Ions And Excitability Flashcards
Why can’t ions passively diffuse across the membrane ?
Because they are charged so they are excluded from the internal lipid environment produced by the bilayer
What pumps allows cells to have difference in ionic composition across their membranes?
Sodium-potassium ATPase
- uses ATP
- pumps 3 sodium ions out for 2 potassium ions in
- creates a high potassium and low sodium intracellular concentrations relative to the extracellular fluid
How is membrane permeability regulated ?
By changing he open probability or number of channels in the membrane to allow the passage of ions
A cell has a high intracellular potassium and chloride concentration in a solution of a more dilute KCl concentration. Some of the potassium channels open so what happens ?
Potassium ions diffuse out of the cell down its concentrati gradient. This movement creates an electrical gradient and the cell gets hyperpolarises so more and more negative until the electrochemical gradient of the potassium reaches equilibrium causing the movement of potassium ions to stop
What effect does the negative electrical potential have on the movement of potassium ions ?
It prevents the potassium ions reaching a concentration equilibrium (equal concentration of potassium ions intracellularly and extracellularly) across the membrane
What is he Nernst equation ?
Allows us to calculate the electrical potential needed to balance a given concentration gradient of an ion
RT/zF * ln Xo/Xi- for cations - for anions it is intracellular divided by extracellular
Why do squid axons have a much higher concentration of ions compared to mammalian cells ?
Because they live in water which is colder and salty so they need them to be higher to prevent them shrivelling up
If a mammalian cell had a resting membrane potential of -80mV which ion do you think it is most permeable to ?
Potassium ions because it is the only ions which can easily cross the membrane and has a negative electrical potential and the membrane potential is closest to the potassium equilibrium potential
How could you test that the membrane potential was most permeable to potassium ions ?
If you removed all the chloride ions the membrane potential shouldn’t change or if you removed all the potassium extracellularly then the membrane potential should become more negative
If the membrane is predominantly permeable to potassium and the interstitial fluid potassium concentration increases what happens to the cell ?
Potassium ions will move into the cell and increase the excitability of the cell -pushing the membrane potential closer to threshold
What direction is the current when there is an inward movement of chloride ions ?
Outward current because the membrane potential is becoming more negative
What the difference between equilibrium potential and reversal potential ?
Reversal potential is measured experimentally whereas equilibrium potential is calculated using Nernst
Theoretically what could the membrane potential become during an action potential but why doesn’t it ?
+70mV
It normally reaches +40mV because there is always some permeability to potassium ions and because as membrane potential becomes more positive it. Amuses potassium channels to open
