Q. A cell swells up when kept in

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Solution:

In a hypotonic solution the concentration of outer solution (in which the cell is placed) is lower than concentration of cell sap. When a cell is placed in hypotonic solution then endosmosis i.e. entry of water from outside to the cell through plasma membrane occurs which makes the cell again turgid and it swells up.
When a cell is placed in hypertonic solution then exosmosis occurs. The protoplasm shrinks and leaves the cell wall and the cell becomes flaccid which is called plasmolysed cell. Thus exosmosis leads to plasmolysis.