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If I place a RBC in 0.5M urea we see haemolysis.

My textbook says that "the solution is hypotonic. Urea enters the RBC down it's concentration gradient. Water follows the movement of urea by osmosis. This influx of water causes lysis".

I am very confused by this, for a number of reasons.

  1. I thought tonicity only applies to non-penetrating solutes (i.e. a solution is hypertonic relative to a cell if the solution has a greater concentration of non-penetrating solutes then the intracellular fluid). If urea can penetrate the membrane, then why is the solution hypotonic? As a penetrating solute isn't it not factored into tonicity.
  2. If urea is a penetrating solute, then won't it just come to an equilibrium across the cell membrane, and hence there will be no net osmosis since the concentration inside the cell will the same as that outside the cell.

I feel like I have a conceptual failing here, but the more I google the more confused I get.

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  • $\begingroup$ It would be helpful if you could quote the entire scenario from your text book. $\endgroup$ – canadianer Feb 22 at 22:40
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I think you have the conceptual understanding down, but my guess is that the textbook is not representing the situation quite correctly. Based on the phrasing of the statement, I can imagine they are describing one of two situations:

  1. Imagine a dropping a cell in to a solution of 0.5 M urea with no other solutes. Urea will diffuse across RBC membranes due to facilitated diffusion. It won't really factor in to this situation much. So, the RBC contains proteins and other solutes that cause water to flow in to the cell and the cell lyses. I don't think this is what the textbook means.

  2. Now, imagine a situation in which you precisely calculated the water potential of a RBC. You mixed up an isotonic solution of urea and some non-penetrating solutes. This solution is perfectly isotonic at first, but as the urea diffuses in to the cell, it alters the water potential of the cell and the outer solution leading to osmosis. I think this is the situation the textbook is trying to describe.

Imagine a third situation now in which a RBC is at equilibrium with a solution. If you just added urea to the solution, it would not affect osmolarity in the long term for the reason you describe.

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