When we say that Solution A is hypertonic, or Solution A is isotonic,
or Solution A is hypotonic, we compare the "water-grabbing ability" of solution A to that of other solutions.
These other solutions are the reference solutions: i.e. the reference points for comparison. We look at how solution A affects the net movement of water out of or into the reference solution.
In Biology,
the reference solutions will be either the extracellular solutions (Blood, extracellular fluids) or the intracellular solutions.
Let's say that we work on cells.
Solution A is hypertonic means:
- Water is drawn out of the cells by Solution A and the cells will shrink (solution A grab water from the cells = A has a Higher "water-grabbing ability").
Solution A is isotonic means:
- There are no net movement of water in or out of the cells and the cells will
not change shape (solution A cannot grab water from the cells = A has the same "water-grabbing ability").
Solution A is hypotonic means:
- Water is drawn inside the cells and the cells
will burst (hemolysis) (Not only solution A cannot grab water from the cells,
but it is loosing its own water to the cells = A has the weaker "water-grabbing ability" as it is its own water that is grabbed by the cell).
These terms are purely descriptive:
they describe where the water is going. They do not explain at all
the reason why.
So, do not explain hemolysis of red blood cell by
by saying that it is because the solution is hypotonic and leave it at that.
It would be like saying: "the cells burst because
water enters the cells and make them burst".
You have to explain why the water enters the cell.
You have to explain why the solution is hypotonic.