 | Term | Definition |
| |
Driving Force |
The difference between the chemical gradient and the electircal gradient. |
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When DF is equal to 0 |
No net movement of Ions---equlibrium reached |
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Nernst Equation |
E = ([RT] / [zF]) In (X1/X2) |
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Nernst Equation at Room Temp |
E = [-58/z] log [conc.in/conc.out] |
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Nernst Equation defines? |
The nernst equation defines equilibrium potential. |
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Equilibrium Potential |
The electrical potential that exactly balances a chemical concentration gradient for a specific ion |
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What is z in Nernst equation |
z represents the valancy of the specific ion |
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Donnan Equilibrium states |
K+(in)*Cl-(in) = K+(out)*Cl-(out) |
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What complicates things with D.E? |
The presence of inpermeable anions in intracellular space |
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Step 1 of DE explanation |
K+ and Cl- move down conc gradients. |
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Step 2 of DE explanation |
Cl- reaches conc equilibrium at 30mM each side. However, large K+ conc grad still exists so K+ still moves out. This means Cl- has to go with it. |
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Step 3 of DE explanation |
As K+ has reached conc grad of 2:1 the Cl- has also managed to reach 2:1 (in opposite direction). The conc grads cause potential. Calculating with Nernst = -17mV |
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Why is Donnan Equilibrium important |
It gives an explanation of resting potentials. |
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What type of cells do not obey donnan effect? |
Nerve cells do not obey donnan effect |
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What equation is Sodium conc used in? |
Goldman-Hodgkin-Katz |
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Driving Force is also |
The difference between equilibrium potential and membrane potential |
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Membrane Potential |
The electrical potential across a cell membrane caused by the movement of ions |
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What happens with an excess of Sodium influx in respect to driving forces |
Higher Na movement would decrease Driving Force. This is because the membrane has moved slightly towards positive (Na is in higher concentration in the extracellular space). This causes the Driving Force for K to increase because there is now a larger differenc between membrane potential and equilibrium potential for K. This leads to more K moving out of cell and less Na into cell. Eventually the driving forces return to normal. |
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Rate of Flow |
The rate of flow for a particular ion species is simply the conductance of the membrane to the species multiplied by the driving force for that species |
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Current |
I = g*(Em-Eek) |
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Ohms Law |
V=IR |
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When the membrane is permeable to more than one ion what is the membrane potential dependent on |
1)concentration gradient 2)Absolute concentration 3)relative permeabilities |
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GHK equation |
E = -58log { ([pK*Kin]/[pK*Kout]) + ([pNa*Nain]/[pNa*Naout]) + ([pCl*Clin]/[pCl*Clout]) } |
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pK+ |
1 |
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pNa+ |
0.01 |
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Does Cl- contribute to Em in most cells? |
No |
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Why is Em close but not equal to EK+ |
This is because the membrane will have a slight pNa which makes the cell more negative than EK+ |
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Can Cl- contribute to Em? |
Yes, cells that have Na+/K+/Cl- co transporter (ascending loop of henle). If Cl- is pumped into cell and the membrane is permeable there will be outward leak of Cl-. This is a depolarizing effect |
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When should Cl- be used in GHK equation? |
Only when Cl- is not passively distributed. |
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How many of each ion is used in Na+/K+ pump |
3Na+ out for every 2K+ in. This hyperpolarizes cell membrane (more negative) |
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