While carrying out functions such as growth, division and synthesis, cells use and produce substances that have to be able to cross cell and organelle membranes.

Semipermeable cell membranes allow some molecules to travel across a concentration gradient from the high-concentration side of the membrane to the low-concentration side through simple diffusion.

Facilitated diffusion lets other important molecules cross in a selective manner in that it uses proteins embedded in the cell membrane to allow certain substances to cross.

The membrane proteins of facilitated diffusion either form openings in the membrane and control what can pass, or they actively carry specific molecules through the membrane. This process is especially important for controlling the flow of ions because many cell functions depend on the presence of certain ions to allow a chemical reaction to proceed.

In addition to ions, the carrier proteins can also facilitate the passage of large molecules such as glucose.

Substances that the cell produces or that it needs can be transported across cell and organelle membranes in several ways. Passive transport doesn't require an energy input and uses the concentration gradient to power the movement of molecules.

In the simple diffusion type of passive transport, diffusion takes place across a semipermeable membrane from the side with a higher concentration of the transported substance to the side with a low concentration. The substance passes through the membrane down the concentration gradient, but some molecules are blocked.

If blocked molecules have to cross the membrane because they are needed on the other side, facilitated diffusion can transport specific molecules.

The diffusion method works through membrane-embedded proteins but still relies on the concentration gradient to power molecular movement across the membrane. It doesn't require energy, but the proteins can be selective about which molecules they transport.

Sometimes molecules have to be transported across membranes from a side with a low concentration to the side that has a high concentration. This goes against the concentration gradient and requires energy.

Cells that carry out active transport have produced energy and have stored it in adenosine triphosphate (ATP) molecules.

Active transport is based on proteins similar to those used for facilitated diffusion, but they use energy from ATP to carry molecules across the membrane against the concentration gradient.

After forming a bond with the molecule to be transported, they use a phosphate group from ATP to change shape and deposit the molecule on the other side of the membrane.

Cell membranes can allow the passage of many small molecules, but charged ions and larger molecules are generally blocked. Facilitated diffusion is a method by which such substances can enter and leave the cells. Carrier proteins embedded in the membrane can facilitate the passage of ions in two ways.

Some proteins are arranged around a central passage and create a hole in the plasma membrane of the cell, opening up a path through the fatty acids of the membrane's interior. Specific ions can pass through such openings, but the carrier proteins are designed to let only one kind of ion pass.

Other proteins don't form openings but transport large molecules through the cell membranes. The transfer is still powered by a concentration gradient, but the carrier proteins actively link to the substance they are transporting.

The part of the protein that is outside the cell membrane in the extracellular space binds to the molecule of the substance to be transported and then releases it into the cell interior.

Normally the hydrophobic non-polar fatty acids of the membranes block the passage of charged polar molecules such as sodium ions. The carrier proteins that provide openings for such ions attract the ions and facilitate their passage through ion channels.

They may be designed for and let pass only sodium ions but not others such as potassium ions. Carrier protein openings may also control the flow of ions, shutting down when the cell doesn't need more ions.

For the transport of glucose molecules, which are normally too big to pass through the membrane, glucose transporter proteins have a site where they can bind to the glucose molecules. They attach themselves and facilitate the transport of glucose across the cell membrane. The location of a carrier protein becomes a permeable gap in the membrane that doesn't allow the glucose molecule to cross elsewhere.

Cells in multicellular organisms have to coordinate their activities, such as when to grow and when to divide. The cells accomplish this coordination by signaling what kind of activity they are engaged in and what is needed, releasing signaling chemicals. Facilitated diffusion helps with cell signaling.

Signals can be local or long distance, affecting cells in the immediate neighborhood or cells in other organs and tissues. In each case, signaling molecules travel between cells and have to either enter target cells or attach to their membrane to deliver their signal.

Facilitated diffusion proteins can allow these signaling molecules to enter cells as needed and close the communication loop.

Because facilitated diffusion is a passive transport mechanism, it is governed by factors in the immediate environment where the transport is taking place.

There are four such factors:

• Concentration: Facilitated diffusion relies on the potential energy represented by the concentration gradient. A greater difference between the high and low concentration sides means a higher gradient and faster diffusion.
• Carrier protein capacity: The rate of binding between the substance to be transferred and the protein along with the transfer speed affects the rate of diffusion.
• Number of carrier protein sites: More sites means higher diffusion capacity and faster diffusion.
• Temperature: Chemical reactions are temperature-dependent, and a higher temperature means faster reaction progress and more rapid diffusion.

While cells can control the number of carrier protein sites, the carrier protein capacity is fixed, and the cell has a limited ability to control the process temperature and the substance concentration outside the cell. The ability to close off carrier protein site activity becomes important for controlling cell processes.

Simple diffusion takes care of cell needs in terms of small non-polar molecules, but other important substances can't easily cross the membranes. Polar molecules and larger molecules can't diffuse across the semipermeable plasma membranes of cells and organelles because the interior layer of lipids and fatty acids blocks them.

Facilitated diffusion allows substances with polar or large molecules to enter and exit the cells in a controlled manner.

Glucose and amino acids, for example, are large molecules that play a key role in cell functions. Glucose is an important nutrient, and amino acids are used for many cell processes, including cell division.

For these processes to proceed, facilitated diffusion allows the molecules to pass through cell membranes and membranes of organelles such as the nucleus.

Even smaller molecules such as oxygen can benefit from facilitated diffusion. Although oxygen can diffuse across membranes, facilitated diffusion through carrier proteins increases the rate of transfer and helps with the functions of blood cells and muscles.

Overall, these membrane-embedded proteins play a vital role in a variety of cell processes.

Other topics:

• Carbon dioxide
• Red blood cells