Membrane potential electrochemical

Membrane Potentials Ion-selective electrodes, such as the glass pH electrode, function by using a membrane that reacts selectively with a single ion. figure 11.10 shows a generic diagram for a potentiometric electrochemical cell equipped with an ion-selective electrode. The shorthand notation for this cell is... 

The net electrochemical driving force is determined by two factors, the electrical potential difference across the cell membrane and the concentration gradient of the permeant ion across the membrane. Changing either one can change the net driving force. The membrane potential of a cell is defined as the inside potential minus the outside, i.e. the potential difference across the cell membrane. It results from the separation of charge across the cell membrane. 

The sodium channels are very selective for Na+ over K+, allowing Na+ influx down the electrochemical gradient to generate positive membrane potentials. The sodium channels are also permeable to Li+ and NH4+. The narrowest portion of the channel pore is estimated to be rectangular (3.1 x 5.2 A). 

Mitchell s chemiosmotic theory postulates that the energy from oxidation of components in the respiratory chain is coupled to the translocation of hydrogen ions (protons, H+) from the inside to the outside of the inner mitochondrial membrane. The electrochemical potential difference resulting from the asymmetric dis-... 

The individual steps of the multistep chemical reduction of COj with the aid of NADPHj require an energy supply. This supply is secured by participation of ATP molecules in these steps. The chloroplasts of plants contain few mitochondria. Hence, the ATP molecules are formed in plants not by oxidative phosphorylation of ADP but by a phosphorylation reaction coupled with the individual steps of the photosynthesis reaction, particularly with the steps in the transition from PSII to PSI. The mechanism of ATP synthesis evidently is similar to the electrochemical mechanism involved in their formation by oxidative phosphorylation owing to concentration gradients of the hydrogen ions between the two sides of internal chloroplast membranes, a certain membrane potential develops on account of which the ATP can be synthesized from ADP. Three molecules of ATP are involved in the reaction per molecule of COj. 

A significant development in the methodology of potentiometry that paved the way for its utility in bioanalysis was the discovery of the ion selective electrode (ISE). Conceptually, the ISE involves the measurement of a membrane potential. The response of the electrochemical cell is therefore based on an interaction between the membrane and the analyte that alters the potential across the membrane. The selectivity of the potential response to the analyte depends on the specificity of the membrane interaction for the analyte. 

The second model of a biological membrane is the liposome (lipid vesicle), formed by dispersing a lipid in an aqueous solution by sonication. In this way, small liposomes with a single BLM are formed (Fig. 6.11), with a diameter of about 50 nm. Electrochemical measurements cannot be carried out directly on liposomes because of their small dimensions. After addition of a lipid-soluble ion (such as the tetraphenylphosphonium ion) to the bathing solution, however, its distribution between this solution and the liposome is measured, yielding the membrane potential according to Eq. 

The two terms in the final expression of Equation 9.11 can compete against one another and with only a small membrane potential the concentration gradient chemical potential can be overcome. Electrochemical energy is thus harnessed to allow nutrients to flow into a cell and increase the local concentration. 

The membrane potential and the concentration gradient can reinforce each other or they can be in opposition to each other. The total force tending to move a molecule or ion through a membrane is called the electrochemical potential. When the concentration gradient and the electrical potential work to oppose each other, the stronger effect wins. If someone forces you to get quantitative (this may be a physiologist rather than a biochemist), see the section on the Nernst equation at the end of the chapter. 

Pumps move ions and molecules up their electrochemical gradient. Pumps require energy, usually in the form of ATP hydrolysis. Sodium-potassium ATPase is an example of a pump. Cells maintain a higher concentration of potassium inside the cell than they do outside the cell. Sodium is maintained low inside, high outside. Sodium-potassium ATPase pumps three sodium ions from inside the cell to outside. This is the unfavorable direction—Na+ moves from low concentration to a higher one and against the membrane potential. At the same time, it also... 

Fig. 4.1. Cellular model illustrating cell types in vascular wall involved in vasorelaxation induced by SERMs. Putative targets of SERMs are indicated within cyan tags. SERMs directly affect L-type VDCC, BK fil subunit in smooth muscle cells, and ER in endothelial cells. L-type VDCC L-type voltage-dependent calcium channel BK calcium-activated large conductance K+ channel PKG protein kinase G eNOS endothelial nitric oxide synthase GC soluble guanylate cyclase cGMP cyclic GM P V electrochemical membrane potential ER estrogen receptor. See text for further details...

It should be noted that the electrochemical gradient across the mitochondrial membrane is not novel or unusual membrane potential across the plasma membrane of many cells is set up, due to the movement of K+ ions across the membrane. This is particularly important in muscle or nerve (Figure 9.8). 

Although artificial lipid membranes are almost impermeable to ions, biological membranes contain ion channels that selectively allow individual ion types to pass through (see p. 222). Whether an ion can cross this type of membrane, and if so in which direction, depends on the electrochemical gradient—i.e., on the concentrations of the ion on each side of the membrane (the concentration gradient) and on the difference in the electrical potential between the interior and exterior, the membrane potential. 

Kamo, N., Muratsugu, M., Hongoh, R. and Kobatake, Y., (1979) Membrane potential of mitochondria measured with an electrode sensitive to tetraphenyl phosphonium and relationship between proton electrochemical potential and phosphorylation potential in steady state. Journal of Membrane Biology, 49 (2), 105-121. 

To produce membrane depolarization, a current stimulus of sufficient intensity to exceed the outward K+ current must be appUed to the cell. If the depolarizing stimulus raises the membrane potential above a threshold value, sodium channels within the sarcolemmal membrane change their conformation and open their ion-selective pore, allowing Na to enter the cell driven by the electrochemical gradient. The open sodium channels raise the membrane potential toward the equilibrium potential of sodium (-f65 mV) and set into motion the intricate and precisely coordinated series of ion channel openings and closings leading to the characteristic action potential. 

It is, of course, not easy to make statements about the relative contributions of phase boundary and diffusion potentials. Since the electrochemical behavior of membranes is generally reflected by the total membrane potential, we did not try to differentiate in this respect. The models described in my report may, however, approximate the selectivity of certain membrane systems in the equilibrium domain even when assuming the absence of diffusion potentials. 

---