Sodium concentration gradient

Sodium concentration gradients may serve other purposes as well. Thus, in nerve cells, the outflow and inflow of Na+ cause the propagation of currents along the axons. We had occasion to observe this in the case of the visual impulse (Chapter 6). Additionally, Na+ drives a H+ exchange antiport whose function is... 

The Na-2C1-K co-transporter binds and transports one Na, one K and two Cl ions to the cell interior, driven by the inwardly directed sodium concentration gradient established by the Na, K-ATPase pump. 

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]

Sodium trimetaphosphate was used as an eluting agent for the removal of heavy metals such as Pb, Cd, Co, Cu, Fe, Ni, Zn and Cr from aqueous solutions. Distribution coefficients of these elements have been determined regarding five different concentrations of sodium trimeta phosphate (3T0 M 5T0 M 0.01 M 0.05 M 0.1 M) on this resin. By considering these distribution coefficients, the separation of heavy metals has been performed using a concentration gradient of 3T0 - 5T0 M sodium trimetaphosphate. Qualitative and quantitative determinations were realized by ICP-AES. 

It has been seen that this resin has also some important advantages over the other resins in the literature like high total ion exchange capacity, easy synthesis, lower cost, simple regeneration. Furthermore, very good sepai ations were obtained using a concentration gradient of elution. In these elutions, very low concentrations of sodium trimetaphosphate were used. As a result, the resin synthesized can be used as an adsorbent for the effective removal of Pb, Cd, Co, Cu, Fe, Ni, Zn and Cr from aqueous solutions. 

Sodium SGLTl -dependent unidirectionai transporter Small intestine and kidney Active uptake of glucose from lumen of intestine and reabsorption of glucose in proximal tubule of kidney against a concentration gradient... 

Ordinarily, when the current pulse is over, the excess charges will be drained through the passive transport channels, and by operation of the sodium-potassium pumps the original values of membrane potential and of the concentration gradients will be reestablished. However, when in the case of depolarization the negative value of cp has dropped below a certain threshold value, which is about -50 mV, the picture changes drastically Excitation of the membrane occurs. When the current is turned off, the membrane potential not only fails to be restored but continues to... 

With active transport, energy is expended to move a substance against its concentration gradient from an area of low concentration to an area of high concentration. This process is used to accumulate a substance on one side of the plasma membrane or the other. The most common example of active transport is the sodium-potassium pump that involves the activity of Na+-K+ ATPase, an intrinsic membrane protein. For each ATP molecule hydrolyzed by Na+-K+ ATPase, this pump moves three Na+ ions out of the cell and two K+ ions into it. As will be discussed further in the next chapter, the activity of this pump contributes to the difference in composition of the extracellular and intracellular fluids necessary for nerve and muscle cells to function. 

The other, even more important effect of the Na+-K+ pump is that it maintains the concentration differences for sodium and potassium by accumulating Na+ ions outside the cell and K+ ions inside the cell. As previously discussed, the passive diffusion of these ions down their concentration gradients is predominantly responsible for generating the resting membrane potential. Sodium diffuses inward and potassium diffuses outward. The continuous activity of the pump returns the Na+ ions to the ECF and the K+ ions to the ICF. Therefore, it can be said that the pump also makes an indirect contribution to generation of the resting membrane potential. 

Aldosterone acts on the distal tubule of the nephron to increase sodium reabsorption. The mechanism of action involves an increase in the number of sodium-permeable channels on the luminal surface of the distal tubule and an increase in the activity of the Na+-K+ ATPase pump on the basilar surface of the tubule. Sodium diffuses down its concentration gradient out of the lumen and into the tubular cells. The pump then actively removes the sodium from cells of the distal tubule and into the extracellular fluid so that it may diffuse into the surrounding capillaries and return to the circulation. Due to its osmotic effects, the retention of sodium is accompanied by the retention of water. In other words, wherever sodium goes, water follows. As a result, aldosterone is very important in regulation of blood volume and blood pressure. The retention of sodium and water expands the blood volume and, consequently, increases mean arterial pressure. 

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). 

Because the transport of sodium is an active process, it is used to accumulate NaCl in the interstitial fluid of the medulla. In fact, this activity is involved in the initial establishment of the vertical osmotic gradient. Furthermore, sodium is actively transported out of the tubular epithelial cells up its concentration gradient until the filtrate is 200 mOsm/1 less concentrated than the surrounding interstitial fluid. This difference between the filtrate and the interstitial fluid is referred to as the horizontal osmotic gradient. Because the filtrate at the end of the Loop of Henle has an osmolarity of 100 mOsm/1, the kidneys have the ability to produce urine that is significantly more dilute than the plasma. 

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