Facilitated diffusion
Facilitated diffusion is the movement of lipid insoluble molecules across the phospholipid bilayer by the use of carrier proteins[1].
Basic 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[1]. 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[2].
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[1].
Carrier proteins0
These are proteins that span the plasma membrane (transmembrane proteins) and are also known as permeases[3]. 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[4] to release the molecules on the other side of the membrane (where there is a lower concentration of the molecules)[5].
Factors affecting rate of facilitated diffusion
- Difference in concentration between the two sides of the membrane.
- 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. [6]
- The time taken for the molecule to bind with the carrier protein.
- Type of carrier protein utilized as some carriers are also specific to similar shaped molecules.
References
- ↑ 1.0 1.1 1.2 Dee Unglaub Silverthorn (2010). Human Physiology. 5th Edition. Pearson Internation Edition. Page 145-146.
- ↑ Alberts et al (2002). Molecular Biology of the Cell. 4th Edition. US Garland Science. Page 618.
- ↑ Marieb E. (2004) Human Anatomy & Physiology, 6th edition, San Francisco: Pearson Education, inc. page 72-73
- ↑ Marieb E. (2004) Human Anatomy & Physiology, 6th edition, San Francisco: Pearson Education, inc. page 72-73
- ↑ 5.0 5.1 Barry G. Hinwood (1992). A Textbook of Science for the Health Professions. Nelson Thornes. Page 255-256.
- ↑ Marieb E. (2004) Human Anatomy & Physiology, 6th edition, San Francisco: Pearson Education, inc. page 72-73