Facilitative diffusion, glucose

Cl cotransporter), (5) ion antiports (Na /H exchange), (6) facilitated diffusion (glucose via CLUT-1), (7) active transport (P-gp), (8) active antiport transport (Na /K ATPase), and (9) endocytosis (receptor as insulin or transferrin or adsorption mediated). Adapted from Huber etal. [47]. 

Glucose passes into an erythrocyte via glucose permease by facilitated diffusion. Glucose flows using its concentration gradient via passive transport. (Adapted from Lehninger, Principles of Biochemistry, Third Edition, by David L. Nelson and Michael M. Cox. 1982, 1992, 2000 by Worth Publishers. Used with permission ofW. H. Freeman and Company.)... 

Glucose Transport in Erythrocytes Occurs by Facilitated Diffusion... 

Phloretin is the aglycon of phlorizin and inhibits the facilitated diffusion of glucose catalyzed by GLUT1 or GLUT4. It has been used to terminate the uptake of glucose in timed assays with isolated membranes or reconstituted transporters. 

Physiologically muscle-derived NO regulates skeletal muscle contractility and exercise-induced glucose uptake. nNOS is located at the plasma membrane of skeletal muscle and facilitates diffusion of NO to the vasculature to regulate muscle perfusion. 

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 history of observations of efflux associated with PTS carriers is nearly as old as PTS itself. Gachelin [82] reported that A -ethylmaleimide inactivation of a-methyl-glucoside transport and phosphorylation in E. coli was accompanied by the appearance of a facilitated diffusion movement of both a-methylglucoside and glucose in both directions, uptake and efflux. His results could not discriminate, however, between one carrier operating in two different modes, active transport for the native carrier and facilitated diffusion for the alkylated carrier, or two distinct carriers. Haguenauer and Kepes [83] went on to show that alkylation of the carrier was not even necessary to achieve efflux NaF treatment which inhibits P-enolpyruvate synthesis was sufficient but this study did not address the question of one carrier or two. 

By far the most complete study of the kinetics of mammalian passive glucose transporters has been done on the GLUT-1 isoform in the human erythrocyte. The transport of glucose in this cell type is a classic example of facilitated diffusion, the... 

Glucose and galactose enter the absorptive cells by way of secondary active transport. Cotransport carrier molecules associated with the disaccharidases in the brush border transport the monosaccharide and a Na+ ion from the lumen of the small intestine into the absorptive cell. This process is referred to as "secondary" because the cotransport carriers operate passively and do not require energy. However, they do require a concentration gradient for the transport of Na+ ions into the cell. This gradient is established by the active transport of Na+ ions out of the absorptive cell at the basolateral surface. Fructose enters the absorptive cells by way of facilitated diffusion. All monosaccharide molecules exit the absorptive cells by way of facilitated diffusion and enter the blood capillaries. 

Facilitated diffusion of glucose across the blood-brain barrier and into brain cells is catalyzed by GLUT1,- 2 and -3, products of the SLC2 gene superfamily 90... 

Glucose transporters in muscle and fat tissue operate by facilitated diffusion. 

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.

Facilitated diffusion is a mechanism of transmembrane transfer that is carrier mediated but not energy-dependent. The carrier molecule is usually a transmembrane protein, which binds molecules and releases them on the other side of the membrane. It is an important mechanism for endogenous substances, such as glucose. 

Facilitated diffusion is a carrier-mediated mechanism of transplacental transfer. It is important for endogenous substances, such as glucose. It is also the mechanism for some drugs, e.g. the antibiotic cefalexin. Active transport is a carrier-mediated process that requires energy against an electrochemical or concentration gradient. Amino acids and calcium are transported by this mechanism. Only a few drugs, such as a-methyidopa and 5-fluorouracil, are transferred by active transport. 

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... 

Glucose cannot diffuse directly into cells, but enters by one of two transport mechanisms a Na+-independent, facilitated diffusion transport system or a Na+-monosaccharide co-transporter system. 

---