laya533

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I am a little confused about how do we get the negative amount for cell membrane potential. Is it due to having more positive charges outside the cell? I ran to this problem b/c I did not get the answer to the following Q:

The extracellular K level has increased. What will happen to membrane potential.?
In the answers it is described that b/c the exterior k is more than interior so the K does not have to go out and so the membrane potential will become more positive.
But: is memrane potential only described relative to inside k ? isnt it differencr between out and in?if it is the difference, so when out has increased it mean we have less difference between in and out and so the membrane potential should increase.
I think I am confusing you all . Please help me. Thanks
 

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laya533 said:
I am a little confused about how do we get the negative amount for cell membrane potential. Is it due to having more positive charges outside the cell? I ran to this problem b/c I did not get the answer to the following Q:

The extracellular K level has increased. What will happen to membrane potential.?
In the answers it is described that b/c the exterior k is more than interior so the K does not have to go out and so the membrane potential will become more positive.
But: is memrane potential only described relative to inside k ? isnt it differencr between out and in?if it is the difference, so when out has increased it mean we have less difference between in and out and so the membrane potential should increase.
I think I am confusing you all . Please help me. Thanks
The way it works is that an equillibrium exists between the Na+/K+ pump, and the "leak" rates of K+ and Na+ along their respective electrochemical gradients. Typically Na+ has a net electrochemical gradient favoring flow INTO the cell, while K+ has a chemical gradient favoring flow OUT of the cell (for K+ it's only a chemical gradient that favors a flowing out of the cell, since the inside of the cell is negative (making it's electrical gradient favoring staying put inside the cell- since opposites attract). But the K+ CHEMICAL concentration gradient is a slightly larger force favoring movement outwards.

So, if "all the sudden" there becomes a large concentration of K+ extracellularly, the chemical gradient favoring K+ to move from out of the cell no longer exists. In fact, the gradient will then favor K+ moving into the cell. And this will cause an imbalance in the equillibrium stated above, since K+ will now want to "leak" into the cell (through leaky channells), therefore making the cell less negative, and more positive.

Also, keep in mind that DNA is always inside the cell. DNA has all those negative phosphate groups, so the inside of the cell should always be considered to be negative at rest.

I hope this helps. I believe the situation is actually a bit more complex, but the above should be good "logic" for the MCAT.
 

medanthgirl

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yup- it's something the mcat loves b/c it's literally a combo of physics/bio/and chem all in one concept, i've heard that's true of other things, like blood pressure, etc.
 
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laya533

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So let me see if I have got it right:
If K increases inside the potential become more +
If Na increase inside the potential becomes more +
If K leak out more than Na leak in the potential becomes more - (like in a normal resting cell)
If pump of K/Na does not work,(K does not come in and Na does not go out) so potential becimes more -
right?
if all these are true then tell me if the potential becomes more positive, will threshold change or remain the same?
thanks
 

Daichi Katase

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laya533 said:
So let me see if I have got it right:
if all these are true then tell me if the potential becomes more positive, will threshold change or remain the same?
thanks
no...threshold remains the same. that is why the leaky cardiac muscles are able to self stimulate.
 

Turkeyman

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The super basic way i think of it is that proteins, macromolecules, and other components of the cell give the intracellular environment a negative charge naturally. So in effect, there have to be positive ions inside to counteract this large negative, though not fully, so the internal membrane potential at rest is still around -65 mV or so. Whenever you lose positive charge (K+ flows out of the cell), you are letting those damn macromolecules have their damn cake and eat it too(bastards), so the membrane potential goes even more negative.

When postive charge flows INTO the cell, aka Na+ during an action potential...you are overtaking those damn macromolecules and their negative charges (rebellion!) and going positive.

And just remember after any flux of ions across the membrane, K+ leakage channels and the Na+/K+ pump re-establish the natural negative membrane potential.

But yeah...sorry...to answer your initial question --> We have a negative membrane potential simply due to the fact that we do things in terms of the internal charge of the cell, and of course for the reasons above.

Lastly, the threshold is something that has to do with action potentials, and that never changes for a certain type of cell. It is simply the level of positive charge increase(depolarization) needed for an action potential spike, which is an all-or-nothing occurrence.

Again, this is a really dumbed down explanation...and it's 7am and I haven't slept yet tonight so I'm gonna go get some shuteye! hooray! :good:
 

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laya533 said:
I am a little confused about how do we get the negative amount for cell membrane potential. Is it due to having more positive charges outside the cell? I ran to this problem b/c I did not get the answer to the following Q:

The extracellular K level has increased. What will happen to membrane potential.?
In the answers it is described that b/c the exterior k is more than interior so the K does not have to go out and so the membrane potential will become more positive.
But: is memrane potential only described relative to inside k ? isnt it differencr between out and in?if it is the difference, so when out has increased it mean we have less difference between in and out and so the membrane potential should increase.
I think I am confusing you all . Please help me. Thanks
Only potassium is permeable to the cell membrane in the resting state.
It has a high internal concentration, but it can leave.

Once the concentration gradient has equalized, it remains with a negative charge on in inside, due to the missing positive K.
 

petesays

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i've always thought of the potential thing this way:
that the greater the difference in concentration, the higher the potential, the greater 'potential' for the system to do work.
i asked a prof. one time about this, saying, "can i think of potential as potential to do work?" and he said that i was exactly right.