Membrane Transportation: Passive Processes

Introduction to Membrane Transport

Plasma membranes are selectively permeable, allowing only certain substances to pass. Transport is categorized into passive processes (requiring no energy, moving substances down a concentration gradient) and active processes (requiring energy, often moving substances against a gradient). Passive transport includes simple diffusion, facilitated diffusion, and osmosis.

Simple Diffusion

Simple diffusion allows small, nonpolar, lipid-soluble (hydrophobic) molecules to pass directly through the lipid bilayer from high to low concentration. Clinical relevance is highlighted in burn patients, where membrane damage leads to abnormal leakage of intracellular substances.

Facilitated Diffusion

Facilitated diffusion assists hydrophilic molecules like glucose and ions that cannot pass through the lipid bilayer alone. It relies on specific membrane proteins and is subject to saturation and regulation. It occurs via channel-mediated diffusion (using leakage or gated channels) or carrier-mediated diffusion (where proteins change shape to transport substances).

Osmosis

Osmosis refers specifically to the movement of water across the plasma membrane. Because water is polar and not lipid-soluble, it primarily passes through specialized leakage channels called aquaporins, which remain permanently open to facilitate water movement.