How is it a solution of 300mOsM NaCl is both isotonic and isosmotic to a red blood cell with an osmolarity of 300mOsM? Yet a solution of 300mOsM urea is only isosmotic to that red blood cell?
Areb't they the same thing since both solutions are 300mOsM?
To understand why a solution of 300mOsM NaCl is both isotonic and isosmotic to a red blood cell with an osmolarity of 300mOsM, while a solution of 300mOsM urea is only isosmotic, let's break down the terms involved.
Isotonicity refers to two solutions having the same concentration of solutes, causing no net movement of water across a semipermeable membrane. In this case, the red blood cell has an osmolarity of 300mOsM. For a solution to be isotonic with the red blood cell, it should have the same osmolarity, meaning it will have the same concentration of solutes. Therefore, a solution with 300mOsM NaCl will be isotonic to the red blood cell.
Isosmoticity, on the other hand, refers to two solutions having the same osmolarity, regardless of the specific solutes involved. So, any solution with an osmolarity of 300mOsM will be isosmotic to the red blood cell, regardless of its composition.
Now, the reason why a solution of 300mOsM NaCl is both isotonic and isosmotic to the red blood cell is because the red blood cell maintains its normal shape and volume when placed in this solution. Since NaCl dissociates into Na+ and Cl- ions, both ions can freely move across the cell membrane through ion channels to maintain the balance of charge. This allows the red blood cell to achieve equilibrium with the solution, making it isotonic and isosmotic.
On the other hand, a solution of 300mOsM urea is only isosmotic to the red blood cell. Urea is a small, non-ionic molecule that can freely permeate the cell membrane through simple diffusion. It does not require specific channels or transporters like Na+ or Cl- ions. Because urea is membrane-permeable, it readily diffuses into and out of the red blood cell, leading to no net movement of water and maintaining an isosmotic state. However, since urea does not exert the same osmotic pressure as ions, it cannot maintain isotonicity and the red blood cell may undergo shape changes or lysis when placed in a urea solution.