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Homework Help: Osmolarity of water in red blood cells?

  1. Feb 18, 2013 #1
    I am confused on how osmolarity of cells with water would cause a cell to swell, shrink, or burst. I know that a cell has a solution concentration of 300 mosm. In a lab that was done, we analyzed the impact of a red blood cell in solutions containing different osmolarities. For example, when a red blood cell was placed into a 400 mM ethanol penetrating solution and 0 non-pentrating solutes, the cell would burst and shrink as the water doesn't leave the cell. In another example, when the cell was placed into a 1400 mM solution of NaCl, the cell was found to shrink because water moved down it's concentration gradient with the higher osmolarity of the outer solution. Finally, when a red blood cell was placed into a solution containing a 700 mM penetrating amount of urea and a 700 mM nonpentrating amount of NaCl, the urea moved into the cell and the cell was found to shrink. I'm really confused on why this would be the case. Is it because the urea moving into the cell makes the molar concentration of solute outside the cell lower then in the cell so the water would move down its concentration gradient for that reason? Any help is very much appreciated.
  2. jcsd
  3. Feb 20, 2013 #2
    I can see why you would be confused about adding urea to a cell and it getting smaller.
    When fluid enters a cell, it be comes hypotonic, causing it to expand or lyse
    When fluid leaves a cell, the cell becomes hypertonic, causing it to shrink.

    Fluid exchange will occur when cellular concentrations are not favorable with external and internal equilibrium conditions. Molecular weight should also be taken into consideration, as larger molecules will not be able to permeate the cell membrane.

    By comparing the internal and external concentrations, we should be able to understand the direction of flow with respect to the concentration gradient.

    Urea will bond via electrostatic interactions with several H2O molecules. Since we have used urea to tie up the external water molecules, there is a decreased external concentration of water. Thus, the internal water molecules from the erythrocyte are in higher concentration. The water volume from the RBC is decreases as the water follows the concentration gradient outside of the cell into the system.
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