Facilitated diffusion

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Carrier protein showing a conformational change
Facilitated diffusion is the movement of lipid insoluble molecules across the phospholipid bilayer by the use of carrier proteins[1].

Contents

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 action of facilitated diffusion is spontaneous, however, the rate of the diffusion differs according to how permeable a membrane is for each substance. Most membranes are selectively permeable so different membranes have different permeability. For example, water is diffused freely across cell membranes. Other substances, however, must pass through the lipid bilayer first.[2] 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[3].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].

Facilitated is characterised by the following:

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[4]. 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[5] to release the molecules on the other side of the membrane (where there is a lower concentration of the molecules)[6].

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 intracellular messenger-gated ion channel[7]. 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[8].

There are three forms of transport can with facilitated diffusion ;

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 transport is limited by the number of carrier proteins present in the membrane[9].
  3. The time taken for the molecule to bind to the carrier protein.
  4. Type of carrier protein utilized as some carriers are also specific to similarly shaped molecules[6].
  5. The affinity of the carrier protein for its substrate molecule[10].

References

  1. 1.0 1.1 1.2 Dee Unglaub Silverthorn (2010). Human Physiology. 5th Edition. Pearson Internation Edition. Page 145-146.
  2. ThoughtCo. Regina Bailey. Diffusion and Passive Transport. Updated October 23 2017.[cited 5/12/17]; Available from: https://www.thoughtco.com/diffusion-and-passive-transport-373399
  3. Alberts et al (2002). Molecular Biology of the Cell. 4th Edition. US Garland Science. Page 618.
  4. Marieb E. (2004) Human Anatomy and Physiology, 6th edition, San Francisco: Pearson Education, inc. page 72-73
  5. Marieb E. (2004) Human Anatomy and Physiology, 6th edition, San Francisco: Pearson Education, inc. page 72-73
  6. 6.0 6.1 Barry G. Hinwood (1992). A Textbook of Science for the Health Professions. Nelson Thornes. Page 255-256.
  7. http://www.vivo.colostate.edu/hbooks/cmb/cells/pmemb/diffusion_f.html
  8. http://zoologysangamnercollege.yolasite.com/resources/TRANSPORT%20ACROSS%20MEMBRANE.pdf
  9. Marieb E. (2004) Human Anatomy and Physiology, 6th edition, San Francisco: Pearson Education, inc. page 72-73
  10. Alberts et al. (2008). Molecular Biology of the Cell. Fifth Edition. US Garland Science
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