The cell shrinks because of the movement of water away from the cell into the more concentrated salt solution.
Osmosis is the movement of water from one region to another through a semi - permeable membrane. A hypertonic solution contains more solute than the cell.
In this case, the external salt solution is hypertonic because it contains a 8% salt concentration whereas the cell contains only a 0.9% salt concentration. As such, water moves out of the cell into the solution.
The cell shrinks because of the movement of water away from the cell into the more concentrated salt solution.
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