Here's why:
* Diffusion: The movement of molecules from an area of high concentration to an area of low concentration. The cell membrane allows some molecules to pass through freely (like water and small, non-polar molecules), while others are blocked (like large molecules and charged ions).
* Osmosis: The movement of water molecules across a semi-permeable membrane from an area of high water concentration to an area of low water concentration. The cell membrane controls the flow of water in and out of the cell, maintaining its internal environment.
Therefore, the cell membrane acts as a selectively permeable barrier, allowing certain substances to pass through while restricting others. This selective permeability is crucial for maintaining the cell's internal environment and carrying out essential functions.
