Membrane facilitated diffusion

Demonstration of catalytic activities of E-IIs other than vectorial phosphorylation, like phosphorylation of free substrate at the cytoplasmic side of the membrane and facilitated diffusion catalyzed by E-IIs and phosphorylated E-IIs. 

In addition to faster solute transport rates, the major experimental features of membrane-facilitated transport that distinguish it from membrane diffusion include (1) specificity and selectivity (2) saturability (3) inhibition, activation, and cooperativity (4) transmembrane effects and (5) greater temperature sensitivity than is characteristic of membrane diffusion [42],... 

Two distinguishing features of gastrointestinal active and facilitated transport processes are that they are capacity-limited and inhibitable. Passive transcellular solute flux is proportional to mucosal solute concentration (C), where the proportionality constant is the ratio of the product of membrane diffusion coefficient (Dm) and distribution coefficient (Kd) to the length of the transcellular pathway (Lm). 

Facilitated transport of penicilHn-G in a SLM system using tetrabutyl ammonium hydrogen sulfate and various amines as carriers and dichloromethane, butyl acetate, etc., as the solvents has been reported [57,58]. Tertiary and secondary amines were found to be more efficient carriers in view of their easy accessibility for back extraction, the extraction being faciUtated by co-transport of a proton. The effects of flow rates, carrier concentrations, initial penicilHn-G concentration, and pH of feed and stripping phases on transport rate of penicillin-G was investigated. Under optimized pH conditions, i. e., extraction at pH 6.0-6.5 and re-extraction at pH 7.0, no decomposition of peniciUin-G occurred. The same SLM system has been applied for selective separation of penicilHn-G from a mixture containing phenyl acetic acid with a maximum separation factor of 1.8 under a liquid membrane diffusion controlled mechanism [59]. Tsikas et al. [60] studied the combined extraction of peniciUin-G and enzymatic hydrolysis of 6-aminopenicillanic acid (6-APA) in a hollow fiber carrier (Amberlite LA-2) mediated SLM system. 

Membrane proteins lower the activation energy for transport of polar compounds and ions by providing an alternative path through the bilayer for specific solutes. Proteins that bring about this facilitated diffusion, or passive transport, are not enzymes in the usual sense their substrates are moved from one compartment to another, but are not chemically altered. Membrane proteins that speed the movement of a solute across a membrane by facilitating diffusion are called transporters or permeases. 

Figure 11.1 Schematic examples of passive diffusion, facilitated transport and coupled transport. The facilitated transport example shows permeation of oxygen across a membrane using hemoglobin as the carrier agent. The coupled transport example shows permeation of copper and hydrogen ions across a membrane using a reactive oxime as the carrier agent...

Question The movement of an ionized solute across certain types of membranes does not always give rise to equal concentrations of the ion on both sides of the membrane, even when only simple diffusion, facilitated diffusion, or both are available. How can this be explained ... 

The transport proteins involved are intrinsic, that is, they span the membrane. They are solute-speciflc, for example, they have binding sites for the specific molecule or ion that they transport. They also have a defined capacity of how many solutes they can transport. An example of facilitated diffusion is glucose that permeates through cell membranes rather slowly via conventional diffusion because it is not soluble in the membrane. However, glucose passes quickly across a cell membrane via facilitated diffusion. 

Keywords Active transport Adsoiptive endocytosis and transcytosis Brain endothelial cell Cytokine Endothelin Facilitated diffusion Inmiune cell Neurovascular unit Trans-membrane diffusion... 

C. Passive diffusion needs a change in free energy across the membrane, but facilitated diffusion does not. 

Several types of absorptive mechanisms exist for nutrients including active transport, passive diffusion, facilitated diffusion, and endocytosis. End-ocytosis occurs when the outer plasma membrane surrounds soluble or particulate nutrients in the GI tract and engulfs the contents. This process is similar to phagocytosis. 

The placenta is both a transport and a metabolizing organ. Transport is accomplished by simple diffusion, facilitated diffusion, active transport across membranes, and by special processes such as pinocytosis, phagocytosis, specific transport molecules, and channels in the barrier . The placenta also contains a full complement of mixed function oxidases located in the microsomal and mitochondrial subcellular fractions capable of induction and metabolism of endogenous and exogenous chemicals. 

For the absorption of carbohydrates, amino acids, and peptides, a variety of transport systems following facilitated diffusion and active mechanisms have been identified on a molecular and functional level. D-Glucose is mainly absorbed via the Na -dependent transporter SGLTl in the brush-border membrane of enterocytes [18-20]. It is transported across the basolateral membrane by facilitated diffusion via the hexose transporter GLUT-2. Besides SGLTl, the Na +-independent transport protein GLUT-5 is localized in the apical enterocyte membrane, recognizing fructose as a substrate [21]. 

Fructose transport is distinct from glucose-galactose transport and requires a specific saturable membrane carrier (facilitated diffusion). 

The three ways by which substances, including drugs, can cross cell membranes are simple diffusion, facilitated diffusion and active transport. 

Many nutrients and a few drugs can pass across the cell membrane by facilitated diffusion. In this case, in addition to the concentration gradient, a membrane protein acts as a carrier to transport a substance from one side of the membrane to the other. Carrier proteins are specitic and only transport molecules that they recognize . Glucose enters many body cells by facilitated diffusion and the process appears to be more efficient than simple diffusion. 

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