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Module 1 Lecture 2 (1.3-1.4) Dr. Fuller Diffusion of Electrolytes
Terms in this set (42)
What governs diffusion through the membrane?
permeability times concentration gradient
What are electrolytes?
sodium, potassium, chloride; charged species
What things must be considered when discussing the diffusion of electrolytes?
small ions, such as sodium, potassium, chloride respond to chemical gradients in the same way as do non electrolytes such as glucose. However, these ions are also charged, and so not only the chemical concentration, but the electrical potential also has to be considered when thinking about how these ions move.
What governs diffusion for charged species?
both electrical and chemical forces govern diffusion
What happens when K+ acetate(-) wants to diffuse across the membrane?
Potassium is very small and moves very quickly whereas the acetate is large and much slower to move. Once you have these 2 paired in a solution the potassium is going to move very quickly but because we can't violate electroneutrality, its also going to want to take acetate with it. Potassium is positively charged whereas acetate is negatively charged. Thus they form a dipole where K+ is in front and acetate is lagging behind. They do still move across the membrane as a pair however.
Why would potassium move with acetate across a membrane when both are paired in a solution?
Because it can't violate the law of electroneutrality. The concentration gradient is still the driving force of this however.
What is a dipole? What does a series of dipole generate?
a dipole is a pair of electric charges separated by a small distance. A series of dipoles will generate a diffusion potential. Eventually equilibrium will be reached and Cs1=Cs2. This would mean there is no potential difference across the membrane.
T or F. If K+ and acetate were paired together in a solution, there is a point in time in which one acetate (part of the dipole) is on one side of the membrane while the K+ (part of the dipole) is on the other side of the membrane.
TRUE; however they do cross over together. The K+ is just a little ahead of the acetate. This actually creates a diffusion potential
How is a diffusion potential generated?
If K+ and acetate were paired together in a solution, there is a point in time in which one acetate (part of the dipole) is on one side of the membrane while the K+ (part of the dipole) is on the other side of the membrane. This is what forms the diffusion potential
What would happen if you had K+ and acetate together in a solution and you have 2 compartments but the membrane in between them is only permeable to potassium?
Under those conditions potassium would continue to diffuse until enough potassium is on the 2nd side of the membrane to retard the further movement of potassium. This is because the charge on the ion opposes further movement (like repels like). At that point, further movement of ion will stop, irrespective of the concentration gradient. This point is called the equilibrium potential. This situation only occurs when the membrane is permeable to only a single ion in a dipole pair.
When does equilibrium potential occur?
the charge on the ion opposes further movement (like repels like). At that point, further movement of ion will stop, irrespective of the concentration gradient. This point is called the equilibrium potential. This situation only occurs when the membrane is permeable to only a single ion in a dipole pair.
T or F. You cannot violate electroneutrality
FALSE, you can but its in such a microscopic way that you don't change the bulk solutions electroneutrality. This happens only at a very minimal level.
When the membrane is permeable to only one of the ions in a dipole a __________ is reached. Here the chemical and electrical ___________
an equilibrium potential is reached. Here the chemical and electrical driving forces are equal and opposite. The Nerst equation is the membrane voltage at which these 2 driving forces are equal and opposite.
What is the Nernst equation and when would you use it?
This is used when a membrane is only permeable to one ion, but there is technically 2 oppositely charged ions in a solution. However, only one ion can cross the membrane. When the membrane is permeable to only one of the ions in a dipole a equilibrium potential is reached. Here the chemical and electrical equal and opposite. The Nerst equation is the membrane voltage at which these 2 driving forces are equal and opposite. There also must be NO NET OR FURTHER MOVEMENT OF ION
Which of the following are invariant?
A) Diffusion potentials
B) Equilibrium potentials
C) Both A and B
D) Neither A or B
The answer is B; equilibrium potentials do not change with time. A diffusion potential changes with time because when you have 2 ions that are permeable; as the ions diffuse down their gradients and the gradients equilibrate then eventually that diffusion potential is going to drop to zero because you have equal amounts of ions on both sides of the membrane.
T or F. Cells do not get to equilibrium potential.
This is actually true
________ is satisfied for ions at equilibrium and is used to compute the electrical force that is equal and opposite to the concentration force.
A) Diffusion potential
B) Starling Forces equation
C) Nerst equation
D) Fick's first law of diffusion
The answer is C.
Most of the negative ions inside of biological cells are which of the following?
