Here's why:

* Cell Membrane Structure: The cell membrane is a selectively permeable barrier. It allows some substances to pass through while blocking others.

* Osmosis Definition: Osmosis is the movement of water molecules across a semi-permeable membrane from a region of high water concentration to a region of low water concentration.

* Water Movement: The cell membrane controls the movement of water in and out of the cell, driven by the concentration gradient of water (and dissolved solutes). This movement is essential for maintaining the cell's internal environment and volume.

Let me know if you'd like more details about osmosis or the cell membrane!