2.7 Facilitated Diffusion

Cells have membranes that allow them to establish and maintain internal environments that are different from their external environments. Explain how the structure of a molecule affect its ability to pass through the plasma membrane. Membrane proteins are required for facilitated diffusion of charged and large polar molecules through a membrane--Large quantities of water pass through aquaporins. Charged ions, including Na+ and K+, require channel proteins to move through the membrane. Membrane proteins are necessary for active transport. Metabolic energy (such as from ATP) is required for active transport of molecules and.or ions across the membrane and to establish and maintain concentration gradients. The Na+/K+ ATPase contributes to the maintenance of the membrane potential.

facilitated diffusion

passive transport of large polar molecules and ions with the help of transport proteins— channel proteins and carrier proteins

channel proteins

type of transport proteins which make hydrophilic tunnels across the membrane, only accepting one (or a few) type(s) or molecules for transport (ex. aquaporins)

• membranes may become polarized by the movement of ions across the membrane

aquaporins

type of channel protein allowing large quantities of water to pass through a membrane

carrier proteins

type of membrane protein which binds to molecules and changes shape to shuttle them across the cell’s membrane at the solute-binding site

• generally much slower than channel proteins, need to shape and reset

• facilitated diffusion when ATP not required to change conformation

• active transportation when ATP needed to change conformation

active transport requirements

membrane proteins and metabolic energy (such as from ATP)