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Thread: quick physiology question (osmolarity + tonicity)

  1. #1 quick physiology question (osmolarity + tonicity) 
    Forum Freshman
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    i encountered a simple question for which i cant seem to find a direct simple answer..

    "if a cell is isoosmotic and therefore isotonic. the intercellular and extracellular fluid contain the same amount of what?"

    of the 4 options the only reasonable one was water. which is the correct answer.

    but this is confusing. if the cell is isoosmotic that means that the osmolarity is equal on both sides, which essentially mean the concentration (times van hoffs factor ofc..) is equal on boths sides?

    - when the concentration isnt in equilibrium the only substance that passes should be water....so how could it be equal amouns of water on each side?


    thanks for any replies in advance


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  3. #2  
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    The question is a bit badly worded in my opinion. Of course there isn't the same volume of water inside a cell as in the whole interstitium, but the CONCENTRATION is the same.

    Also, if they're not in equilibrium water isn't the only substance that can pass through the membrane as some solutes can also diffuse across the membrane to help balance the equilibrium. This is the difference between osmolarity and tonicity, osmolarity is the difference in osmoles, tonicity is the difference in osmoles of substances that cannot diffuse across the membrane, and therefore will cause a net movement of water.


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  4. #3  
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    Ah I think I understand. Thank for clarifying that up
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