy increase induced by H+ transfer through the same membrane in a countercurrent. 

Together with proteins, phospholipids are the most important structural components of biological membranes. Since mobility of the lipid segments fa vors molecular transport through a membrane and thereby increases its permeability, a marked increase in 7] along a lipid-fatty acid chain also reflects a more efficient molecular diffusion through the lipid layer of a membrane [175]. 

In Section 4.1, the mass transport through a membrane layer as a bioreactor was discussed. Now, in this section, the concentration distribution in the feed side of the... 

When oxygen is transported through a membrane in a steady state, there is no net charge current and the flux of oxygen anions, O2-, can be written using the ambipolar diffusion expression, as follows [49]... 

If transport through a membrane involving both surface reaction (dissociation) and diffusion was limited by surface reactions, then n = 1. If transport was diffusion-limited, then n = 0.5. Intermediate values of n (0.5proton transport membranes was modeled with the same form of the equations used to model diffusion membranes, Eqns. 3 and 4. Values of k H2 and n were determined from Eqn. 3. The concentration of hydrogen on the permeate-side was insignificant relative to the concentration on the retentate-side. Therefore was equated to... 

Figure 5. Transport through a membrane when solubilizing micelles are present on both sides. The dye may cross as if it were unsolubilized (bottom) and perhaps also solubilized in micelles (middle). Monomeric surfactant may also cross (top) but does not affect dye transport...

It illustrates that the permeability P is equal to the flux Q of permeant that is transported through a membrane of thickness L under the influence of a difference Ay in potential. As shown in Table 18.1 the potential y may be a pressure (N/m2), a mass concentration (kg/m3) or a molar concentration (mol/m3). This, of course, influences the dimensions of flux and permeability. 

Fig. 4.2. Water transport within a hypha. Water moving between points A and B (plant fungus) or E and F (soihfungus) must pass through a fungal membrane, a major limit to transport (see text). However in fungi, water may not necessarily have to transport through a membrane between cells as no membrane separates individual cells . Thus, rapid equilibration in xp can occur both within cells (e.g. B-C or D-E) and between C and D. Structures as large nuclei can be seen moving between cells squeezing between the walls (see Alexopolis et al., 1996).

The membrane in a broad sense is a thin layer that separates two distinctively different phases, i.e., gas/gas, gas/liquid, or liquid/liquid. No characteristic requirement, such as polymer, solid, etc., applies to the nature of materials that function as a membrane. A liquid or a dynamically formed interface could also function as a membrane. Although the selective transport through a membrane is an important feature of membranes, it is not necessarily included in the broad definition of the membrane. The overall transport characteristics of a membrane depends on both the transport characteristics of the bulk phase of membrane and the interfacial characteristics between the bulk phase and the contacting phase or phases, including the concentration polarization at the interface. The term membrane is preferentially used for high-throughput membranes, and membranes with very low throughput are often expressed by the term barrier. ... 

In the domain where the entity that is transported through a membrane is immiscible or not completely soluble in the contacting (exit) phase, such as the case of gas phase air or oxygen in water, the interfacial factor becomes overwhelmingly important over the transport characteristics of the bulk membrane phase, which is empty space. It is important to recognize that the surface of macroporous membrane consists of the solid phase and the gas phase (in the pore diameter exposed to the interface), and the interfacial aspect of the solid surface dominates the behavior of the gas phase that expands out of the pore. 

The mode of transport through a membrane may be passive, active, or facilitated type. In passive transport, the membrane acts as a barrier and permeation of the components is determined by their diffusivity and concentration in the membrane or just by their size. In facilitated transport along with the chemical potential gradient, the mass transport is coupled to specific carrier components in the membrane. In active transport driving force for transport is achieved by a chemical reaction in the membrane phase. 

The membrane is essentially a barrier, that separates two phases and restricts transport of various chemicals in a selective maimer. A membrane can be homogeneous or heterogeneous, symmetric or asymmetric, sofid or liquid it can carry positive or negative charges or can be neutral. Transport through a membrane can be affected by convection or by diffusion of individual molecules, and induced by the chemical gradient or electrical gradient. 

Schematic representation of Ca transport through a membrane by a Ca +-ATPase molecule. denotes membrane potentials.

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