Membranes ionic gradient

The electrical signals are carried by the movement of charged ions across the cell membrane. This makes use of the potential energy stored across the cell membrane in the form of ionic gradients. Concentration gradients for the principal ions across a typical nerve cell membrane are indicated in Fig. 2.1(a). The cell interior has a high concentration of K+ ions and a low concentration of Na+, Cl and Ca + ions relative to the exterior. 

Figure 2.1 (a) Resting ionic gradients across a nerve cell membrane. Concentrations [ ] are in... 

The excitable membrane of nerve axons, like the membrane of cardiac muscle (see Chapter 14) and neuronal cell bodies (see Chapter 21), maintains a resting transmembrane potential of -90 to -60 mV. During excitation, the sodium channels open, and a fast inward sodium current quickly depolarizes the membrane toward the sodium equilibrium potential (+40 mV). As a result of this depolarization process, the sodium channels close (inactivate) and potassium channels open. The outward flow of potassium repolarizes the membrane toward the potassium equilibrium potential (about -95 mV) repolarization returns the sodium channels to the rested state with a characteristic recovery time that determines the refractory period. The transmembrane ionic gradients are maintained by the sodium pump. These ionic fluxes are similar to, but simpler than, those in heart muscle, and local anesthetics have similar effects in both tissues. 

A membrane is a semipermeable barrier whose function is to compartmentalize metabolic processes, maintain pH differences on either side, control osmotic pressure and ionic gradients, provide a surface or environment for the stabilization of active biomolecules, provide tissue discrimination, and allow selective access as well as egress to specific metabolites. 

All of these experiments suggested that the PolyP content in yeast cells depended strongly on the energetic status of the cells, including the ionic gradients on the membranes. 

Energy for transport by pumps may be derived from ATP hydrolysis or from the ionic gradients of another ion. This is the case of the sodium/calcium exchanger, which uses the electrical energy stored in the sodium gradient across the plasma membrane to transport calcium out of the cell while harnessing the energy from concerted sodium influx (Komuro et al, 1992). 

Thus, the Gibbs Free energy expression for such an ionic gradient across the membrane consisting of an positively charged species C, would be of the form of the following expression ... 

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