Passive Transport

Passive transport covers every route by which a substance crosses a membrane using only its own kinetic energy, moving down a concentration gradient without any cost in cellular ATP. The simplest form is plain diffusion - the same everyday process by which perfume molecules spread out to fill an entire closed room - which works well for small, non-polar or lipid-soluble molecules that can slip through the membrane unassisted, but is far too slow to rely on over any real distance. Polar molecules such as ions, sugars and amino acids cannot cross a membrane by diffusion alone, so they instead rely on facilitated diffusion, in which specific membrane transport proteins - either channel proteins forming a pore, or carrier proteins that bind and physically escort the molecule across - assist the crossing, still without spending any ATP. Because these transport proteins are limited in number and specific to particular molecules, facilitated diffusion, unlike simple diffusion, shows real selectivity and can become saturated once every available carrier is occupied. The same carrier proteins can also move two different molecules together in a single coupled step …