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.
- 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.
- 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.