Passive transport of ions

The composition of ICF can differ markedly from that of the ECF because of the separation of these compartments by the ceU membrane. The composition differences are a consequence of both the Gibbs-Donnan equilibrium and active and passive transport of ions. 

The channels provide no energy and permit only passive transport of ions. When a sodium channel is open, sodium ions flow from the region of high [Na" ] outside a cell to the region of low [Na" ] inside. Conversely, potassium ions flow in the opposite direction. The ion gradients are established by active pumps that require energy. 

In 1952, Alan Hodgkin and Andrew Huxley pubHshed a paper showing how a nonlinear empirical model of the membrane processes could be constructed [Hodgkin and Huxley, 1952]. In the five decades since their work, the Hodgkin-Huxley (abbreviated HH) paradigm of modeling cell membranes has been enormously successful. While the concept of ion channels was not estabhshed when they performed their work, one of their main contributions was the idea that ion-selective processes existed in the membrane. It is now known that most of the passive transport of ions across cell membranes is accompHshed by ion-selective channels. In addition to constructing a nonhnear model, they also estabhshed a method to incorporate experimental data into a nonlinear mathematical membrane model. 

Vm acting across the membrane, with the outside of the cell at the more positive potential. It should be noted that Eq. (1) does not take account of the loss of free energy due to leakages of ions back across the membrane. In excitable tissues such as nerves, electrical impulses are produced as a result of the sudden increase in back-leakage of sodium and potassium ions, which causes a transient collapse of the transmembrane potential gradient. Some of the physical aspects associated with the membrane potential and associated diffuse electrical double layer, together with the active and passive transport of ions, will be discussed in this article. 

Fig. 2. Schematic representation of relevant electrolyte transport through the renal tubule, depicting the osmolar gradient ia medullary iaterstitial fluid ia ywOj yW where represents active transport, —passive transport, hoth active and passive transport, and passive transport of H2O ia the presence of ADH, ia A, the cortex, and B, the medulla. An osmole equals a mole of solute divided by the number of ions formed per molecule of the solute. Thus one mole of sodium chloride is equivalent to two osmoles, ie, lAfNaCl = 2 Osm NaCl. ADH = antidiuretic hormone.

In the first papers dealing with SEI electrodes it was suggested that the passivating layer consists of one or two layers [1, 2], The first one (the SEI) is thin and compact the second (if it exists), on top of the SEI, is a more porous, or structurally open, layer that suppresses the mass transport of ions in the electrolyte filling the pores of this layer. 

It is clearly impossible to give a comprehensive overview of this rapidly expanding field. I have chosen a few experts in their field to discuss one (class of) transport protein(s) in detail. In the first five chapters pumps involved in primary active transport are discussed. These proteins use direct chemical energy, mostly ATP, to drive transport. The next three chapters describe carriers which either transport metabolites passively or by secondary active transport. In the last three chapters channels are described which allow selective passive transport of particular ions. The progress in the latter field would be unthinkable without the development of the patch clamp technique. The combination of this technique with molecular biological approaches has yielded very detailed information of the structure-function relationship of these channels. 

Thus, the ideas above do not suffice for an interpretation of all experimental results. These ideas include the assumption that the ions move in the membrane only under the effect of concentration and potential gradients (diffusion and migration), and that transport of one sort of ions is independent of the transport of other sorts of ions. This transport of ions under the effect of external forces has been named passive ionic transport. 

Models of lipid bilayers have been employed widely to investigate diffusion properties across membranes through assisted and non-assisted mechanisms. Simple monovalent ions, e.g., Na+, K+, and Cl, have been shown to play a crucial role in intercellular communication. In order to enter the cell, the ion must preliminarily permeate the membrane that acts as an impervious wall towards the cytoplasm. Passive transport of Na+ and Cl ions across membranes has been investigated using a model lipid bilayer that undergoes severe deformations upon translocation of the ions across the aqueous interface [126]. This process is accompanied by thinning defects in the membrane and the formation of water fingers that ensure appropriate hydration of the ion as it permeates the hydrophobic environment. 

Palytoxin targets the sodium-potassium pump protein by binding to the molecule in such a way that the molecule is locked in a position where it allows passive transport of both the sodium and potassium ions, thereby destroying the ion gradient that is essential for most cells. 

