Cellular activity is the basis of all life. Even the largest and most complex organisms on Earth are sustained by the biological processes carried out by trillions of microscopic cells. Individual cells fulfill their biological functions by transporting various materials to and from their multicellular hosts. Some substances that cannot readily pass through the cell membrane use a fascinating transport method called facilitated diffusion.
The Skin of a Cell
Cells are enclosed by a thin layer known as the plasma membrane. This membrane keeps the cell intact by containing cellular fluid, or cytoplasm, and specialized structures called organelles. The plasma membrane also regulates the substances that enter or exit the interior of the cell. Cells have a variety of methods for moving molecules through the cell membrane, and these methods fall into one of two general categories: passive transport and active transport. A cell must expend energy to accomplish active transport, while passive transport does not require cellular energy. Facilitated diffusion is an example of passive transport.
Molecules Flow from High to Low
Diffusion is a process in which molecules naturally flow from areas of high concentration to areas of low concentration. Some molecules, however, cannot freely enter or exit a cell under the influence of a concentration gradient because they are not compatible with the cell's plasma membrane, which is less permeable to molecules that are large, polar, electrically charged, or lipid-insoluble. With facilitated diffusion, the cell can "help" some of these molecules pass through the plasma membrane by binding them to special carrier proteins or by opening channels between the cell and the surrounding environment.
Glucose is a sugar molecule that serves as the fundamental energy source for many cells. Outside the cell, the bloodstream constantly supplies glucose, and inside the cell, glucose is continually consumed through the process of cellular metabolism. As a result, the concentration of glucose outside the cell remains higher than the concentration inside the cell, but the glucose molecule is too large to pass through the plasma membrane unassisted. Thus, the cell provides glucose-specific carrier proteins that bind to glucose molecules and allow them to enter the cell.
Facilitated diffusion via carrier proteins is common for a variety of larger molecules that cannot easily pass through the plasma membrane. Examples include fructose and galactose, which are monosaccharides like glucose; amino acids, from which proteins are made; and nucleosides, which are the building blocks of DNA and RNA. A different type of facilitated diffusion involves channel proteins, which do not bind to molecules but rather open a channel that allows for the rapid transport of smaller molecules and ions, such as sodium, potassium, calcium and chlorine.