I gave this example that seems to function without any ATP !Deoxyribose said:My point is simply that without the plasma membrane, integral membrane proteins such as ion channels and pumps, and ATP, living cells could not establish a resting membrane potential.
http://en.wikipedia.org/wiki/Action_potentialMy point is simply that without the plasma membrane, integral membrane proteins such as ion channels and pumps, and ATP, living cells could not establish a resting membrane potential.
Ions cross the cell membrane under two influences: diffusion and electric fields. A simple example wherein two solutions - A and B - are separated by a porous barrier illustrates that diffusion will ensure that they will eventually mix into equal solutions. This mixing occurs because of the difference in their concentrations. The region with high concentration will diffuse out toward the region with low concentration. To extend the example, let solution A have 30 sodium ions and 30 chloride ions. Also, let solution B have only 20 sodium ions and 20 chloride ions. Assuming the barrier allows both types of ions to travel through it, then a steady state will be reached whereby both solutions have 25 sodium ions and 25 chloride ions. If, however, the porous barrier is selective to which ions are let through, then diffusion alone will not determine the resulting solution. Returning to the previous example, let's now construct a barrier that is permeable only to sodium ions. Since solution B has a lower concentration of both sodium and chloride, the barrier will attract both ions from solution A. However, only sodium will travel through the barrier. This will result in an accumulation of sodium in solution B. Since sodium has a positive charge, this accumulation will make solution B more positive relative to solution A. Positive sodium ions will be less likely to travel to the now-more-positive B solution. This constitutes the second factor controlling ion flow, namely electric fields. The point at which this electric field completely counteracts the force due to diffusion is called the equilibrium potential. At this point, the net flow of this specific ion (in this case sodium) is zero.
alxm said:Pumps do function. They're using energy, be it ATP (K/Na pumps) or some other reaction (COX).
You don't necessarily have a case where there's an equilibrium between electrical and concentration gradients, or any equilibrium at all. The inner mitrochondrial membrane, for instance, is not at equilibrium electrically or concentration-wise. (Although more so with respect to concentration because a certain amount of ion exchange occurs.)
Deoxyribose said:but in a living cell the concentration gradient is formed by ion pumps such as Na/K-ATPase moving ions against their concentration gradient.
Not 10 seconds as stated and the contribution of Na K pump is often < to 10 % of the resting potential.Contribution of the Na+ pump to resting axonal potential is estimated at -7 mV. Ouabain (10 microM to 10 mM) evoked a dose-dependent depolarization that was maximal at >/=1 mM, depolarizing the nerves to approximately 35-40% of control after 60 min.
somasimple said:http://www.ncbi.nlm.nih.gov/pubmed/9325376
Not 10 seconds as stated and the contribution of Na K pump is often < to 10 % of the resting potential.
Notice that I said the concentration gradient is formed by ion pumps such as Na/K-ATPase, not that the concentration gradient is formed by Na/K-ATPase.Deoxyribose said:but in a living cell the concentration gradient is formed by ion pumps such as Na/K-ATPase moving ions against their concentration gradient.
This does show that Na/K-ATPase is not the only ion pump involved in maintaining the resting membrane potential. It also shows that ATP is necessary to power those ion pumps.Inhibiting energy metabolism (CN- and iodoacetate) during high-dose ouabain (1-10 mM) exposure caused an additional depolarization, suggesting additional ATP-dependent, ouabain-insensitive ion transport systems.
In addition, maintenance of membrane potential is critically dependent on continuous Na+ pump activity due to the relatively high exchange of Na+ (via the above mentioned routes) and K+ across the membrane of resting optic axons.
To sustain electrogenesis, transmembrane K+ and Na+ gradients maintain axons in a polarized state and provide energy for signaling, respectively. These electrochemical gradients are established by energy-dependent ion transport systems, the most important of which is the Na+,K+-ATPase
http://www.ncbi.nlm.nih.gov/pubmed/6320455The inward movement of sodium ions and the outward movement of potassium ions are passive and the reverse movements against the electrochemical gradients require the activity of a metabolism-driven Na+/K+-pump.
Pumped and transported components of ionic flux have been added to passive electrodiffusive components.
A plot of the membrane potential versus log [K]o with an electrogenic Na pump present gives a curve with slopes both greater than and less than 58 mV per 10-fold concentration change. Over a middle range of [K]o values, the slope is 58 mV. The slope of Em versus log [K]o curves is, therefore, not a very sensitive test for the presence of an electrogenic pump.
Ion pumps influence the action potential only by establishing the relative ratio of intracellular and extracellular ion concentrations. The action potential involves mainly the opening and closing of ion channels, not ion pumps. If the ion pumps are turned off by removing their energy source, or by adding an inhibitor such as ouabain, the axon can still fire hundreds of thousands of action potentials before their amplitudes begin to decay significantly.[23] In particular, ion pumps play no significant role in the repolarization of the membrane after an action potential.[10]