Cell Membrane & Transport

How cells control what enters and exits

The plasma membrane is a phospholipid bilayer with embedded proteins — the fluid mosaic model (Singer & Nicolson, 1972). Transport mechanisms: Simple diffusion — small nonpolar molecules (O₂, CO₂, steroids) move down their concentration gradient through the lipid bilayer. Facilitated diffusion — ions and polar molecules use channel proteins (ion channels, aquaporins) or carrier proteins (glucose transporters) to cross the membrane, still down the gradient, no ATP needed. Osmosis — water moves across a semipermeable membrane from low to high solute concentration (high to low water potential). Active transport — the Na⁺/K⁺ ATPase uses ATP to pump 3 Na⁺ out and 2 K⁺ in against their gradients, maintaining electrochemical gradients. Endocytosis and exocytosis move large molecules via vesicles.