The cell membrane, also called the plasma membrane, of eukaryotic cells is composed of a phospholipid bilayer. These phospholipids are composed of a polar head, made up of a phosphate group, and two non-polar fatty acid tails. This amphipathic nature of phospholipids creates a semi-permeable membrane, fluid enough to allow for the growth and movement of the cell, but solid enough to hold the shape of the cell.
The phospholipids of the membrane are arranged tail to tail, creating a protective, hydrophobic region in-between the bilayer. This hydrophobic area prevents many types of molecules from entering the cell; including large, polar or charged molecules. This is an important feature for the regulation of substances and concentrations within and without of the cell; however, it poses a problem because most vital molecules are unable to pass through the membrane via simple diffusion. Water, for example, is vital to a cells survival. Water is also a polar molecule. How, then, does the cell allow water and similar molecules to pass in and out? By using a process termed facilitated diffusion.
There are many ways for a cell to transport molecules across its membrane. Diffusion is an example of passive transport. Passive means that no energy is required. Facilitated diffusion is a specific type of passive transport specific to large molecules, such as glucose, polar molecules, such as water, or ions, such as Na+.
Facilitated diffusion is performed by various types of proteins that are embedded within the cell membrane. While there are hundreds of different proteins throughout the cell, only two types are found associated with facilitated diffusion: channel proteins and carrier proteins.
Channel proteins typically are used to transport ions in and out of the cell. Channel proteins come in two forms, open channels and gated channels. Open channels are simply trans-membrane proteins that form a hydrophilic pore in the membrane allowing the charged (and therefore hydrophilic) ions to flow in and out of the cell along their respective concentration gradients. Gated channels are slightly more specific. Found commonly in the nervous system, gated channels open and close in response to a specific stimulus. This stimulus can be either electrical, i.e. a buildup of charge from accumulated ions, or molecular, meaning that a second substance binds to the protein, opening the channel thus allowing the original substance to flow through the channel.
Carrier proteins transport molecule by changing their shape, or conformation, to move a specific molecule across the membrane. A buildup of some substance on one side of the carrier protein causes a conformational change in the protein allowing the substance to be moved across the plasma membrane.