a stupid quest about electrochem gradient

[nope80]

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I'm not sure i completely understand this. in the case of osmosis, how does the electro part come in? I thought water just moves based on amounts on either side of the membrane?
I also don't understand the electro part when it applies to ions. If you have more Cl- on one side of the membrane it will want to move down its concentration gradient until both sides have equal amounts so then wheres the electro part?

this is really simple i think but i just dont get it...

thanks!!

 

[UCLAstudent]

For an uncharged species (i.e. glucose), you only have to concern yourself with the concentration portion of the gradient (i.e., the relative concentrations on either side of a membrane). A gradient doesn't have to include an electric component. In this case, equilibrium would be reached with equal concentrations of the species on each side of the membrane. However, for a charged species, you also have to take into account that like charges repel each other. A negatively charged Cl- doesn't WANT to be next to another Cl-, so that will influence the gradient.

As a simple example, say that on one side of a membrane, you have lots of Na+ which cannot diffuse through the membrane. Say that on the other side of the membrane, you have lots of Cl- that CAN freely diffuse. The concentration of Cl- will adjust until it is GREATER on the Na+ side, since the excess positive charges can relieve/soothe some of the buildup of negative charge on the other side of the membrane. If there were no Na+ present, the concentrations of Cl- would eventually balance out on both sides of the membrane, since there would be no "incentive" to have a greater concentration on either side. Hope this helps! 🙂

 

[cfdavid]

A couple things to keep in mind: Try thinking of it this way. This is strictly for MCAT purposes and is very general, and is the "normal" state of affairs with respect to a cell. Things change when cells undergo stimulations (actions potentials etc.), but it all relates.

1) "salt"(Na+ Cl-) is always in higher concentrations outside of the cell.

2) Potassium K+ is always higher inside of the cell.

The inner membrane of the cell is slightly negative compared to the outside (so much Na+ on outside-but forget about the Cl-). Think of all the negative DNA, RNA, and proteins INSIDE the cell, making it neg.

Remember the Na+/K+ pump. Thus Na+ is pumped out. K+ is kept/pumped in.

So, given that the cell has all the negatively charged DNA, RNA and proteins inside of it (making it neg), K+ likes being there from an electrical standpoint (opposites attract). However, it has hardly any concentration on the outside. So, chemically, it wants to move out to balance it's concentration gradient. Thus, as it turns out, the NET electrochemical gradient for K+ is slightly favoring moving out of the cell (slightly favors concentration/chemical gradient over it's electrical gradient).

Also, Na+, since it's highly concentrated outside of the cell, with much less conc. inside, as well as the fact that it's positive and the inside of the cell is neg. follows BOTH it's electro and chemical gradient to flow into the cell.

You can apply the same logic to the other common ions. I hope this helps.