Facilitated diffusion channels

Facilitated diffusion (channel), molecule moves down its electrochemical gradient. Active transport (pump) molecule moves up its electrochemical gradient (requires energy input). Pumps use energy (usually ATP hydrolysis). Na+ high outside/K+ high inside. 

Facilitated diffusion within organisms takes place when carriers or proteins residing within membranes—ion channels, for instance—organize the movement of ions from one location to another. This diffusion type is a kinetic, not thermodynamic, effect in which a for the transfer is lowered and the rate of diffusion is accelerated. Facilitated diffusion channels organize ion movements in both directions, and the process can be inhibited both competitively and noncompetitively. It is known that most cells maintain open channels for K+ most of the time and closed channels for other ions. Potassium-ion-dependent enzymes include NaVK+ ATPases (to be discussed in Section 5.4.1), pyruvate kinases, and dioldehydratases (not to be discussed further). 

Channel proteins transport water or specific types of Ions and hydrophilic small molecules down their concentration or electric potential gradients. Such protein-assisted transport sometimes is referred to as facilitated diffusion. Channel proteins form a hydrophilic passageway across the membrane through which multiple water molecules or ions move simultaneously, single file at a very rapid rate. Some ion chan-... 

Several polymers were found to fit all or most of the above criteria and were used to prepare the carrier films. Many polymers have been used for this purpose, viz., ethyl cellulose, poly(y-benzyl glutamate), poly(vinyl acetate), cellulose acetate phthalate, and the copolymer of methyl vinyl ether with maleic anhydride. In addition to the base polymers, plasticizers were often needed to impart a suitable degree of flexibility. Plasticizers, which are found to be compatible with polymeric materials include, acetylated monoglycerides, esters of phthalic acid such as dibutyl tartarate, etc. An excipient was usually incorporated into the matrix of the carrier films. The excipients used were water-soluble materials, which are capable of creating channels in the polymer matrix and facilitate diffusion of the drug. PEGs of different molecular weights were used for this purpose. 

Facilitated diffusion (i.e./ via antibiotic carriers or membrane channels). 

The distinction between facilitated diffusion through channels and carrier-mediated transport is somewhat artificial/ but may be justified on the basis of specificity. For example/ 3-lactams in general can pass through nonselective bacterial outer membrane porin (e.g./ OmpF) channels via passive diffusion/ whereas imipenem (and related zwitterionic carbapenems) can also utilize OprD channels/ which preferentially recognize basic amino acids and dipeptides. The identification of mutants that selectively confer imipenem resistance suggests that more intimate protein-drug associations are involved in carrier-mediated transport than in facilitated diffusion/ which may be limited only by pore diameter. 

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. 

Cr(VI) can more easily pass through cell membranes than Cr(III) — in a process similar to sulfate and phosphate species, chromate enters cells via facilitated diffusion through nonspecific anion channels. 

Facilitated diffusion Large molecules (e. g. glucose, protein, organic ions) cannot diffuse across the cell membrane barrier unaided. Mechanisms in the form of channels or transport proteins (= carriers) are therefore required to enable molecules to penetrate the impermeable... 

B. Facilitated diffusion uses protein channels, but passive diffusion does not. 

The transport of molecules across the plasma membrane of a cell occurs by three major mechanisms simple diffusion across the membrane, facilitated diffusion through channels or carriers, and active transport of molecules by carriers and pumps. The latter form of transport requires energy from the cell and is divided into two major types primary active and secondary active (1,2). 

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). 

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