Here's why:
* Structure: The cell membrane is composed of a phospholipid bilayer, which has a hydrophobic (water-repelling) interior and hydrophilic (water-attracting) exterior. This structure allows for the passage of small, nonpolar molecules like oxygen and carbon dioxide, but blocks large, polar molecules like sugars and ions.
* Proteins: Embedded within the membrane are various proteins that play a crucial role in selective permeability:
* Channel proteins: Act like tunnels, allowing specific ions to pass through.
* Carrier proteins: Bind to specific molecules and transport them across the membrane.
* Receptor proteins: Bind to signaling molecules and initiate cellular responses.
In summary: The cell membrane's unique structure and embedded proteins work together to control which substances enter and exit the cell, ensuring the cell's proper functioning and maintaining a stable internal environment.
