Question #3

Q: Do I need to use the Nernst equation to calculate the overall electrical force/gradient?


A: You are supposed to use other resources and what you've learned from lecture. So yes, you can use the Nernst equation and other outside resources to answer the last part of question 3.

Homework questions 1 and 2

These questions are similar to Lab questions 14 and 15.  The membrane is permeable to K+.

Homework question 3

You are to answer this question based on the cell's overall electrical gradient, not Cl's individual electrical gradient.

Q:    I am wondering if you can clarify Q15 on the bean lab we did this past week. 

This was my answer to that question: 

The cell will become more positive, because it is gaining K+..is this correct?

The resting membrane potential difference will decrease..this is what I do not understand. If the difference decreases how is it going closer to zero, when it is already negative?

A:   Yes, the membrane potential in question 15 does become more positive.

As for the second part of your question, this concept is a bit tricky to understand at first, so I'm glad you asked this question!

Remember that every time we say "membrane potential" we actually mean "resting membrane potential difference" (see page 17 of lab manual for explanation). Don't forget that "difference" because that is the most important part (I always have to remind myself!).  

The membrane potential simply measures the difference in charge between the ICF and the ECF. 

Because we always set the ECF charge to 0, a positive membrane potential means the ICF is more positive than the ECF. A negative membrane potential means that the ICF is more negative than the ECF. 

But, in this case, it doesn't actually matter what sign (+ or -) the membrane potential is, the most important thing is how big the number (i.e. absolute value) is. The larger the number, the larger the difference in charge between the two sides of the membrane. If the membrane potential is 0mV, that means there is no difference in charge between the ICF and the ECF.

So in this case, we initially have a negative membrane potential (lets say -10mV, just as an example). This means the ICF is more negative than the ECF. 

Then suddenly the membrane potential becomes more positive (Let's say from -10mV to -8mV). This means that the the ICF becomes less negative, right? So the absolute value ( which is 8) of the membrane potential becomes smaller, i.e. closer to 0.

You can also look at it another way. If you use the number line on page 17. As you can see, -8mV is closer to 0 than -10mV.

  I---I---I---I---I---I---I---I---I---I---I---I

-10     -8                               0

Q: When I am looking at the membrane potential moving closer or farther away from zero mV to determine if is depolarize or hyperpolarize, am I supposes to looking at the zero mV pertain to the ECF?  

A: In general, yes, consider the ECF to be 0mv.

However, in this exercise, you don't need to know the exact number of mV on each side. You just have to know if the difference between the two sides is becoming larger or smaller.

Q:  What is repolarizes?  What is action potentials and when does it occur so that all cells are firing it?  Is the resting membrane potential of the cell, the ICF?  In question 3, do I have to use the information that Cl concentration in the ECF is 150mM and the Cl concentration in the ICF is 15mM, so that I can solve this problem?  Lastly, is it alright that you explain to me about the open and closing of Cl channels?  I really don't understand it.  The only thing I get from it is that it needs to be trigger into being the open state.  I'm assuming that the Cl channel only allows Cl to enter into the cell. 

A: My answers to your questions are below:

What is repolarizes?  What is action potentials?

This animation explains it better than I can through email (see link below). I also expect that Dr. Rechs will get to action potentials next week, if he hasn't already. There is also an explanation in your text book.

http://outreach.mcb.harvard.edu/animations/actionpotential.swf

 and when does it occur so that all cells are firing it? 

To fire an action potential, the cells need a stimulus.

 Is the resting membrane potential of the cell, the ICF?  

I'm sorry, I don't quite understand what you are asking here. The ICF simple means "intracellular fluid." Resting membrane potential is explained in detail on page 17 of your lab manual. 

In question 3, do I have to use the information that Cl concentration in the ECF is 150mM and the Cl concentration in the ICF is 15mM, so that I can solve this problem? 

Yes, you can use those numbers if you want. The important thing is that you know that there is a higher Cl- concentration in the ECF than the ICF.

Lastly, is it alright that you explain to me about the open and closing of Cl channels?  The only thing I get from it is that it needs to be trigger into being the open state.  

The only thing you need to know is that the Cl channels are normally closed and GABA opens these Cl channels. GABA is the trigger.

I'm assuming that the Cl channel only allows Cl to enter into the cell.  

Yes, that's right.