yulin week 2-3 - Recap on Nernst Equation Approximate...

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Recap on Nernst Equation Approximate equilibrium potentials for ions K+: -90 mV Na+: +61 mV Cl-: -61 mV Ca++: +122 mV * What the Nernst equation is doing is essentially a conversion of chemical gradient to the corresponding electrical gradient that is required to completely offset this chemical gradient. Therefore, if the membrane potential equals to the equilibrium potential for an ion species, there would be no net movement for this ion species across the membrane.
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What does a positive or negative value of equilibrium potential indicate? The relative chemical concentration gradient of an ion species across the cell membrane. The outside of the cell is the “0” The direction of an electrical potential always goes from “+” to “-” A cell has a -90 mV equilibrium potential for potassium means that an inward electrical gradient needs to be set up in order to balance the chemical concentration gradient, which means this cell has an outward chemical concentration gradient (more potassium inside than outside).
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Question Suppose you are doing research on an alien cell that is mostly permeable to an unkown element X+. You were told that the equilibrium potential for X+ ion across the membrane is +50 mV. What can you tell about the concentration gradient for X+ across the membrane? In other words, are X+ more concentrated on the inside or the outside?
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Answer A +50 mV potential difference indicates that the direction of electrical potential is outwards (pointing from +50 mV to 0 mV), which means that the concentration gradient is pointing inwards. X+ carries a positive charge, so it is more concentrated on the outside (if X carries a negative charge, then it is more concentrated on the inside). Alternative way of solving this problem Substitute +50mV and Z=+1 into nernst equation For the purpose of getting a good grade, it is always easier to simply remember when and how to use an equation. However, in order to learn, one has to make intuitive sense of the logic behind the equations, a process that takes time but can be rewarding.
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Membrane Potential
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What is membrane potential? In a typical resting neuron, the cell membrane has an uneven distribution of charge across the membrane (ex. with greater negative charge on the inside of the plasma membrane). The resting membrane potential is a result of this voltage difference across the membrane, and it can be measured by a voltmeter. The value of resting membrane potential is determined by all permeable ion species that can cross the membrane. A typical neuron has a -70mv resting membrane potential.
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How is membrane potential established and maintained? What does one need in order to establish membrane potential across a barrier?
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