if water moves in both direction (inside and outside a cell), that is, from higher to lower concentration and lower to higher concentration, shouldn’t all solutions be isotonic solutions? Why is it that in some solutions the cell will gain more net water and in some it will lose more net water?
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The terms isotonic, hyper/hypotonic are terms that tell us the relative difference in salt concentration between two containers of salt water.
Let’s say that we have two tanks with 1L of water. In tank A, we dissolve 10g of salt, while in tank B we dissolve 50g of salt. Tank B has more dissolved particles so we say that liquid in tank B is hypertonic when compared to tank A.
An experimentally shown fact is that inside a cell, you can find a liquid whose osmolarity is 250mOsm/L. That tells us that the water inside a cell has 0.25 moles of salt in a liter of that fluid in a cell.
Now, lets put that cell in a hypertonic solution. As we’ve said, hypertonic means that it has more salt than the cell, meaning that its osmolarity could be 500mOsm/L. Because there’s more salt outside of the cell, the water inside the cell will passively diffuse outwards in order to “dilute” the water outside of the cell and bring down its oslomarity to lowe than 500mOsm/L. However, the osmolarity i.e. the salt content in the cell will increase, because the cell is passively loosing water. So in theory, the water outside and inside the cell should difuse through the cell until the salt concentrations inside and outside level out, which should be at the exact midpoint, which is (250+500)/2 = 375mOsm/L. When salt concentration reaches that value both inside and outside, net osmosis will stop and the solutions will be isotonic to one another.
However, if we test that out, we will see that the salt concentration in the solution outside the cell will never get down to such a degree. The reason why is because there’s MUCH MORE water outside of the cell, than it is inside of the cell. So even if the cell gives all of its water to the solution outside, it would barely make a fifference in diluting the salt in the outside solution. It would be like you eating a very salty soup, thinking that it’s too salty, and solving the problem by dropping one drop of water inside a 5 liter pot. That obviously won’t make the soup any less salty.
So the answer is that the cell will “try” to reach a balance with the outside solution, but because the solution has a much bigger volume than the volume of the fluid inside of the cell, the cell can’t change the salt concentration of the outside solution, but it can only react to it. That’s why if we put a cell in a very salty soup, the cell will shrivel up; if we put it in pure water, the cell will explode.
If you have further questions, please don’t hesitate to ask.
Well if you have a cell and it has several electrolytes dissolved inside, these can all be different concentrations compared to the outside solution of plasma with all of the electrolytes interacting with water generate osmotic pressure in both directions. Cells will concentrate certain ions when others are low. They are able to find an equilibrium of sorts. But just because there is an osmotic equilibrium doesn’t mean that any of the specific types of electrolytes are isotonic to themselves
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