Transport of substances across a biological membrane from an area of higher concentration to an area of lower concentration by means of a carrier molecule. Since the substances move along the direction of their concentration gradients, energy is not required.
Diffusion refers to the net movement of molecules from higher to lower concentration. Diffusion may be a simple diffusion or facilitated diffusion. A simple diffusion is one in which that occurs unassisted. Facilitated diffusion in contrast is an assisted diffusion in a way that it requires a carrier molecule. For instance, polar molecules diffuse across a cell membrane through carrier molecules in the cell membrane. For example, polar molecules and charged ions dissolved in water cannot diffuse freely across cell membrane due to its hydrophobic lipids. They can only be transported across membranes by proteins forming transmembrane channels. Larger molecules are transported by transmembrane carrier proteins, such as permeases that change their conformation as the molecules are carried through, for example glucose or amino acids.
- facilitated transport
- passive-mediated transport
- passive transport
- carrier protein
- Selectively-permeable membrane
- Channel forming ionophore