Facilitated diffusion

Carrier protein showing a conformational change

Facilitated diffusion is the movement of lipid insoluble molecules across the phospholipid bilayer by the use of carrier proteinsBasic Features

The bilayer consists of lipids therefore only lipid-soluble molecules can pass through. This is mainly small polar uncharged molecules and small hydrophobic molecules. Facilitated diffusion is a passive process that requires no use of external energy. The molecules move across the membrane from an area of high concentration to an area of low concentration. If the solute carries a net charge for example, molecules will move down its electrochemical gradient.
Sugars and amino acids are examples of molecules that move across the plasma membrane using facilitated diffusion. Also, GLUT transporters are a group of carrier proteins that move glucose sugars and associated hexose sugars across the plasma membrane.

Facilitated is characterised by the following :

• High rate of transport.
• Saturation which leads to a decrease in transport across the membrane might occur as there are a limited number of carriers which might be fully active. 
• Specifity as carriers are specific for substances they transport .
• Competition with substances with structural similarity as substrate might occur.
• Temperature sensitivity and carriers can be denatured at high temperatures.
• Inhibition with drugs might occur.

Two major types of facilitated diffusion:

1. Carrier proteins

These are proteins that span the plasma membrane (transmembrane proteins) and are also known as permeases. Each protein carrier is specific to bind to a complementary molecule. On one side of the membrane (higher concentration of molecules), the molecules bind to the carrier protein. The carrier then changes conformational shape, moving the binding site from one side of the membrane to the other to release the molecules on the other side of the membrane (where there is a lower concentration of the molecules).

2. Ion channel proteins

Unlike carrier proteins, ion channel proteins have gate to control the passage of substances across the cell membrane down their electrochemical gradient usually by the ligand-gated ion channel, voltage-gated ion channel or intracelullar messenger-gated ion channel. Ion channels are integral membrane proteins (hydrophobic region) that do not require binding of solutes but only allow specific types of solutes to pass through the protein channel. 

There are three forms of transport can with facilitated diffusion ;

• Uniport which transports only a single substance for example , glucose transporter.
• Antiport which exchanges one molecule for another for example, sodium calcium exchanger (Na⁺/Ca²⁺).
• Symport which cotransports two or more substances all in one direction for example, sodium potassium chloride cotransporter (Na⁺/K⁺/Cl^-).

Factors affecting rate of facilitated diffusion

1. Difference in concentration between the two sides of the membrane.
2. The frequency of carrier proteins available on the plasma membrane: When all carrier proteins are holding molecules they are known to be 'saturated' and are working at their maximal rate, so the rate of tranport is limited by the number of carrier proteins present in the membrane .
3. The time taken for the molecule to bind with the carrier protein.
4. Type of carrier protein utilized as some carriers are also specific to similar shaped molecules.
5. The affinity of the carrier protein for its substrate molecule.

References

      9. Alberts et al (2008). Molecular Biology of the Cell. Fifth Edition. US Garland Science.