Facilitated diffusion transport uniport

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... 

B) In secondary translocation reactions, primary bond exchanges or the transfer of electrons do not occur. This class of reactions may be divided into three sub-classes (7) In non-coupled solute translocation, or facilitated diffusion, or uniport reactions, a single solute equilibrates across an osmotic barrier. (2) In sym-coupled solute translocation, or cotransport" , or symport reactions, two solutes equilibrate across an osmotic barrier, and the translocation of one solute is coupled to the translocation of the other in the same direction, (i) In anti-coupled solute translocation, or counter-transport , or antiport reactions, two solutes equilibrate across an osmotic barrier, and the translocation of one solute is coupled to the translocation of the other in the opposite direction. 

Figure 4-6. Mechanism of facilitated diffusion mediated by a glucose transporter. This is an example of uniport. The reversible interconversion between conformations of the transporter in which the glucosebinding site is alternately exposed to the exterior and interior of the cell is called a ping-pong mechanism.

Thus, a specific carrier molecule is involved, but the process relies on a concentration gradient, as does passive diffusion. The transport of glucose out of intestinal cells into the bloodstream occurs via facilitated diffusion and uses a uniport. 

The GLUT transporters, such as GLUT1 of erythrocytes, carry glucose into cells by facilitated diffusion. These transporters are uniporters, carrying only one substrate. Symporters permit simultaneous passage of two... 

A large family of transmembrane facilitators from bacteria and eukaryotes appear to consist largely of 12 transmembrane helices with intervening cytosolic and extracellular loops. Some of these transporters facilitate simple uniport diffusion, but others participate in active transport of the symport or antiport type.393 394 Several hundred members of the family are known.395... 

Figure 5.12 Transport proteins in cell membranes, (a) Energy-dependent, ATP-powered ion pumps such as the Na /K exchange ATPase (b) channels, gated or non-gated, which permit diffusion through an aqueous pathway, such as voltage-gated Na channels (c) passive, facilitated transport systems, which can act in uniport, symport, or antiport modes, such as the glucose transporter. The filled arrow indicates gradient of molecules indicated by the filled symbol.

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