Facilitated diffusion passive

Facilitated diffusion Passive diffusion across a membrane facilitated by membrane transporter proteins. 

Facilitated diffusion passive transport, the movement of specific compounds across a biomembrane from higher to lower concentration, but at a rate greater than simple diffusion. F. d. is saturable, meaning that above a certain concentration, the rate is not dependent on the substrate concentration. Furthermore, it is stereospecific and susceptible to competitive inhibition. Together, these properties indicate that the process is mediated by a carrier or pore protein in the membrane. F.d. differs from Active transport (see) in not requiring energy. A class of substances called lonophores (see) mimic the carriers of F.d. by making membranes permeable to certain ions. Antibiotics that act in this way are called transport antibiotics. 

FIGURE 10.3 Passive diffusion and facilitated diffusion may be distinguished graphically. The plots for facilitated diffusion are similar to plots of enzyme-catalyzed processes (Chapter 14) and they display saturation behav-... 

Does this transport operate by passive diffusion or by facilitated diffusion ... 

Molecules can passively traverse the bilayer down electrochemical gradients by simple diffusion ot by facilitated diffusion. This spontaneous movement toward equilibrium contrasts with active transport, which requires energy because it constitutes movement against an electrochemical gradient. Figure 41-8 provides a schematic representation of these mechanisms. 

Figure 41-11. A comparison of the kinetics of carrier-mediated (facilitated) diffusion with passive diffusion. The rate of movement in the latter is directly proportionate to solute concentration, whereas the process is saturable when carriers are involved. The concentration at half-maximal velocity is equal to the binding constant (KJ of the carrier for the solute. maximal rate.)...

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. 

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]. 

The transport mechanisms that operate in distribution and elimination processes of drugs, drug-carrier conjugates and pro-drugs include convective transport (for example, by blood flow), passive diffusion, facilitated diffusion and active transport by carrier proteins, and, in the case of macromolecules, endocytosis. The kinetics of the particular transport processes depend on the mechanism involved. For example, convective transport is governed by fluid flow and passive diffusion is governed by the concentration gradient, whereas facilitated diffusion, active transport and endocytosis obey saturable MichaeUs-Menten kinetics. 

Carrier-mediated passage of a molecular entity across a membrane (or other barrier). Facilitated transport follows saturation kinetics ie, the rate of transport at elevated concentrations of the transportable substrate reaches a maximum that reflects the concentration of carriers/transporters. In this respect, the kinetics resemble the Michaelis-Menten behavior of enzyme-catalyzed reactions. Facilitated diffusion systems are often stereo-specific, and they are subject to competitive inhibition. Facilitated transport systems are also distinguished from active transport systems which work against a concentration barrier and require a source of free energy. Simple diffusion often occurs in parallel to facilitated diffusion, and one must correct facilitated transport for the basal rate. This is usually evident when a plot of transport rate versus substrate concentration reaches a limiting nonzero rate at saturating substrate While the term passive transport has been used synonymously with facilitated transport, others have suggested that this term may be confused with or mistaken for simple diffusion. See Membrane Transport Kinetics... 

Passive transport or facilitated diffusion has no energy requirement and is defined as transport of molecules down their concentration gradient (high to low concentration). 

In iron-deficient rats, nickel enhanced the absorption of iron (Nielsen 1980 Nielsen et al. 1980, 1984). This effect of nickel was only observed when ferric sulfate was given. No interaction was observed when iron was given as a 60% ferric/40% ferrous sulfate mixture. It has been proposed that nickel facilitates the passive diffusion of ferric ions by stabilizing the transport ligand (Nielsen 1980). 

Figure 12.4 Mechanism of action of Na+/K+symport inhibitors (thiazides) on the distal convoluted tubule. As in the other parts of the nephron, Na+movement is powered by the energy-requiring sodium pump (P) in the basolateral membrane which exchanges intracellular Na+for K-i-in the extracellular fluid (ECF). The transport of Na-rand Cl- into the cell from the filtrate against the prevailing electrochemical gradient is facilitated by the symporter (S). The Na-Hons are then transported by the pump mechanism described above and the Cl- ions diffuse passively Into the ECF through ion channels in the basolateral membrane. Thiazide diuretics inhibit the symporter by disabling the Cl- binding site with the loss of Na-rand Cl- in the urine.

Passive transport requires no overt energy expenditure because the substances moving across the membrane are going from an area of higher concentration to one of lower concentration. The two types of passive transport are simple diffusion (osmosis) and facilitated diffusion. Molecules which may undergo simple diffusion can be found in Table 4. They are... 

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... 

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. 

Passive diffusion of compounds into the tubules is proportional to the concentration in the bloodstream, so the greater the amount in the blood, the greater will be the rate of elimination. However, when excretion is mediated via active transport or facilitated diffusion, which involves the use of specific carriers, the rate of elimination is constant, and the carrier... 

Passive diffusion, active transport, facilitated diffusion, phago-/pinocytosis, and filtration. 

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. 

A single NPC can mediate both import and export (Keminer et al., 1999). There are two ways to move through the NPC passive diffusion and facilitated translocation. Passive diffusion does not require specific interactions to take place. For small molecules it is fast, but becomes inefficient as the size of the molecules approach 20-40kDa (Paine et al., 1975). In contrast, facilitated translocation is a selective process that... 

Drugs and other substances that pass through biologic membranes usually do so via passive diffusion, active transport, facilitated diffusion, or some special process such as endocytosis (Fig. 2-2). Each of these mechanisms is discussed here. 

Passive facilitated diffusion involves movement down a concentration gradient without an input of energy. This mechanism, which may be highly selective for specific conformational structures, is necessary for transport of endogenous compounds whose rate of transport by simple diffusion would otherwise be too slow. The classical example of facilitated diffusion is transport of glucose into red blood cells. 

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