View Full Version : Nernst Equation Help Needed

enigma00183

05-19-2008, 06:14 PM

I'm not sure if this is the right place to put it, maybe someone here can help me with the problems below:

1. Chamber A has a 100 mM solution of KCl. Chamber B has a 1 M solution of KCl. Chamber B is -70 mV relative to Chamber A. What is the net force and direction of K+?

2. What is the net force and direction of Cl-?

1a. 8.5 mV from Chamber A to Chamber B

2a. 131.5 mV from Chamber B to Chamber A

I can calculate the equalibrium potentials just fine, but I can not figure out for the life of me the net force or direction by using the Nernst Equation. However, I can figure out the directionality intuitively, but I still have to get the correct net force to get the problem correct. Answers are hidden (just need to highlight).

I know this seems like a really dumb problem, but I have an exam next week and my prof can't explain it well enough...if that makes any sense. TIA!

drymor

06-02-2008, 12:16 PM

1. Chamber A has a 100 mM solution of KCl. Chamber B has a 1 M solution of KCl. Chamber B is -70 mV relative to Chamber A. What is the net force and direction of K+?

2. What is the net force and direction of Cl-?

So If you wanna know the Net Force you need 2 things :

1.-Concentracion Force o = E (ion)

2.- Electrical Force = Em

To get # 1

E ion you need to use Nerst Equation

thus,

Eion = -61 mV Concetration of ion Intracellular

----------- x log10 ----------------------------------

z Concentration of ion Extracellular

where z would be the charge of ion and valence.

so in your case

E k+ = -61 mV 100

--------- x log10 -------- =>

z 1 since the ion is K+ (potasium)

Z = + (positively charge) 1 (valence of potassium) = +1

Ek+ = -61 mV

--------- x 2 =>

+1

EK+ = -122mV

so the concentracion force would have a value of

122 mv

The direcction of this force will be determine facing the concentration in both chamber, since chamber A has a higher concentration, potassium would flow from A to B.

So :

Concentracion Force :

Value = 122 mV

Direction = A------->B

To get # 2

You need to pay attention to the membrane resting potential.

so in this case it is - 70 mV

Em value would be 70 mV

to Know direction, now you suppose to face the principal that says, like charges attracts each other and different charges repels each other. Therefore, since the potencial depend on B, this negative charges that are in B will attract the potassium to his side since potassium his a cation ( positively charge ion), so direction would be:

A -------> B

so Electric Force :

Value : 70 mV

Direction : A-------> B

Now to calculate Net force:

We needed Concetration Force and his direction, Electrical force and his direction. Now the principal says that if you have to forces and you want to know the total force, if there have same direction, you sum both and direction would be the same direction of both. But, if they have different direction, you take the least value force the highest value and direction would be the direction of the highest value one.

Since in our case they have same direction, we will sum both, and direction would be direction of both.

so

Net Force = 122 mV + 70 mV= 192 mV

Direction = A----------> B

For chloride, the net force is O mV, since chloride move across the membrane with both Na+ and K+ as NaCl and KCl so it is on a equilibrium state on both chambers always.

The only things you could determine is the Electrical force of Chloride using the same principal we mention before, since we have a membrane potential 0 - 70 mV depending on B chamber, so B chamber repels Cl ion since it is an anion (negatively charge ion ), therefore:

Direction : B----------A

I hope a help you a lil bit,

Good luck in studies

Drymor

siheg

08-02-2008, 07:40 AM

God, I hope theres not alot of math is med school. I want Bio!

CombatMedic89

11-19-2008, 05:11 AM

... face the principal that says, like charges attracts each other and different charges repels each other.Therefore, ...

It should be like charges repels each other and different charges attracts each other.

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