Osmolarity and Tonicity Concepts

Q)  I'm trying to wrap my head around osmolarity. when comparing penetrating solutes to non penetrating solutes, since the penetrating solutes are allowed to move through the membrane - wouldn't that be diffusion, since the solutes are freely moving from high to low concentration - thus water would not need to move in or out of the cell? I understand that with non penetrating solutes, since the solutes are not able to move freely, water moves to dilute the more concentrated non penetrating solutes. What i am confused about is why the movement of penetrating solutes is tied with osmolarity...to clarify my viewpoint - the movement of penetrating particles from high to low is "diffusion"...


I'm reading the book as i'm emailing you, but this might answer my question...

if you have a mixture of penetrating solutes and non penetrating solutes then thats where the penetrating solutes come into play with osmolarity - which in this case tells us about tonicity? whereas if you just have penetrating solutes in a cell and solution, the movement of the penetrating solutes is just simple diffusion and thus tonicity is irrelevant...?

sorry if this is confusing, but i'm having a hard time trying to tie everything together

A)  Yes, penetrating particles move from one compartment to the other by simple, passive diffusion. 

Osmolarity depends on the number of particles in each compartment, not on whether they are penetrating or non-penetrating.

Tonicity (whether or not the cell shrinks or swells) is dependent on the ICF and ECF concentration of non-penetrating particles. If there areonly penetrating particles present, then there will be no effect on tonicity.

However, if there is a mixture of penetrating and non-penetrating particles, both will may have an effect on tonicity.

If you remember the two examples in class today, I had a cell in an iso-osmotic but hypotonic solution, and another in a hypo-osmotic and isotonic solution. In both examples there were penetrating and non-penetrating particles. In those examples we had to see what was going on with the penetrating particles to understand the effect of tonicity on the cell. 

Also, remember that osmolarity of a solution (whether you call a solution iso-, hypo-, or hyperosmotic) depends on the number of particles BEFORE putting the cell into solution, whereas tonicity of a solution (whether you call it iso-, hypo-, or hypertonic) depends on what happens to the cell AFTER it is put in solution and the particles balance themselves out between the two compartments.