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When does osmosis stop

  1. Feb 12, 2018 #1
    I have a question about osmosis that goes a bit deeper than most basic textbooks so I cannot find the answer.Here's the problem:

    Imagine an animal cell, say a red blood cell, in a slightly hypotonic solution. The water starts to flow in osmotically. The concentration of solute in the cell decreases. The cell slowly starts to swell but does not burst.

    The question is: Does this water entering increases the pressure on the membrane (similar to turgor in plant cells only less, because the cell can increase in volume - comparable to elastic energy that must be overcome when blowing a baloon...) and does this cause osmosis to stop BEFORE the concentrations come to equilibrum (meaning that cytoplasm still has a bit more solute concentration than outside of cell) or do the concentrations perfectly match?

    Thanx in advance.
  2. jcsd
  3. Feb 12, 2018 #2
    Is it not already defined? Homeostasis. The limits to osmosis isn't assumed to be the elasticity necessarily. I am not a biologist, but in my first year university we shared common courses. Osmoregulation -osmoconformers and osmoregulators. You need to look here i believe -if they still call them that. Not much help but points you in the right direction to help your self.
  4. Feb 12, 2018 #3
    Forget cell's input - let's keep it as simple as possible. Imagine that instead of a real cell we have a phospholipide bilayer - liposome, impermeable to solutes. When does osmosis stop in a SLIGHTLY hypotonic solutions. Do the concentrations match equally or osmotic pressure matches pressure inside of liposome so inside is still more hypertonic than outside?
  5. Feb 12, 2018 #4


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    Since pressure can stop or reverse osmosis, then presumably any small increase in pressure will cause equilibrium while there is still a small difference in concentration.
    The strength and elastic properties of the membrane should determine what happens for the given concentration difference.
    I know nothing about the strength of cell membranes. Are blood cells really much more elastic than plant cells? Can they sustain much pressure?
    Can a cell say double in size? If so, isn't it safe in solutions up to half the concentration of its intracellular fluid.
  6. Feb 12, 2018 #5


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    From Lodish's Molecular Cell Biology textbook:

    If the swelling does not result in bursting, there will be some combination of dilution and osmotic pressure that balances out the initial difference in solute concentration between the inside and outside of the cell. My intuition would be that osmotic pressure is the bigger factor (since cell volume cannot increase too much without bursting), but I'm not sure whether the relative contribution of each has been quantified. Cells may also possess some active transport mechanisms to pump ions out of the cell to help prevent osmotic lysis.

    For cells with a rigid cell wall (e.g. plant cells), it is definitely the case that osmotic pressure is the main factor that stops the net diffusion of water into the cells.
  7. Feb 12, 2018 #6


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    (Edited ot an incomplete sentence here.)
    If your liposome can expand without increase of internal pressure, osmosis stops when the concentrations match.
    If there is any increase in pressure, surely osmosis must stop when the pressure difference equals the osmotic pressure.

    Although osmotic pressures can be large, many atmospheres, if the radius is small small enough, bubbles (like cells) can support large pressure differences with modest skin tension.
  8. Feb 13, 2018 #7
  9. Feb 20, 2018 #8


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    If aquaporins are involved, then the osmosis stops when the acquaporins reach their substrate-saturation point, which may be different from osmotic equilibrium. If acquaprosins are not involved and it is all spontaneous diffusion, then it stops are equilibrium.
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