Uniport systems

Transport systems can be described in a functional sense according to the number of molecules moved and the direction of movement (Figure 41-10) or according to whether movement is toward or away from equilibrium. A uniport system moves one type of molecule bidirectionally. In cotransport systems, the transfer of one solute depends upon the stoichiometric simultaneous or sequential transfer of another solute. A symport moves these solutes in the same direction. Examples are the proton-sugar transporter in bacteria and the Na+ -sugar transporters (for glucose and certain other sugars) and Na -amino acid transporters in mammalian cells. Antiport systems move two molecules in opposite directions (eg, Na in and Ca out). 

Some specific solutes diffuse down electrochemical gradients across membranes more rapidly than might be expected from their size, charge, or partition coefficients. This facilitated diffusion exhibits properties distinct from those of simple diffusion. The rate of facilitated diffusion, a uniport system, can be saturated ie, the number of sites involved in diffusion of the specific solutes appears finite. Many facihtated diffusion systems are stereospecific but, fike simple diffusion, require no metabolic energy. 

Secondary active uniport systems facilitating the permeation of a single solute, dependent on the electrochemical potentials of the solute molecules, are rare in bacteria. Only a glucose uptake system of Zymomonas mobilis has been studied in more detail [101]. 

Figure 9.7 (a) The epithelial brush border cells of the small intestine concentrate glucose from the intestinal lumen in symport with Na+ this is driven by the (Na+-K+)-ATPase located on the capillary side of the cell. The glucose is then exported by a passive uniport system. (From Voet and Voet, 2004. Reproduced with permission from John Wiley Sons., Inc.) (b) Two Na+-binding sites in the LeuT Na+-dependent pump. (From Gouax and MacKinnon, 2005. Copyright (2005) American Association for the Advancement of Science.)... 

Transport of many compounds including drugs across cell membranes is mediated by membrane proteins called carrier proteins or channel proteins. Some of these proteins transport only one substrate molecule at a time across the membrane (uniport systems), while others act as cotransport systems (Figure 9.4). Depending on the direction of the second substrate, the proteins are also called symporters or antiporters, for example, Na /glucose cotransporter, H " /peptide cotransporter, or Na /K antiporter (—Na /K -ATPase). 

A carrier which transports a single molecule in one direction is called a uniport system. Alternatively, a carrier may carry two molecules simultaneously in the same direction, i.e. a symport system. TTiirdly, a carrier may exchange one molecule for another and therefore transport them in opposite directions, i.e an antiport system (Figure 9.4). 

The transport systems of the inner mitochondrial membrane use various mechanisms. Metabolites or ions can be transported alone (uniport, U), together with a second substance (symport, S), or in exchange for another molecule (antiport. A). Active transport—i. e., transport coupled to ATP hydrolysis—does not play an important role in mitochondria. The driving force is usually the proton gradient across the inner membrane (blue star) or the general membrane potential (red star see p. 126). 

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... 

Ultracentrifugation 108,109 Ultracentrifuge 100 analytical 108 optical system, figure 109 Ultrafiltration 100 Ultrasensitive responses 567 Ultrasonic sound 468 UMP (Uridine5-phosphate) 200, 200s,203 Unimolecular processes 457 Uniporters 414 Units, International System 2 Unsatisfied ends of hydrogen bonded chains 64... 

Figure 9.29 Some mammalian (left) and microbial (right) membrane transport systems. (A) Primary electrogenic mechanisms (pumps) creating either a Na+ or a H+ gradient. (B) Secondary active transport systems of the symport type, in which the entry of a nutrient S into the cell is coupled with the entry of either the sodium ions or protons. (D) Various passive ion movements, possibly via channels or uniports. (Reproduced by permission from Serrano R. Plasma Membrane ATPase of Plants and Fungi. Boca Raton CRC Press, 1985, p. 59.)...

Figure 9.4 Transport proteins acting as uniport, symport, or antiport systems.

Many active-transport systems couple the uphill flow of one ion or molecule to the downhill flow of another. These membrane proteins, called secondary transporters or cotransporters, can be classified as antiporters, symporters, and uni porters. Antiporters couple the downhill flow of one type of ion in one direction to the uphill flow of another in the opposite direction. Symporters move both ions in the same direction. Uniporters transport a substrate in either direction, determined by the concentration differences. Studies of the lactose permease from E. coli have been a source of insight into both the structures and the mechanisms of secondary transporters. 

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