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Facilitated diffusion is the movement of lipid insoluble molecules across the phospholipid bilayer by the use of carrier proteins<ref name="null">Dee Unglaub Silverthorn (2010). Human Physiology. 5th Edition. Pearson Internation Edition. Page 145-146.</ref>.  
[[Image:Carier protein.jpg|right|279x121px|Carrier protein showing a conformational change]]Facilitated diffusion is the movement of [[Lipid|lipid]] insoluble molecules across the [[Phospholipid bilayer|phospholipid bilayer]] by the use of carrier proteins<ref name="null">Dee Unglaub Silverthorn (2010). Human Physiology. 5th Edition. Pearson Internation Edition. Page 145-146.</ref>.  


=== Basic Features<br> ===
=== 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<ref name="null">Dee Unglaub Silverthorn (2010). Human Physiology. 5th Edition. Pearson Internation Edition. Page 145-146.</ref>. 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<ref name="second">Alberts et al (2002). Molecular Biology of the Cell. 4th Edition. US Garland Science. Page 618.</ref>.<br>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<ref name="null">Dee Unglaub Silverthorn (2010). Human Physiology. 5th Edition. Pearson Internation Edition. Page 145-146.</ref>.  
The bilayer consists of lipids, therefore only lipid-soluble molecules can pass through it. This is mainly small [[Polar|polar]] uncharged [[Molecules|molecules]] and small [[Hydrophobic|hydrophobic]] molecules. Facilitated diffusion is a passive process that requires no use of external energy<ref name="null">Dee Unglaub Silverthorn (2010). Human Physiology. 5th Edition. Pearson Internation Edition. Page 145-146.</ref>. 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.<ref>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</ref> 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<ref name="second">Alberts et al (2002). Molecular Biology of the Cell. 4th Edition. US Garland Science. Page 618.</ref>.Sugars and amino acids are examples of molecules that move across the [[Plasma membrane|plasma membrane]] using facilitated diffusion. Also, [[GLUT transporters|GLUT transporters]] are a group of carrier proteins that move [[Glucose|glucose]] sugars and associated [[Hexose|hexose]] sugars across the [[Plasma membrane|plasma membrane]]<ref name="null">Dee Unglaub Silverthorn (2010). Human Physiology. 5th Edition. Pearson Internation Edition. Page 145-146.</ref>.  


=== Carrier proteins<br> ===
Facilitated is characterised by the following:


[[Image:Carier protein.jpg|left|279x121px|Carrier protein showing a conformational change]]These are proteins that span the plasma membrane. 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 to release the molecules on the other side of the membrane (where there is a lower concentration of the molecules)<ref name="third">Barry G. Hinwood (1992). A Textbook of Science for the Health Professions. Nelson Thornes. Page 255-256.</ref>.  
*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.  
*Specificity 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.  
*[[Enzyme Inhibitors|Inhibition]] with drugs might occur.


=== Factors affecting rate of facilitated diffusion  ===
=== Two major types of facilitated diffusion: ===


1. Difference in concentration between the two sides of the membrane.<br>2. The frequency of carrier proteins available on the plasma membrane.<br>3. The time taken for the molecule to bind with the carrier protein.<br>4. Type of carrier protein utilized as some carriers are also specific to similar shaped molecules.<br>
==== 1. Carrier proteins ====


<ref name="third">Barry G. Hinwood (1992). A Textbook of Science for the Health Professions. Nelson Thornes. Page 255-256.</ref>  
These are [[Proteins|proteins]] that span the [[Plasma membrane|plasma membrane]] ([[Transmembrane proteins|transmembrane proteins]]) and are also known as [[Permeases|permeases]]<ref>Marieb E. (2004) Human Anatomy and Physiology, 6th edition, San Francisco: Pearson Education, inc. page 72-73</ref>. Each [[Protein|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<ref>Marieb E. (2004) Human Anatomy and Physiology, 6th edition, San Francisco: Pearson Education, inc. page 72-73</ref>,&nbsp;releasing the molecules on the other side of the membrane (where there is a lower concentration of the molecules)<ref name="third">Barry G. Hinwood (1992). A Textbook of Science for the Health Professions. Nelson Thornes. Page 255-256.</ref>.
 
==== 2. Ion channel proteins  ====
 
Unlike carrier proteins, [[Ion channels|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|ligand-gated ion channel]], [[Voltage-gated ion channel|voltage-gated ion channel]] or intracellular messenger-gated ion channel<ref>http://www.vivo.colostate.edu/hbooks/cmb/cells/pmemb/diffusion_f.html</ref>. Ion channels are integral membrane proteins ([[Hydrophobic|hydrophobic]] region) that do not require binding of solutes but only allow specific types of solutes to pass through the protein channel<ref>http://zoologysangamnercollege.yolasite.com/resources/TRANSPORT%20ACROSS%20MEMBRANE.pdf</ref>.
 
There are three forms of transport can with facilitated diffusion&nbsp;;
 
*[[Uniporter|Uniport]] which transports only a single substance for example , glucose transporter.
*[[Antiporter]]Antiport which exchanges one molecule for another for example, sodium calcium exchanger (Na<sup>+</sup>/Ca<sup>2+</sup>).
*[[Symporters|Symport]] which cotransports two or more substances all in one direction for example, [[Sodium|sodium]] [[Potassium|potassium]] [[Chloride|chloride]] cotransporter (Na<sup>+</sup>/K<sup>+</sup>/Cl<sup>-</sup>).
 
=== '''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 transport is limited by the number of carrier proteins present in the membrane<ref>Marieb E. (2004) Human Anatomy and Physiology, 6th edition, San Francisco: Pearson Education, inc. page 72-73</ref>.
#The time taken for the molecule to bind to the carrier protein.
#Type of carrier protein utilized as some carriers are also specific to similarly shaped molecules<ref name="third">Barry G. Hinwood (1992). A Textbook of Science for the Health Professions. Nelson Thornes. Page 255-256.</ref>.
#The affinity of the carrier protein for its substrate molecule<ref>Alberts et al. (2008). Molecular Biology of the Cell. Fifth Edition. US Garland Science</ref>.


=== References  ===
=== References  ===


<references />
<references />

Latest revision as of 11:51, 22 October 2018

Carrier protein showing a conformational change
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].

Basic Features

The bilayer consists of lipids, therefore only lipid-soluble molecules can pass through it. 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:

  • 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.
  • Specificity 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[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], releasing 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 ;

  • Uniport which transports only a single substance for example , glucose transporter.
  • AntiporterAntiport 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 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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