Reflection coefficient (osmosis)

  1. hello ppl,

    i have a question about the reflection coefficient....

    since the RC indicates the permeabilty of a membrane for a specific molecule....doesnt that mean (If the RC between 0 and 1) that the concentration of that molecule on both sides of the membrane will become the same after a while ??i mean at first there will be a concentration difference but the molecules will keep diffusing to the lower concentration side of the membrane till the concentrations become the same and thus there wont be any osmotic pressure more...

  2. jcsd
  3. Andy Resnick

    Andy Resnick 5,943
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    I've forgotten the exact definition of the reflection coefficient, but conceptually, an osmotic jump (for an aqueous solution) is equivalent to a hydrostatic pressure jump. Semipermeable membranes can support on osmotic jump (Donnan equilibrium).
  4. epenguin

    epenguin 2,301
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    Gold Member

    If helpful I think the answer is, in a word, yes! Unless the reflection coefficient is 1.

    The closer it is to 1 the closer thermodynamic equations will be to reality (useful) for short enough times.

    If you think about it probably there is a similar idealisation in most or all thermodynamic treatments. The rigid containers of gases and pistons of theory do not allow any gas to escape, and it and the container are certainly not in true equilibrium with the rest of the universe. In the long run your membrane might dissolve or disintegrate. Or in chemical equilibria an equilibrium between A, B, C and D is treated when they could theoretically form thermodynamically more stable E and F, perhaps in millions of years, just their rate for getting there is so slow it can be ignored. Rather than true equilibrium we are always treating a constrained equilibrium involving only certain processes it seems to me, in your case the devices are noticeably less ideal so this is more obvious.
  5. thx..that helped a lot :)
  6. Andy Resnick

    Andy Resnick 5,943
    Science Advisor
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    I found the reflection coefficient defined here, and seems to imply how selectively permeable the membrane is:

    The reflection coefficient tells you how efficiently an osmotic gradient can be maintained. A reflection coefficient of '1' means the membrane is (selectively) impermeable, while a reflection coefficient of '0' means the membrane is very permeable.

    The osmotic gradient across a membrane can be a steady-state condition, and is in fact how biological membranes function. Osmotic gradients are maintained by a variety of transporter proteins that use the free energy of ATP to drive ions against their concentration gradient. Mitochondria perform the reverse: they synthesize ATP using an osmotic gradient (Peter Mitchel''s chemiosmotic theroy)

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