During diffusion, molecules move from a region where their concentration is low to a region where their concentration is high, until the cell solution is isotonic.

This is false.

It should be molecules move from a region the where concentration is HIGH to a region where their concentration is LOW.

This is a net movement resulting from the random movement. They move down their concentration gradient.

The net movement will end when the solution reaches isotonic state, which means both sides has the same concentration of solute.

Instead, if the molecules move against their concentration gradient, from lower concentration to higher concentration, the method used should be active transport. Though it requires extra energy from for example cellular respiration too.