Sodium ions transport across membranes

Soil as an Electrolyte Most soils are conductive due to the presence of dissolved ions, such as calcium, magnesium, sodium, potassium, (bi) carbonate, some soluble fatty acids, nitrate, phosphate, sulfate, and chloride ions. Most seemingly, dry soils have more than 5% moisture, sufficient to provide a continuous path for these ions to move. This is essential for plants as the roots need access to these nutrients and ion transport across membranes is the means with which they extract them. The most signihcant feature of natural soils, with respect to their contamination and subsequent remediation, is the high ion exchange capacity (Table 33.1). 

As detailed in chapter 17, biological membranes are basically lipid—think fat or oil—in nature with some attached proteins. As such, these thin sheets of phospholipids and proteins are nearly impermeable to charged particles such as sodium, potassium, or chloride ions. While the isolation of the cell interior from the exterior ionic environment is critical in many ways, it is also true that controlled permeability to ions may be critical. In fact, it is the near-impermeability of biological membranes to ions that permits control of ion transport across them by certain, specific proteins. 

Sodium ions A marked and rapid increase in the Na ion transport across the plasma membrane into a nerve or muscle cell, via the Na ion channel, causes a depolarisation of the membrane that initiates a transient flux of electrical activity along the nerve or muscle (that is, an action potential) (Chapters 13 and 14). 

S. H. Stern and M. E. Green, Noise generated during sodium and hydrogen ion transport across a cation exchange membrane, J. Phys. Chem., 77 (1973), p. 1567. 

Fig. 4.3 Uptake of glutamate is associated with ion transport across neural membranes. Ion transport results in entry of 3 sodium ions and 1 proton in the cell whilst 1 potassium ion is transported out (modified from Attwell, 2000)...

The mechanism by which the exergonic methyl-transfer reaction is coupled with vectorial electrogenic Na translocation across the membrane is not known. An electron transport chain appears not to be involved in Na transport. Sodium ion transport... 

A laboratory membrane brine electrolysis cell, designed for automated operation, was constructed ( 1,2). This system enables the measurement of the sodium ion transport number of a membrane under specific sets of conditions using a radiotracer method. In such an experiment, the sodium chloride anolyte solution is doped with 22Na radio-tracer, a timed electrolysis is performed, and the fraction of current carried by sodium ion through the membrane is determined by the amount of radioactivity that has transferred to the sodium hydroxide catholyte solution. The voltage drop across the membrane during electrolysis is simultaneously measured, so that the overall performance of the material can be evaluated. 

One of the most common control mechanisms for enzymes is by phosphorylation. The side-chain hydroxyl groups of serine, threonine, and tyrosine can all form phosphate esters. Transport across membranes provides an important example, such as the sodium-potassium ion pump, which moves potassium into the... 

Nanochannels and ion transport across nanochannels is a very common feature in living cells [1]. Cells contain various internal and external membranes made out of lipid molecules. These are called lipid bilayers. The nanochannels are made of proteins which form a pore on the lipid bilayer. Various kinds of such channels are known with different biological functions. For example, nerve cells contains channels that are permeable to Sodium, Pottasium or Calcium ions. Channels open or close in response to various factors. For example, voltage gated channels open in response to a change in the electrical potential across it. 

Back-diffusion is the transport of co-ions, and an equivalent number of counterions, under the influence of the concentration gradients developed between enriched and depleted compartments during ED. Such back-diffusion counteracts the electrical transport of ions and hence causes a decrease in process efficiency. Back-diffusion depends on the concentration difference across the membrane and the selectivity of the membrane the greater the concentration difference and the lower the selectivity, the greater the back-diffusion. Designers of ED apparatus, therefore, try to minimize concentration differences across membranes and utilize highly selective membranes. Back-diffusion between sodium chloride solutions of zero and one normal is generally [Pg.173]

A well-known example of active transport is the sodium-potassium pump that maintains the imbalance of Na and ions across cytoplasmic membranes. Flere, the movement of ions is coupled to the hydrolysis of ATP to ADP and phosphate by the ATPase enzyme, liberating three Na+ out of the cell and pumping in two K [21-23]. Bacteria, mitochondria, and chloroplasts have a similar ion-driven uptake mechanism, but it works in reverse. Instead of ATP hydrolysis driving ion transport, H gradients across the membranes generate the synthesis of ATP from ADP and phosphate [24-27]. 

The rate of the active transport of sodium ion across frog skin depends both on the electrochemical potential difference between the two sides of this complex membrane (or, more exactly, membrane system) and also on the affinity of the chemical reaction occurring in the membrane. This combination of material flux, a vector, and chemical flux (see Eq. 2.3.26), which is scalar in nature, is possible according to the Curie principle only when the medium in which the chemical reaction occurs is not homogeneous but anisotropic (i.e. has an oriented structure in the direction perpendicular to the surface of the membrane or, as is sometimes stated, has a vectorial character). 

An essential requirement for diffusion of Na+ ions is the creation of a concentration gradient for sodium between the filtrate and intracellular fluid of the epithelial cells. This is accomplished by the active transport ofNa+ ions through the basolateral membrane of the epithelial cells (see Figure 19.4). Sodium is moved across this basolateral membrane and into the interstitial fluid surrounding the tubule by the Na+, K+-ATPase pump. As a result, the concentration of Na+ ions within the epithelial cells is reduced, facilitating the diffusion of Na+ ions into the cells across the luminal membrane. Potassium ions transported into the epithelial cells as a result of this pump diffuse back into the interstitial fluid (proximal tubule and Loop of Henle) or into the tubular lumen for excretion in the urine (distal tubule and collecting duct). 

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