Meares and his collaborators are especially interested in transport processes across biological membranes. They wish to distinguish experimentally between the active and the passive transport of a solute. For that purpose they determined the fluxes of the sodium ions in each direction through the membrane, using the technique of radio-tracers. The ratio of these experimental fluxes was compared with the theoretical ratios. The same is done with regard to the chlorine ions. 

Iontophoresis by definition is the process of transport of ions into or through a tissue by the use of an applied potential difference across the tissue [52], Depending on the physicochemical characteristics of a molecular species, electrorepulsion is usually the primary mechanism of transdermal transport for ions, whereas electroosmosis and increased passive diffusion (as a result of the reduced barrier properties) are more prominent for neutral species [53]. In contrast, enhancement in flux for neutral or weakly charged species during electroporation arises predominantly from the reduced barrier properties of the membrane, whereas direct electrorepulsion is usually of secondary importance [25],... 

Animal cells do not have a cell wall and are, thus, highly susceptible to the effects of shear stress. Cells respond to hydrodynamic stress within minutes, altering their metabolism and the gene expression pattern (Nol-lert et al., 1991). Under sub-lethal levels of shear stress, there is initially an increase in passive transmembrane transport, simultaneously with damage to surface receptors. The plasma membrane is generally the main site for shear damage, and it may lose its capacity to mediate the transport of ions and molecules, so that the cell loses its viability. It has been demonstrated... 

We can now consider some typical nutrient solutes like amino acids and phosphate. Such molecules are ionized, which means that they would not readily cross the permeability barrier of a lipid bilayer. Permeability coefficients of liposome membranes to phosphate and amino acids have been determined [46] and were found to be in the range of 10 11 -10 12 cm/s, similar to ionic solutes such as sodium and chloride ions. From these figures one can estimate that if a primitive microorganism depended on passive transport of phosphate across a lipid bilayer composed of a typical phospholipid, it would require several years to accumulate phosphate sufficient to double its DNA content or pass through one cell cycle. In contrast, a modern bacterial cell can reproduce in as short a time as 20 min. 

Transport of ions across a membrane in the direction of the gradient of the electrochemical potential is called passive. However, transport occurs against the gradient of the electrochemical potential (active transport), (d) How can this be (Bockris)... 

The most difficult and interesting question about H+-ATPase is how chemical reaction (ATP synthesis/ hydrolysis) is coupled with vectorial H+ conduction. The mechanism for the stoichiometric coupling between chemical reaction and vectorial transport of ions is a universal question for ion-motive ATPases. The Fo portion is a passive proton pathway but becomes a regulated pathway after the binding of Fi. Mutant analyses suggest that the y subunit has regulatory role(s) for proton conduction. 

As the estimations above display, the net flows of chloride and bicarbonate ions are negligible, and the transport of ions is passive. 

Several mechanisms for the transport of ions are operative (1) active transport of the ion against a concentration gradient by an ATP-driven membrane carrier, (2) passive carrier facilitated transport, (3) passive diffusion dependent upon the abihfy of the ion or complex to pass the membrane. Sodium,... 

The lipid bilayer of biological membranes, as discussed in Chapter 12. is intrinsically impermeable to ions and polar molecules. Permeability is conferred by two classes of membrane xoXems, pumps and channels. Pumps use a source of free energy such as ATP or light to drive the thermodynamically uphill transport of ions or molecules. Pump action is an example of active transport. Channels, in contrast, enable ions to flow rapidly through membranes in a downhill direction. Channel action illustrates passive transport, or facilitated diffusion. 

Many substances cross biological membranes according to their lipid solubility. Other polar molecules, such as amino acids and glucose, cross the membranes more rapidly than expected according to their solubUity in lipids. Cations, such as Na" and K, also cross membranes rapidly in spite of their hydrophilic nature. This passive transport of substances at higher rates than predicted from their lipid solubility is termed facilitated diffusion. That proteins are directly involved in facilitated diffusion was shown by comparison of experiments with natural membranes and synthetic membranes produced with phospholipid films. With phospholipid films all molecules, except water, diffuse according to lipid solubility and molecular size. Ions are essentially impermeable. The addition of membrane proteins, however, frequently allowed many polar and charged species to penetrate the membrane at rates comparable to natural membranes. 

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 either in the same or a different direction. Uniports are carrier proteins which actively or passively (see facilitated diffusion below) transport one molecule through the membrane. Active transport requires a source of energy, usually ATP which is hydrolysed by the carrier protein, or the co-transport 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. 

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