Biology Forum Cell Biology Iso-osmotic Vs Isotonic :?:

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    • #4082

      We had a Prac a few weeks ago that involved investigating membrane permeability and tonicity. Although i got a good mark for my prac report…there are still areas i JUST CAN’T fully comprehend or am confident about. Such as the concept of Iso-osmotic Vs Isotonic. How is it that a solution that starts of as Iso-Osmotic AND Isotonic…end up NOT Isotonic?


      Rate of Haemolysis refers to the rate at which cells Lysed? ie. burst releasing Haemoglobin

      I dont want to wait till exam period before trying to come to terms with these terminologies 😕

    • #43733

      What? I thought isotonic and isoosmotic were the same thing. Aren’t they?

    • #43768

      No its different.

      Osmolarity is the total sum of molarities of the PARTICLES in a solution right?

      well Iso-OSMOTIC is the term used for two solutions who’s OSMOLARITY are equal……

      Iso-TONIC however is or refers to a solution with the same SOLUTE concentration.

      In a solution, some compounds tend to seperate bringing about a change in Osmolarity. eg. Red Blood Cells in a solution of NaCl 154mOsm. NaCl will seperate into Na+ and Cl- and mOsm will double ie.308mOsm

      consequently…solution is no longer Iso-Osmotic but it is Isotonic because Red Blood cells have 300mOsm so there will be no NET Movement of water as cells can accomodate for a little bit more water….and cells in this solution will neither LYSE or SHRINK etc.

      Iso-osmotic solutions aren’t ALWAYS Iso-tonic for eg. Urea is one solution that can be Iso-osmotic but not Isotonic as mOsm halves as some particles move INTO the blood cell increasing the mOsm of the cell and the Urea becomes Hypotonic….consequently the cell will swell/Lyse.

      This is my understanding of this……..but im not totally confident about it. Thats why i asked, could some one shed some light please? I am hopelessly running in circles trying to understand this. aaaaaaarrrrhhhh 😡

    • #43778


      osmostic= osmolarity

      so solutions with the same osmolarity (active particles)


      iso= same
      tone = pressure

      So solutions which have the same osmostic pressure (active particles + chemical makeup)

      Hope that helps….

    • #43810

      Oh yeah makes sense. But only if there are more than one solutes. If there is only one, isoosmotic is also isotonic and viceversa right?

    • #43812
      quote MrMistery:

      If there is only one….

      If there is only one, how can it be "iso"?



    • #43815

      I am sorry. i meant one substance disolved into water on each side(the same substance)

    • #43826

      Oh….i see….yes, I believe you are right!

    • #86413

      As i understand it, tonicity only compares one solute on differing sides of a membrane.


      Left side of membrane:
      0.3 M NaCl

      Right side of membrane:
      0.5 M NaCl

      the right side is hypertonic to the left side, meaning the right side has more solute

      Iso osmotic however compares many solutes at one time in an interacting environment.

      For example

      On the left side of the membrane:
      0.3 M of X Y and Z

      On right side
      0.3 M of A B and C

      The individual solutes all have a 0.3 M make up, there for they all exert the same osmotic force.

      0.3 = 0.3 , the same osmotic pressure.

      Okay lets change it up a little.

      left side of membrane:
      0.5 M of X Y and Z

      Right side of membrane:
      0.3 M of A B and C

      The left side is now hyper osmostic becasue it is now exerting more osmotic pressure.

      This is my understanding. If anyone can argue this for better of worse please do so as I also have exams……… 😯

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