The main function of a cell membrane is to regulate the passage of substances into and out of the cell. This is essential for maintaining the cell's internal environment and allowing it to carry out its specific functions.

Here's how the structure of the membrane supports this function:

* Phospholipid bilayer: The two layers of lipid molecules (phospholipids) create a selectively permeable barrier. The hydrophobic tails of the phospholipids face inwards, forming a barrier that prevents water-soluble substances from passing through freely. The hydrophilic heads face outwards, interacting with the watery environments inside and outside the cell.

* Proteins: Embedded within the membrane are various proteins that serve different roles:

* Transport proteins: Act as channels or carriers to facilitate the movement of specific molecules across the membrane.

* Receptor proteins: Bind to signaling molecules and relay information to the cell's interior.

* Adhesion proteins: Help cells adhere to each other and to the extracellular matrix.

* Recognition proteins: Identify the cell as belonging to a particular organism or tissue.

Overall, the cell membrane acts as a gatekeeper, controlling which substances enter and exit the cell, ensuring the cell's survival and proper functioning.