Concentration gradient force Vs. electrical gradient force

[ndy890]

Hi Everyone,

I was just learning about action potential generation via electrochemical gradients. I was just wondering, does anyone know whether a +1 unit of concentration gradient is stronger/weaker than a +1 unit of electrical gradient?

For example: If side-A of a split chamber had a net charge of +1, while side-B had a balanced net charge of 0. But side-B had one extra K+ ion than side-A. If the membrane was only permeable to K+ ions, the concentration gradient of side B would cause a force on the K+ ions (on side-B) to go to side-A, but the electrical gradient on side-A would cause a force on K+ ions (on side-A) to go to side-B. Which force is stronger?

What term would I have to google to learn more about the forces generated by each gradient and each marginal unit of increase/decrease of concentration/charge? - This is all very interesting to me! :)

Nate

 

[jim mcnamara]

Actually this is a pretty good first question.

I think you mean osmotic pressure for a concentration gradient. And there is a practical limit to the number of ionized K⁺ can diffuse through a membrane. The actual differences in either of these is only important over small distances across a membrane, limited by how fast Potassium or other ions can diffuse through water. There is also something called active transport - molecules move into or out of a cell against osmotic pressure through special structures in the membrane itself. Let's stop there for now.

Osmosis is your first choice here - video:
https://www.khanacademy.org/science/biology/membranes-and-transport/diffusion-and-osmosis/v/osmosis

The action potential in neurotransmission (how nerves "talk"to one another):
https://en.wikipedia.org/wiki/Neurotransmission This has nice graphics. Note the calcium channels.Now come and ask if you get confused.

 

[ndy890]

I think you mean osmotic pressure for a concentration gradient. And there is a practical limit to the number of ionized K⁺ can diffuse through a membrane. The actual differences in either of these is only important over small distances across a membrane, limited by how fast Potassium or other ions can diffuse through water. There is also something called active transport - molecules move into or out of a cell against osmotic pressure through special structures in the membrane itself.

What is the origin of this 'practical limit' to the number of ionized K+ that can diffuse though a membrane? - isn't any limit of an ion's travel from one side to the other side of the membrane just determined by the electrochemical gradient (the combined forces derived from the concentration/electrical gradients)?

If we were to exclude active transport, and just think about passive transport.. how can i determine the likelihood of whether an efflux of an influx of K+ will occur with my original question of one extra +1 net charge on side-A and one extra K+ on side-B?

My original question arose from the following picture - an example of showing the two forces in action:

 

[jim mcnamara]

Your picture shows an equilibrium state and gives you the results in terms of electric potential.
http://antranik.org/movement-of-substances-across-cell-membranes/

Look for the words maximal flux in the second graphic - that is what I refer to for maximum limits. Think of it as traffic flow during rush hour, no more room for more cars (or ions) on the road sometimes.

 

[ndy890]

Thankyou! This was all very helpful!