Osmosis is the movement of water across a semi-permeable membrane from an area of high water concentration to an area of low water concentration. This process is essential for all living cells to maintain their internal balance and function properly.

While all cells participate in osmosis, the plasma membrane is the specific cell structure that acts as the semi-permeable membrane in this process. It allows water molecules to pass through, but it regulates the movement of other molecules and ions, contributing to the differences in water concentration that drive osmosis.