Facilitated and active transports in membranes

The efficiencies of electrokinetic energy conversion for two operations modes, namely electroosmosis 7jeo and streaming potential Tjsp, are expressed as 

IAip 7 sp JVAP output input (Atpf/R JVAP (10.116) 

The maximum energy conversion efficiency may be related to the merit through the degree of coupling q 

Various substances such as amino acids, organic acids, NaOH, NaCl, carbon dioxide, oxygen, and metals, and various ions such as Cd(II), Cu(II), Co(II), and Fe(III), can be separated by using suitable carrier agents in liquid or solid 

Transport in membranes is mostly a complex and coupled process coupling between the solute and the membrane, and coupling between diffusion and the chemical reaction may play an important role in efficiency. It is important to understand and quantify the coupling to describe the transport in membranes. Kinetic studies may also be helpful. However, thermodynamics might offer a new and rigorous approach toward understanding the coupled transport in composite membranes without the need for detailed examination of the mechanism of diffusion through the solid structure. Table 10.4 shows some of the applications of facilitated transport. 

---

E.L. Cussler, Facilitated and Active Transport, in Polymeric Gas Separation Membranes, D.R. Paul and Y.P. Yampol skii (eds), CRC Press, Boca Raton, FL, pp. 273-300 (1994). 

CusslerEL. Facilitated and active transport. In Paul DR and Yampol skii YP, eds. Polymeric Gas Separation Membranes. Boca Raton, FL CRC Press, 1994 Chap. 6. 

The placenta is both a transport and a metabolizing organ. Transport is accomplished by simple diffusion, facilitated diffusion, active transport across membranes, and by special processes such as pinocytosis, phagocytosis, specific transport molecules, and channels in the barrier . The placenta also contains a full complement of mixed function oxidases located in the microsomal and mitochondrial subcellular fractions capable of induction and metabolism of endogenous and exogenous chemicals. 

Passive diffusion, facilitated and active transport, and diffusion through channels account for most of the molecular transit across membranes. Several other mechanisms involving membrane-associated proteins are also important in the life of a cell, and represent important targets for drug therapy. 

Molecules that cannot pass freely through the lipid bilayer membrane by themselves do so in association with carrier proteins. This involves two processes— facilitated dififrision and active transport—and highly specific transport systems. 

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. 

Materials may be absorbed by a variety of mechanisms. Depending on the nature of the material and the site of absorption, there may be passive diffusion, filtration processes, facilitated diffusion, active transport and the formation of microvesicles for the cell membrane (pinocytosis) (61). Following absorption, materials are transported in the circulation either free or bound to constituents such as plasma proteins or blood cells. The degree of binding of the absorbed material may influence the availability of the material to tissue, or limit its elimination from the body (excretion). After passing from plasma to tissues, materials may have a variety of effects and fates, including no effect on the tissue, production of injury, biochemical conversion (metabolized or biotransformed), or excretion (eg, from liver and kidney). 

Both active and passive fluxes across the cellular membranes can occur simultaneously, but these movements depend on concentrations in different ways (Fig. 3-17). For passive diffusion, the unidirectional component 7jn is proportional to c°, as is indicated by Equation 1.8 for neutral solutes [Jj = Pj(cJ — cj)] and by Equation 3.16 for ions. This proportionality strictly applies only over the range of external concentrations for which the permeability coefficient is essentially independent of concentration, and the membrane potential must not change in the case of charged solutes. Nevertheless, ordinary passive influxes do tend to be proportional to the external concentration, whereas an active influx or the special passive influx known as facilitated diffusion—either of which can be described by a Michaelis-Menten type of formalism—shows saturation effects at higher concentrations. Moreover, facilitated diffusion and active transport exhibit selectivity and competition, whereas ordinary diffusion does not (Fig. 3-17). 

Membrane proteins carry out a wide range of critical functions in cells, and they include passive and active transporters, ion chamiels, many classes of receptors, cellular toxins, proteins involved in membrane trafficking, and the enzymes that facilitate electron transport and oxidative phosphorylation. For example, the voltage-gated ion channels that facilitate the passive diffusion of sodium and potassium across the axonal membrane are responsible for the formation of an action potential. Active transport proteins establish ion gradients and are necessary for the uptake of nutrients into cells. Soluble hormones bind to membrane receptors, which then regulate the internal biochemistry of the cell. 

What is known is that the cellular membranes are extremely fussy about what they will and will not allow to pass through them as has been discussed in Chapter 4, even quite small molecules may have their free diffusion into and out of the cell and its compartments restrained by the membrane. Mechanisms of facilitated diffusion and active transport, which are controlled by the membrane, are involved in these processes - helping, for instance, to keep the interior of the cell in a high K+, low Na+ condition, despite the fact that the extracellular environment - for example, the blood and plasma in mammals - is maintained at high Na+, low K+ levels. 

The term transporter refers to a variety of membrane proteins with diverse functions and structures. Transporters can mediate either facilitated or active transport (Figure 9.1). Facilitated transport involves movement of drug across a membrane down an electrochemical gradient. Conversely, active transporters rely on energycoupling mechanisms (e.g., ATP hydrolysis) to move drug. These transporters can also create ion/solute gradients across membranes (secondary active transport), which in turn drive uphill membrane transport of drugs. 

The basic mechanisms involved in solute transport across the plasma membrane include passive diffusion, facilitated diffusion, and active transport. Active transport can be further subdivided into primary and secondary active transport. These mechanisms are depicted in Figure 2-4. 

We know from our own experiments, and others, that there is an induction of intracellular CA activity, as well [11]. A rapidly induced CA, bound or associated to the plasma membrane is one probable explanation for the acclimation to the 10 pM C02(aq) level. The increased photosynthesis, in C. reinhardtii, could then be caused by a facilitated diffusion of CO2 as has been shown to occure in many other biological systems. In a recent review on DIC transport [12], Smith concludes that when DIC is required by the cell it is "transported" by facilitated diffusion. Active transport is only used when the cells need to regulate the internal pH. This conclusion supports our data on the photosynthetic acclimation to low DIC concentrations in C. reinhardtii.. 

Okadaic acid (OA), being a polyether, presents ionophoretic properties (facilitation of ion transport across membranes) as does CTX. It has been found that OA causes contraction in smooth muscles even in the absence of Ca (Ozaki and Karaki 1987 Shibata 1985). Ozaki and Karaki (1987) studied the mechanism of action of OA compared to calyculin A (another polyether isolated from a marine sponge). The results of this work suggest that OA has two separate effects activation of calcium channels as well as activating contractile elements to induce smooth muscle contraction. Recently, OA, in addition to other compounds from marine sources, has been found to be a tumor promoter that is, an agent that promotes tumor formation on already initiated cells (Fujiki 1988). It has been found that the OA class of tumor promoters bind to their own receptors which are present in particulate as well as cytosol fractions. The mechanism of action of these compounds has been partially elucidated. 

---