4.2.1—Membrane transport processes

Membrane transport is classified by what moves, its direction relative to a gradient, the route across the membrane and whether cellular energy is required. Not every process moves from high concentration to low concentration, and not every process uses a membrane protein.

• Simple diffusion: Small non-polar molecules move directly through the phospholipid bilayer from higher to lower concentration; no transport protein or cellular energy is required.
• Facilitated diffusion: A specific ion or polar molecule moves down its concentration gradient through a channel or carrier; a membrane protein is required, but cellular energy is not.
• Osmosis: Water moves across a partially permeable membrane from higher water potential to lower water potential; the driving force is the water-potential gradient, not ATP.
• Active transport: A specific substance is moved against its concentration gradient through a carrier/pump using energy supplied by the cell, commonly from ATP hydrolysis.
• Endocytosis: The membrane encloses material and pinches off a vesicle to bring bulk material into the cell; this is vesicular transport and requires energy.
• Exocytosis: A vesicle fuses with the cell-surface membrane and releases its contents outside; this is vesicular transport and requires energy.

A gradient can drive net diffusion when the membrane route is available. A hydrophobic bilayer may block a polar solute, so facilitated diffusion supplies a protein route without reversing the gradient. Active transport uses energy to build or maintain a gradient, while endocytosis and exocytosis move material too large or numerous for a channel or carrier by changing membrane shape.

Protein involvement alone does not prove active transport: facilitated diffusion also uses proteins but follows a gradient and does not require cellular energy. Osmosis is water movement, not solute movement, and endocytosis/exocytosis are bulk vesicle processes rather than high-to-low diffusion.