Downhill diffusion

After nucleation. these nuclei grow further in size by downhill diffusion whereas the composition of the continuous phase moves gradually to ards that of the other equilibrium phase. The type of structure obtained after liquid-liquid demixing by nucleation and growth depends on the initial concentration. 

Figure 41-14. The transcellular movement of glucose in an intestinal cell. Glucose follows Na+ across the luminal epithelial membrane. The Na+ gradient that drives this symport is established by Na+ -K+ exchange, which occurs at the basal membrane facing the extra-ceiiuiarfiuid compartment. Glucose at high concentration within the ceii moves "downhill" into the extracel-iuiarfiuid by fadiitated diffusion (a uniport mechanism).

Certain solutes, eg, glucose, enter cells by facilitated diffusion, along a downhill gradient from high to low concentration. Specific carrier molecules, or transporters, are involved in such processes. 

The various proposed components of the permease system are based upon the response of the transport system to genetic or environmental changes. The complex nature postulated for the intact permease system is necessary to account for the various observed phenomena such as facilitated diffusion, active concentration, facilitated efflux, exchange diffusion, and counter transport of one compound driven by the downhill efflux of a second (2). 

The potential step Es is generally terminated by switching the potential to some final value Ef at which R is now oxidized back to O. If this final potential is sufficiently positive, the concentration of R at the electrode surface is made to be essentially zero. Consequently, accumulated R now diffuses to the electrode, where it is consumed by oxidation back to O, the original species in solution. This is illustrated in Figure 3. ID. Since the concentration of R is zero both at the surface and in the bulk of the electrode, R diffuses both toward and away from the interface under the influence of both downhill sides of its concentration-distance profile. For this reason, all of the R originally generated is not oxidized back to O unless the potential is maintained for a considerable length of time. 

Equation 3.28 describes the competitive binding of solutes to a limited number of specific sites. In other words, active processes involving metabolic energy do not have to be invoked if a solute were to diffuse across a membrane only when bound to a carrier, the expression for the influx could also be Equation 3.28. This passive, energetically downhill entry of a solute mediated by a carrier is termed facilitated diffusion. 

Ion channels. These proteins simply facilitate the diffusion of ions downhill their concentration gradients, i.e. they tend to dissipate the concentration gradients established by the transporters. 

The lipid bilayer of biological membranes, as discussed in Chapter 12. is intrinsically impermeable to ions and polar molecules. Permeability is conferred by two classes of membrane xoXems, pumps and channels. Pumps use a source of free energy such as ATP or light to drive the thermodynamically uphill transport of ions or molecules. Pump action is an example of active transport. Channels, in contrast, enable ions to flow rapidly through membranes in a downhill direction. Channel action illustrates passive transport, or facilitated diffusion. 

Furthermore, the two processes likely cooperate with each other. The seesaw energy diagram in Fig. 14.3A (the linear angle dependence is simplification) indicates that forward (from 80° to 120°) fluctuation in the state Fi ADP Pi will decrease the affinity for Pi and thus assist Pi release, and that, once Pi is released, the downhill slope in Fi ADP will assist forward rather than backward fluctuations. Power stroke vs. diffusion and catch is more of conceptual distinction rather than practical. 

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