Here's why:
* Structure: The cell membrane is composed of a phospholipid bilayer with embedded proteins. The phospholipids form a barrier, and the proteins act as channels and pumps, facilitating the movement of specific molecules.
* Selective Permeability: The membrane allows small, non-polar molecules like oxygen and carbon dioxide to pass through freely by simple diffusion. However, it restricts the movement of large, polar molecules like sugars and proteins.
* Active Transport: The cell uses energy to move certain molecules against their concentration gradient, meaning from an area of low concentration to an area of high concentration. This process requires specialized proteins called pumps.
* Passive Transport: Some molecules move across the membrane passively, driven by their concentration gradients. This includes simple diffusion, facilitated diffusion, and osmosis.
In essence, the cell membrane acts as a barrier that allows the cell to maintain its internal environment by regulating the passage of substances. This selective permeability is crucial for the cell's survival and proper functioning.
