Question 20

[yfangl097543]

I don't understand why the K+ has to move with the Cl-. I thought that answer A was plausible because there is excess negative charge on the side with the negatively charged proteins, so that the K+ would move to that side to neutralize the excess negative charge. Isn't that an electrochemical gradient? Why is the movement of Cl- required for the movement of K+? It seems that most of the Cl- wouldn't want to move to the side of the excess negative charge anyway.

[admin]

Consider question 19 and its explanation. At the beginning, there is no electrical gradient but then there is a net movement of Cl- moving from Y to X down its chemical gradient. The moment that one Cl- moves from Y to X, that forms the basis for why K+ would move from Y to X. You can argue that K+ is simply going down an electrical gradient (which is what you mentioned); however, it is clearly moving against its chemical gradient thus answer choice A would be incorrect.

[wallstng1442]

Why can't an active transport mechanism be a likely explanation? If a solute is moving from an area of lower concentration to one of greater concentration, I'm thinking active transport. I understand the reasoning for the keyed answer, but how am I to know to use the previous question's answer for this one (especially since I got the previous question wrong!)?

[admin]

[asfino12217]

why can't k+ move to x because of the electrical gradient?

[jonathan.g4101]

but electrochemical gradient is comprised of the combination of electrical and chemical gradient.

If something moves passively it is because of electrochemical gradient, regardless if it is up concentration gradient. That would only mean that the electrical gradient outweighs the chemical gradient.

[pperng4757]

[jellywing_2058]

As admin pointed it out, the K+ goes down the electrical gradient, but NOT down the chemical gradient.

Option A says

[quadalpha]

[nemil.shah3570]

Does "the negative charge of a nondiffusible anion hinders diffusion of the diffusible cations and favors diffusion of the diffusible anions" (from the passage Paragraph 2 Sentence 2) hinder the K+ from diffusing across its electrical gradient?

So K+ wouldn't be able to move across the membrane?

[admin]

The presence of proteins would hinder the diffusion of K from X to Y, not from Y to X. That is why the normal concentration of K is higher inside the cell, compared with the extracellular fluid.