Forces acting on ions separated by a membrane
- ) Chemical driving force: ion’s own chemical concentration gradient exerts this force on this ion causing the ion to move from area of high concentration to area of low concentration. This force is only the result of the ion’s concentration gradient
- ) Electrical driving force: as ion’s move down their chemical concentration gradient, they generate electrical forces depending on their charge and the amount of negative/positive charges inside relative to outside
What does the electrical force exert on positive ions when the membrane potential is negative? Positive? What will this force do to negative ions?
- When negative: positive cations are drawn into the cell, negative ions pushed out
- When positive: positive cations are pushed out of the cell, negative ions brought in
What does the chemical driving force for: K, Na, Cl- and Ca do to these ions given the EC and IC concentrations seen in vitro?
- K: pushes it out
- Na: brings it in
- Cl: brings it in
- Ca: brings it in
Is the inside of a cell negative or positive in relation to the outside?
- Negative
Describe the results of the following movements to the membrane potential of a cell: A.) positive ion moving out B.) positive ion moving in C.) negative ion moving in D.) negative ion moving out
- A.) more negative membrane potential
- B.) less negative membrane potential
- C.) more negative membrane potential
- D.) less negative membrane potential
True/False. The Equilibrium potential gives us the strength of the electrical force on the ion.
- False, gives us the strength of the chemical force on the ion d/t concentration gradient
What does electrochemical equilibrium E(sub)x represent?
- Voltage where chemical driving force against ion equals electrical driving force
What is the idea of net charge neutrality?
- Macroscopic regions of a solution must have equal numbers of positive and negative charges, therefore membrane potential is only generated by an extremely small charge imbalance across a membrane and represent a small proportion of ions
What channels and ion primarily determines the resting membrane potential of a cell?
- More K channels on cells in real life, therefore K ion. Vrest sits between ENa and EK, but much closer to Ek
True/False. At rest, there is a constant efflux of K out of the cell and a constant efflux of Na into the cell with a net flux of K out.
- False, the efflux and influx described here is occurring, but the are counterbalanced to no net ion flux. Pump functions to restore ion concentration gradients while these fluxes are happening.
Why is the sodium/potassium pump electrogenic?
- Pumps 3 sodiums out for every 2 potassiums in – therefore creating a net negative.
What is the Nernst potential?
- Membrane potential that would prevent any net diffusion of the ion
Nernst equation
E(ion) = - RT/zF ln([inside]/[outside]) = - 60/z log([inside]/[outside])
- For positive ion, should be +ve
- For negative ion, should be –ve
- ** Z = charge
What does +ve equilibrium potential mean for direction a positive ion would move?
- Positive ion would move inwards making the inside of the cell positive. Remember the positive E in this case refers to the inside of the cell
What does –ve equilibrium potential mean for direction a positive ion would move?
- Positive ion would move outwards leaving the inside of the cell negative. Remember the negative E in this case refers to the inside of the cell.
Calculate: a.) log1, b.) log10, c.) log100, d.) log0.1
- a.) 0
- b.) 1
- c.) 2
- d.) -1
• just remember log base 10 is taking the number in front of it and looking at its x10 power. 10 can be written 1.0 x 10^1, so answer to log10 = 1. 0.1 can be written 1 x 10^-1, so log0.1 = -1
If K was 100 times higher inside the cell than outside, what would happen to its equilibrium potential? What is the effect of this on the RMP?
- Esub(K)= -60/z log (100/1) = -60/1 x 2 = -120 mV
- Concentration gradient would be steeper d/t hypokalemic condition and K would strongly want to move outside of the cell causing hyperpolarization.
If Na was 10 times higher inside the cell than outside, what would happen to its equilibrium potential? What is the effect of this on the RMP?
- Esub(Na)= -60/z log (10/1) = -60 x 1 = -60 mV
- Concentration gradient would be reversed d/t hyponatremic condition and K would strongly want to move outside of the cell causing hyperpolarization.
Limitation of Nernst equation? Alternative?
- Calculates equilibrium potential of single ion without taking into account the relative permeability of membrane to an ion.
- Goldman-Hodgkin-Katz equation can calculate equilibrium potential across a membrane by using multiple ions and also taking into account the relative permeability of the membrane to said ions.
What will happen to the membrane potential in a cell if the EC fluid starts increasing concentration of K?
- Concentration gradient slope for K will become less steep and K will want to stay in the cell, causing the inside of the cell to be less negative, ie. Cell will depolarize.
What is normal blood potassium level? What occurs when these levels occurs?
- 3.5-5.0 mM
- hyperkalemia = increase – leads to abnormal function of muscle, heart and nerve
What is the contribution of calcium to RMP?
- Does not contribute to RMP as cell is impermeable to it at rest
As the external concentration of K is raised, the EsubK becomes: 1.) more negative / less negative. This makes the membrane potential become 2.) more negative / less negative.
- 1.) less negative (K stays inside, making inside more positive or less negative)
- 2.) less negative (depolarizes the cell)
As the external concentration of K decreases, the EsubK becomes: 1.) more negative / less negative. This makes the membrane potential become 2.) more negative / less negative.
- 1.) more negative (K is driven out cell more, making inside more negative/less positive)
- 2.) more negative (hyperpolarizes the cell)