B) Acetate -
The answer is D, proteins
Lets say we insert a K+ leaky channel into the membrane of a biological cell, making the cell freely permeable to only K+. What is going to happen to the K+ along with any other typical ions found in and outside of a biological cell?
K+ starts to move out of the cell down its concentration gradient. The A- (generally proteins) cannot follow because the cell is not permeable to A-. Then the K+ builds up a little excess positive charge outside the cell. There will also be a little negative charge buildup inside the cell. this will continue until the tendency of K+ wanting to leave the cell balances the tendency of K+ wanting to go back into the cell. The K+ is being drive back in by the net negative charge that is now present inside the cell. This will pull K+ in the opposite direction of its concentration gradient.
What would be reached if the concentration gradient sending K+ out of the cell is exactly opposed by the electrical gradient pulling K+ into the cell? What is driving the influx and efflux respectively in this situation?
This would be electrochemical equilibrium. The efflux is due to concentration gradient while the influx is due to electrical gradient.
T or F. The generation of a membrane potential does violate electroneutrality.
FALSE; even when there is potential difference across a membrane, charge balance of the bulk solution is maintained. This is because potential differences are created by the separation of just a few charges adjacent to the membrane.
If theres only a K+ channel in a biological membrane, would the whole cell get to the Nerst equilibrium of K+?
YES it would
How much ion yields a membrane potential of 100mV?
10^-12 M (1 picomole) of ion (very very small amount)
When we record membrane potentials for a cell.. are they positive or negative? Why is this?
This is always going to be negative. The reason is that the membrane permeability for any real cell is much greater to K+ than any other positive ion like Na+. Because K+ dominates that permeability, the membrane potential will always be closer to the Nerst equilibrium potential.
What are reversal potentials? Why are they called that?
These equilibrium potentials are sometimes referred to as reversal potentials. If we have a membrane and is only permeable to Na+, Na+ has diffused across the membrane and built up a net positive charge on the inside. If you take a back tray or something and feed even a tiny amount of positive charge into that same compartment, this is going to drive the movement of Na+ back across the membrane. Thus we have reversed the flow.
What does z stand for in the Nerst equation?
It stands for valence of the ion (positive or negative and then number).
What is the answer to log 10/100?
This would be -1
What is the answer to log 100/10?
This would be +1
What is the Goldman-Hodgkin-Katz equation? What does it determine?
The constant field equation allows us to compute voltage across a membrane permeable to more than one ion. This is like a real life equation that can calculate membrane potential. Nerst isn't exactly realistic.
What are the main ions that are the most important in determining membrane potential?
Na+, K+ and Cl-; these are the ones used in the GHK equation.
What would you have to change the Nerst equation to in order to calculate the membrane equilibrium potential for Calcium?
We would need to change z to +2
How do you calculate the driving force on an ion across a cell membrane?
You take the membrane potential (Vm) minus the equilibrium potential (Ei) for that ion
-- this is what the electrical force driving the ion into or out of the cell
If a cation has a VDf (sign of driving force) that is negative what will the direction of ion flow be going?
Into the cell
If a cation has a positive VDf (sign of driving force) what will the direction of ion flow be going?
That ion would be moving out of the cell (Happy cat(ions) go outside)
If a anion has a VDf (sign of driving force) that is negative what will the direction of ion flow be going?
it would be going out of the cell
If a anion has a positive VDf (sign of driving force) what will the direction of ion flow be going?
It would be going into the cell
How do we measure membrane potential differences in real life?
We stick an electrodes into the cell; this records the voltage in the cell. Then we have an electrode in the solution (called ground) that is basically a reference electrode. A voltmeter measures the difference between the 2. The membrane potential can change over time as shown in the chart.
What is resting membrane potential?
-70mV; this is close to K+ equilibrium potential (its the most permeable ion)
What happens if we go below the membrane potential difference? Like if we become more negative than the resting membrane potential?
This is called hyperpolarization
What happens if we become more positive than the resting membrane potential?
depolarization would occur
A membrane potential in some neurons is around -90 mv. If K+ has a -93 mV equilibrium potential what does this mean?
This means that this membrane is essentially the only permeable ion to these membranes.
A membrane potential in a specialized cell is around -40 mv. If K+ has a -93 mV equilibrium potential what does this mean?
This means that the membrane is mostly permeable to K+, but there are other ions involved (or that are permeable)
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