# Understanding intra and extracelullar concentrations (membrane potential)

I have 4 question (not homework)

What happens to the cells membrane potential if:

a) Na+ outside rises by 40mM

b) K+ inside rises by 10mM

c) K+ outside rises by 10mM

d) A- (impermeable ion) inside rises by 100mM

e) Na+ outside decline by 50mM

a – hyperpolarisering (more negative)

b – Depolarisering (more positive)

c – Hyperpolarisering

d – Depolarisering

e - Depolarisering

I think it like this:

Membrane potential is the DIFFERENCE in electrical potential inside and outside the cell. When we say a cell has a resting potential of -60mV we mean that the cell has 60mV LESS than the extracellular space.

So when I am asked what happens if all of a sudden, the extracellular space increases in mV (first question) then I can say that the difference is now GREATER than 60 mV, it is now in fact 100 mV, as the extracellular space has increased by 40 mV, then plussing the already established 60 mV.

Is this the correct way to think about these problems?

Afaik you can describe the potentials between the two sides of the cell membrane using the Nernst-equation. Yepp wikipedia writes the same here.

Membrane potential is a very important thing, because human cells require stable internal concentrations of Na+, K+, Ca2+, etc... in order to work properly. These concentration can depend on species, for example plants cells work completely different than animal cells, they have higher Na+ than K+ in their cytoplasm. So every human cell has to maintain a membrane potential if they don't want to die. It requires a lot of ATP (energy) to do so... Human cells typically maintain this potential against the blood plasma. Cytoplasm has around 120mM K+ and 15mM Na+, while the blood plasma has about 5mM K+ and 140mM Na+. You can count the membrane potential from these concentrations using the Nernst equation (a short presentation about how). I don't remember the exact mV value, does not matter. So human cells are working hardly on keeping the Na+ outside and K+ inside, and they can possibly die because of membrane potential changes (e.g. when they require too much energy to maintain the internal concentrations and/or osmolarity).

Excitable cells, e.g. nerve cells or muscle cells are very special because they can change their membrane potential rapidly by sending out K+, Ca+ or sending in Na+. This can create small electric discharges, which can be used to send electric signals or contract muscles, etc... Different cells have different discharge diagrams, e.g. axons have something like this:

• Figure 1 - axon electric discharge diagram - source

and muscles have something like this:

• Figure 2 - muscle electric discharge diagrams - source

Summa summarum it can depend on the cell type what happens, but basicly you can count the new potential using the Nernst equation by the concentration changes you mentioned...

• According to answers from the Nernst equation, I seem to have all my answers flipped...What am I misunderstanding about the basics of membrane potential? How is membrane potential explained if not the way I see it?
– Paze
Oct 16, 2014 at 19:49
• I mean if I put more POSITIVE ions in my cell, I should get a LESS negative membrane potential?! But it seems to be the other way around??
– Paze
Oct 16, 2014 at 19:52
• I put this into a new question as it is boggling me. Feel free to answer it if you like: biology.stackexchange.com/questions/23221/…
– Paze
Oct 16, 2014 at 20:30