Osmolarity VS. Tonicity

I know that there was a lot of confusion regarding the question that was dropped on the exam regarding Osmolarity vs. Tonicity.  Please know that it was not dropped because of poor performance on the question, but due to the fact that one word choice was not clear and made the question an unfair and unclear question.  Obviously when it was originally written and discussed amongst my department it appeared clear but upon further review I was in agreement that the word choice was not correct and that the question should be dropped and the concept re-evaluated at a later time. 

PLEASE NOTE deleting questions is NOT for students who did not study the concept or did poorly, it is due to the validity of a question itself.  Fortunately for you as students you have a committee that is specifically charged with determining when a question is not valid or has multiple correct answers.  That is the sole purpose of the committee to ensure that ALL EXAMS at RUSM are fair and just to you as students.  When a question is given that is deemed not valid it will be removed from that particular exam and the concept will be re-tested on mini 3.  If on mini 3 or the final a question is deemed not valid credit will be given to all.  The design of that is to ensure that we are assessing you fairly and determining your success based upon that fact not unfair or unjust questions.  As you know you have participation in this process by appealing questions within the exam and your exam question review representative participates in the meetings to ensure that your voices are heard.

Due to the fact that there WAS poor performance on the concept, however, that I believe due to some confusion on the concept I thought I would address that in this post so as to clear up any confusion you still have regarding osmolarity vs. tonicity and hopefully will all perform well on the concept when it re-appears on mini 3. 

Osmolarity vs. Tonicity

Remember from the lecture that in order to discuss the movement of water we need to know how many particles are in a solution.  This is because it is the particles attract water, therefore wherever there are more particles the water will be driven to move.  If particles are across a membrane from water and cannot cross the membrane then the water will move towards the particles (down the concentration gradient for water).  If the particles CAN cross the membrane then they will move and water will follow the movement of those particles.

Now, to determine the osmolarity or tonicity of a solution we need to know a few things:
1.  What is the definition of tonicity?
2. What is the definition of osmolarity?
3.  How do you determine how to determine the tonicity or osmolarity of a solution?  In other words, WHAT do we compare it to.  If calling something hyper/hypo/iso is a comparative word, we must be comparing it TO something, so what do we compare it to?

1.  Tonicity of a solution was defined scientifically by what happens to cells in a solution.  If the cells took up water from a solution (i.e. swelled), then the solution was hypotonic and if the cells lost water to a solution (i.e. shrunk) then the solution was hypertonic.  If no change in cell size was observed then the solution was isotonic.  This is because the number of particles in that solution that could not cross the plasma membrane were being being compared to the particles within the cell (that could also not cross the plasma membrane).  Therefore we can simplify the definition of tonicity to be the number of particles in a solution that CANNOT cross a plasma membrane (and therefore will attract water).  An example of this would be if a red blood cell (typically 300 mOsmol/L) was placed into a solution of 100 mmol/L of NaCl (so 200 mOsmol/L because inorganic salts separate into ions in solution) then the cell would swell (the solution is hypotonic), if it was placed into a solution of 200 mmol/L  of NaCl (400 mOsmol/L) then the cell would shrink (the solution is hypertonic), and if it was placed into a solution of 150 mmol/L of NaCl (300 mOsmol/L) the size of the cell would not change (the solution is isotonic).

2.  Osmolarity, in contrast to tonicity, measures ALL of the particles per volume in the solution.  Scientifically osmolarity was determined by an osmometer which is a machine that you put a solution into and it pushes ONLY water through a very specific membrane and then counts all of the particles that could not go through the membrane.  That way it determines ALL of the particles that are within a solution.  Biologically, however, some of those particles CAN go through the biological membrane (and therefore have an osmotic coefficient of 0, ex. Urea) and others CANNOT go through the biological membrane (and therefore have an osmotic coefficient of 1, ex. any ions or Na+) or have a biological membrane permeability somewhere in between.  So the osmolarity of a solution will be the total number of solutes within the solution and if it is hypo/hyper/isosmotic will be compared to whatever you are comparing it to.  Examples of osmolar solutions would be 100 mmol/L of NaCl would have an osmolarity of 200mOsmol/L.  100 mmol/L of NaCl plus 100 mmol/L of urea would have an osmolarity of 300 mOsmol/L because you add up ALL of the particles. 

3.  In order to determine the state of hyper/hypo/isosmolarity OR hyper/hypo/isotonicity of a solution you need to be comparing it TO something.  Typically we compare a solution to the state of a cell which has 300mOsmol/L osmolarity (or to the intracellular solution within the body which will have the same status as the state of a single cell).  Remember that cells typically are made up of particles and proteins that CANNOT move across the plasma membrane.  Therefore, to determine the hyper/hypo/isotonicity of a solution compared to the inside of a cell you would be looking for the number of particles in that solution that cannot cross the plasma membrane (ex. ions) and compare that number to 300 mOsmol/L.  So, for example a solution that has 100 mmol/L of NaCl (200 mOsmol/L) would still be hypotonic as it was above.  To determine the hyper/hypo/isosmolarity of a solution, in contrast to tonicity, you would need to look at ALL of the particles in the solution.  So, for the same example above (100 mmol/L of NaCl or 200 mOsmol/L of NaCl) it would be hyposmotic.  If you had a solution with particles that had osmotic coefficients of both 0 and 1 (ex. Urea AND NaCl or including particles that CAN cross the plasma membrane and particles that CANNOT) then the osmolarity and tonicity of that solution would be different.  For example, if a solution had 150 mmol/L of NaCl and 100 mmol/L of Urea the tonicity of the solution would be 300 mOsmol/L for only the NaCl.  The osmolarity of the solution, however, would be both NaCl and Urea and would therefore be 400 mOsmol/L.  If we were to compare the osmolarity and tonicity of this solution to that of a typical cell (300 mOsmol/L), then it would be isotonic (both have tonicities of 300 mOsmol/L) but hyperosmotic (the solution has an osmolarity of 400 mOsmol/L while the cell has an osmolarity of 300 mOsmol/L).  Therefore, for all solutions that have both particles that CAN cross the plasma membrane and particles that CANNOT, you would need to compare their osmolarity and tonicity separately (and therefore the state of hypo/hyper/isosmolarity and hypo/hyper/isotonicity) because some of the particles would contribute to the tonicity while ALL of the particles would contribute to the osmolarity.

Hopefully this clears up a lot of confusion that you have regarding osmolarity vs. tonicity.  If, however, you are still having troubles conceptualizing this concept please feel free to come and discuss it with me or to discuss it with me and your fellow classmates in electronic format below.