Lecture 5 – IONIC BASIS OF MEMBRANE POTENTIAL Flashcards
Concentration Gradients:
- A hole in the membrane separating two solutions
- One high concentration and one low concentration – to no net movement
- Molecules will move from high to low by diffusion – leading to equilibrium
- Only the concentration gradient determines the direction of movement
Ion Gradients in Cells:
- Inside of cell is negative
- Lots of negatively charged ions outside of cells BUT it is because there is a lot of negatively charged proteins inside the cell that balance out the negative and positive ions across the membrane
- Sodium and calcium will move into the cell
- Cell interior is -70mV negatively charged with respect to the outside
What direction do ions move?
- Energy (work) due to electrical gradient
2. Energy (work) due to concentration
Energy (work) due to electrical gradient
- A definition of a volt: if the electrical potential is 1 VOLT, it takes 1 JOULE of work to move 1 COULOMB of charge
There are F Coulombs of charge in 1 moles of univalent ions
- To move 1 mole of z-valent ion through a membrane potential of Vm Volts takes:
- EQUATION: Work (JOULES) = z. F. V(m)
Energy (work) due to concentration
- To move 1 mole of substance from a concentration c(i) inside the cell to c(o) outside the cell takes:
- EQUATION: Work (JOULES) = R.T.ln (ci/co)
- Ln 1 = 0
- So, if c(i) = c(o) then no work is needed
R is the gas constant and T is temperature
TOTAL WORK = z.F.V(m) + R.T.ln(ci/co)
- Work > 0
- Energy is needed to move ions across the membrane by active transport - Work < 0
- Energy is released when ion moves across the membrane which occurs spontaneously - Work = 0
- No energy is required or released at equilibrium
The Nernst Equation:
- Calculate the K equilibrium potential (Ek)
- When resting potential (RP) = value defined by Nernst Equation then K ions are at equilibrium and no tendency for them to move
- Potential is defined as Ek
- The equilibrium potential is the voltage at which the membrane potential balances the concentration gradient
- Also known as the reversal potential where there is no net movement of the ion
RP = 0mv
K+ leaves down concentration gradient (no electrical)
K+ exit makes the inside of the cell more negative
Movements of ions required are too small to affect concentration
RP is negative
K+ still leaves down concentration gradient but electrical gradient opposes this and slows it
Further K+ exit increases electrical gradient
RP = (-)80mV = Nernst Equation
Electrical gradient exactly balances the concentration gradient
No net K+ movement
Equilibrium
Na+ can also cross the membrane: consequences for RMP
With a negative RMP Na+ will enter down both electrical and concentration gradients
- Na+ would only be at equilibrium at Na+ equilibrium potential
- Permeability to Na+ means that RMP is more positive than Ek
Pk»_space; PNa so RMP is much closer to Ek than ENa
- If PNa increases, RMP will become more positive
- Use Nernst Equation ->
- Potassium membrane is -80mV so is more negative but neither is really at equilibrium
At rest, most cells are much more permeable to potassium than other ions
- With finite permeability to Na+ and K+, the RP is not an equilibrium but matches net fluxes of Na+ and K+ which is restored by active transport (Na/K pump) and powered by ATP
- small, but finite, permeability to sodium ions moves the RMP to a value slightly more positive than Ek (Big idea 2)
Capacitors:
- device for storing energy via separation of electrical charge
- charge stored on two electrically charged plates separated by an insulator – attracted to each other
- used in situations where it is necessary to capture and discharge a high voltage quickly
- defibrillators are a good example (also camera flash, Tasers and lightning)
Capacitors (extra):
- hardly any ion movement is required to charge the capacitor and set up the membrane potential
- there is almost no change in ion concentration
this has important consequences
