Difference between revisions of "Diffusion - Physiology"

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==Introduction==
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|linkpage =Transport Across Membranes - Physiology
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|linktext =TRANSPORT ACROSS MEMBRANES
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Gases or liquids can be unevenly distributed between two areas.  If one area has a higher concentration than the other then the differance between these two areas is termed the '''concentration gradient'''.  The equality is then corrected by the movement of the molecules down this so called gradient from the region of high concentration to that of low.  This process is '''passive''' as the molecules do not have to be forced to do this and it is reffered to as '''diffusion'''.
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==Introduction==
 
  
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If gases or liquids are unevenly distributed over time this unequality is corrected by the movement of molecules from the region of high concentration to that of low.  This is achieved by the process of diffusion.  This works by the random '''thermal movement''' of molecules.  If there is a gas present in an air tight room and then a door is opened into the next room there laws of probability state that some of the randomly moving molecules will escape through the door into the adjoining room and that eventually the concentrations in both rooms with be approximately the same.  In the period between the foor opening and the rooms having the same concentratione net movement will be from the original room to the new room because as there are more molecules in the original room the probability of movement from here is increased.
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This works by the random '''thermal movement''' of molecules.  If there is a gas present in an air tight room and then a door is opened into the next room where a lower concentration of the same gas is present the laws of probability state that more of the randomly moving molecules will escape through the door from the area of high concentration than will escape back through the door from the area of low and that eventually the concentrations in both rooms with be approximately the same.  The net movement therefore will be from the room with a high concentration to that of a low concentration.
  
 
==Ficks Law==
 
==Ficks Law==
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[[Image:simpdifdiag.jpg|right|thumb|250px|<small><center>A Simple Schematic Diagram of Diffusion Across a Cell Membrane(Courtesy of Mariana Ruiz Villarreal)</center></small>]]
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<big>'''Q = DA((C<sub>1</sub> - C<sub>2</sub>) &#247; L)'''</big>
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'''Q''' = the transport rate of a substance by diffusion
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'''D''' = The diffusion coefficient
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'''A''' = The cross sectional area which the substance is to diffuse across
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'''C<sub>1</sub> C<sub>2</sub>''' = The concentrations in the two areas 
  
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  <big>'''Q = DA((C<sub>1</sub> - C<sub>2</sub>) &#247; L)'''</big>
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'''L''' = distance separating the two areas
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  '''Q''' = the transport rate of a substance by diffusion
 
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  '''D''' = The diffusion coefficent
 
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  '''A''' = The cross sectional area which the substance is to diffuse accross
 
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  '''C<sub>1</sub> C<sub>2</sub>''' = The concentrations in the two areas 
 
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  '''L''' = distance seperating the two areas
 
  
 
Ficks law demonstrates that the rate of diffusion can be maximised by increasing the area over which diffusion can occur, minimising the distance it has to occur over and increasing the gradient it occurs along.  In the body it tends to be the former two which are taken into account when adapting surfaces for diffusion to occur over.
 
Ficks law demonstrates that the rate of diffusion can be maximised by increasing the area over which diffusion can occur, minimising the distance it has to occur over and increasing the gradient it occurs along.  In the body it tends to be the former two which are taken into account when adapting surfaces for diffusion to occur over.
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==Diffusion as a Transport Mechanism==
 
==Diffusion as a Transport Mechanism==
  
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Diffusion is very efficent over short distances and does not requrie energy.  As such it is used in a variety of body systems where gradients and membranes suited to allow it are found.  However over large distances diffusion is very slow hence the need for a circulatory system to carry say oxygen around the body.  Although oxygen enters the blood in the lungs and leaves the blood in the tissue by diffusion the circulatory system is needed to fill the gap in between.
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Diffusion is very efficient over short distances and does not require energy.  As such it is used in a variety of body systems where gradients and membranes suited to it are found.  However over large distances diffusion is very slow.
  
 
==Diffusion Through the Lipid Bilayer==
 
==Diffusion Through the Lipid Bilayer==
  
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* Lipid soluble substances are able to diffuse freely across the lipid bilayer of cells.   
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* Lipid soluble substances are able to diffuse freely across the [[Phospholipid Bilayer - Anatomy & Physiology|lipid bilayer of cells.]]  
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* This obviously occurs passively down the concentration gradient.
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* This occurs passively down the concentration gradient.
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==Diffusion Through Water Filled Protein Channels==
 
  
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Hydrophillic ions and molecules are unable to diffuse across the lipid bilayer.  However if concentration gradients allow it they are sometimes able to diffuse across the membrane using water filled protein channels.  These tend to be only large enough for small ions to pass through and so are perhaps more commonly known as '''ion channels'''.  As well as concentration gradients, electrical gradients can cause the movement of ions.  They tend to move towards areas of the charge opposite to themselves.  The two factors of concentration and electrical are termed collectively as the electrochemical gradient and it is their combined effect which determines the movement of ions through channels.  The channels are usually specific to one or a few ions and can be opened and closed by the cell depending on what process ions are required for.
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[[Category:Transport Across Membranes]]

Latest revision as of 14:41, 5 July 2012


Introduction

Gases or liquids can be unevenly distributed between two areas. If one area has a higher concentration than the other then the differance between these two areas is termed the concentration gradient. The equality is then corrected by the movement of the molecules down this so called gradient from the region of high concentration to that of low. This process is passive as the molecules do not have to be forced to do this and it is reffered to as diffusion.


This works by the random thermal movement of molecules. If there is a gas present in an air tight room and then a door is opened into the next room where a lower concentration of the same gas is present the laws of probability state that more of the randomly moving molecules will escape through the door from the area of high concentration than will escape back through the door from the area of low and that eventually the concentrations in both rooms with be approximately the same. The net movement therefore will be from the room with a high concentration to that of a low concentration.

Ficks Law

A Simple Schematic Diagram of Diffusion Across a Cell Membrane(Courtesy of Mariana Ruiz Villarreal)

Q = DA((C1 - C2) ÷ L)


Q = the transport rate of a substance by diffusion

D = The diffusion coefficient

A = The cross sectional area which the substance is to diffuse across

C1 C2 = The concentrations in the two areas

L = distance separating the two areas

Ficks law demonstrates that the rate of diffusion can be maximised by increasing the area over which diffusion can occur, minimising the distance it has to occur over and increasing the gradient it occurs along. In the body it tends to be the former two which are taken into account when adapting surfaces for diffusion to occur over.

Diffusion as a Transport Mechanism

Diffusion is very efficient over short distances and does not require energy. As such it is used in a variety of body systems where gradients and membranes suited to it are found. However over large distances diffusion is very slow.

Diffusion Through the Lipid Bilayer

  • Lipid soluble substances are able to diffuse freely across the lipid bilayer of cells.
  • This occurs passively down the concentration gradient.


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