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I've been reading a book called Principles of Human Physiology by Stanfield 5th edition, and was reading a chapter on membrane transport when I came across a figure question which will be posted below.

It says "As time passes, does the one-way flux from side 2 to side 1 increase, decrease, or stay the same?"

I have unfortunately been unable to comprehend why and how the answer is "Increase". I would suppose it stays the same, correct? There is no possible way that a flux would occur, unless by individual molecules (regularly) passing through each side on a equal basis.

It was a topic on Simple diffusion.

P.S. This book, as i've noticed hasnt been quite explaining some things in a clear-cut way to understand as a beginner, and also it has had some mistake within it.

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  • $\begingroup$ Does it help to emphasize that the question is asking about flux from side 2 to side 1? It's not asking about flux from side 1 to side 2 or net flux. $\endgroup$
    – canadianer
    Commented Sep 12, 2015 at 3:10
  • $\begingroup$ Keep in mind that diffusion is just the random movement of particles. Over time, the solute will randomly distribute throughout the entire available volume. Under initial conditions, there is no solute in side 2 and so it cannot move to side 1 (thus flux from side 2 to side 1 is 0). At equilibrium, the net flux is 0 but there is still flux from side 1 to 2 and from side 2 to 1 (which is equal and opposite) due to the random motion of the solute. So, flux from side 2 to 1 goes from 0 at initial conditions to some non-zero value at equilibrium (ie it increases). $\endgroup$
    – canadianer
    Commented Sep 12, 2015 at 3:17
  • $\begingroup$ @canadianer You can post that as an answer. However this is a physical chemistry question and off-topic. $\endgroup$
    – WYSIWYG
    Commented Sep 12, 2015 at 5:28

2 Answers 2

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Diffusion is just the random movement of particles. Over time, the solute will randomly distribute throughout the entire available volume. Under initial conditions, there is no solute in side 2 and so it cannot move to side 1 (thus flux from side 2 to side 1 is 0). At equilibrium, the net flux is 0 but there is still flux from side 1 to 2 and from side 2 to 1 (which is equal and opposite) due to the random motion of the solute. So, flux from side 2 to 1 goes from 0 at initial conditions to some non-zero value at equilibrium (ie it increases).

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This is also what is refered to as steady-state. Or dynamical equilibrium. Overall, the net flux is zero, but still there's constantly flux from side 2 to side 1, even more so when more solutes are in side 2.

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