The cell membrane

THE CELL MEMBRANE

The cell membrane (plasma membrane) is a selectively permeable barrier surrounding the cell. It separates the intracellular environment from the extracellular environment and helps maintain homeostasis.

It follows the fluid mosaic model and is mainly composed of phospholipids, proteins, cholesterol, and carbohydrates.

Phospholipids have hydrophilic heads and hydrophobic tails, forming a bilayer.

Major functions:

• Controls movement of substances

• Enables cell communication

• Maintains cell homeostasis

• Participates in electrical signaling

Main transport mechanisms:

• Simple diffusion — high → low concentration

• Facilitated diffusion — uses membrane proteins, no ATP

• Osmosis — movement of water

• Active transport — requires energy

Example: Na⁺/K⁺ ATPase moves 3 Na⁺ out and 2 K⁺ into the cell using ATP.