Why does osmolarity matter?
Osmolarity matters because cells cannot survive if the osmolarity if their surroundings is much different from their own.
Water moves across a membrane from a lower osmolarity to a higher osmolarity. In other words, it moves from the dilute side to the concentrated side.
The dilute side loses water and becomes more concentrated. The concentrated side gains water to become more dilute.
Cell survival depends on balancing water uptake and loss . Cells can burst if they take on too much water, or collapse if they lose too much.
Cell membranes in general are permeable to water. Thus, the osmolality of the extracellular fluid (ECF) is approximately equal to that of the intracellular fluid (ICF).
This is important, because it shows that changes in ECF osmolarity have a great effect on ICF osmolarity.
If the osmolarity of the ECF becomes too low (hypotonic), water will fill the cells. This increases their volume and may lead to their rupture (cytolysis).
If the osmolarity of the ECF becomes too high (hypertonic), water will leave the cells. This decreases volume and may lead to their shrinking (crenation).
Hope this helps.