Transfer through membranes active transport

There are, however, various types of active transport systems, involving protein carriers and known as uniports, symports, and antiports as indicated in Figure 3.7. Thus, symports and antiports involve the transport of two different molecules in either the same or a different direction. Uniports are carrier proteins, which actively or passively (see section "Facilitated Diffusion") transport one molecule through the membrane. Active transport requires a source of energy, usually ATP, which is hydrolyzed by the carrier protein, or the cotransport of ions such as Na+ or H+ down their electrochemical gradients. The transport proteins usually seem to traverse the lipid bilayer and appear to function like membrane-bound enzymes. Thus, the protein carrier has a specific binding site for the solute or solutes to be transferred. For example, with the Na+/K+ ATPase antiport, the solute (Na+) binds to the carrier on one side of... 

Biochemically, most quaternary ammonium compounds function as receptor-specific mediators. Because of their hydrophilic nature, small molecule quaternaries caimot penetrate the alkyl region of bdayer membranes and must activate receptors located at the cell surface. Quaternary ammonium compounds also function biochemically as messengers, which are generated at the inner surface of a plasma membrane or in a cytoplasm in response to a signal. They may also be transferred through the membrane by an active transport system. 

In addition to the passive diffusional processes over lipid membranes or between cells, substances can be transferred through the lipid phase of biological membranes through specialized systems, i.e., active transport and facilitated diffusion. Until recently, the active transport component has been discussed only for nutrients or endogenous substances (e.g., amino acids, sugars, bile acids, small peptides), and seemed not to play any major role in the absorption of pharmaceuticals. However, sufficient evidence has now been gathered to recognize the involvement of transporters in the disposition of pharmaceuticals in the body [50, 127]. 

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. 

The processes of pharmacokinetics all involve the transfer of a drug across membranes, beginning with the cell membrane, and sometimes involving single or multiple layers of cells. Drugs can cross these membranes through passive or active transport. 

Derive an equation for the phenomenological description of active transport of a substance through a membrane (Section 2.3.2) for the case of conjugate substance transfer through the membrane and chemical processes far from equilibrium (i.e., at Arij > RT). 

The high-affinity pathway involves oxidation of Fe to Fe by the ferroxidase FET3 and subsequent transport of Fe " " across the plasma membrane by the permease FTRl. FET3p is a member of the family of multicopper oxidases, which include ascorbate oxidase, laccase, and ceruloplasmin (see Chapter 14), and does not become functional until it is loaded with copper intracellularly through the activities of the copper chaperone ATX Ip and the copper transporter CCC2p. It appears that Fe " " produced by FET3 is transferred directly to FTRl, and does not equilibrate with the bulk phase, as is illustrated in Fig. 7.13. This is almost certainly achieved by the classic metabolite-channeling mechanism, a common feature of multifunctional enzymes. 

It is not uncommon for drug compounds to be able to perform very well in a variety of microtiter plate-based assays, but when transferred to in vivo assays, they cannot reach the therapeutic target site. The molecule must permeate through a number of cell membranes made up of phospholipid bilayers, which can increase the passage of highly charged polar molecules. Among the most common means by which a molecule can cross such a membrane are transcellular routes such as passive diffusion, carrier-mediated active transport, and metabolic enzymes, paracellular... 

The various forms of the sugars must also be considered in such processes as transport through membranes. If one form is exclusively (or even preferentially) transferred, the concentrations on each side of the membrane will not be expressed by the total concentrations (as usually measured), especially if the rates of mutarotation are different on both sides of the membrane. Keston has explained active transport of sugars across the membranes of the kidney and intestine on this basis, but assumed that an active form (the aldehydo form) is present in only a small proportion.305 320 The concept seems theoretically sound under non-equilibrium, steady-state conditions, but it cannot be applied to the reversal of the concentration gradient of a particular anomer. 

How and when DNA delivered by non-viral vectors is released are challenging questions. Nature has isolated the nucleus behind a double-bilayer membrane with tightly regulated pores (called nuclear pores) that allow import and export of a specific set of biomolecules. DNA can entiy into the nucleus by three possible routes (i) entry during mitosis when the nuclear envelope breaks down (ii) transport through nuclear pores and (iii) active transport across the nuclear membrane by using kariophilic proteins as transfer carriers. 

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