osmosis of a cell membrain

Osmosis causes water to move from hypertonic solution to hypotonic solution.
Hypertonic solution is a solution where the solute concentration is higher in comparsion to another solution.
Hypotonic is the opposite.

So, when we put the cell in a hypotonic solution (so that the cytosol will be hypertonic), water will move inside the cell.
On the other hand, when we put the cell in a hypertonic solution (so that the cytosol will be hypotonic) water will move out of the cell.

The cell also have some mechanisms to keep the water in its cytosol about constant. However, in extreme hypertonic or hypotonic conditions, the water in the cytosol may change noticeably. For example, see this picture:
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