Diffusion - Physiology

()Map TRANSPORT ACROSS MEMBRANES (Map)

Introduction

If gases or liquids are unevenly distributed, this inequality will be 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 the 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 door opening and the rooms having the same concentration net movement will be from the original room to the new room. This is because as there are more molecules in the original room there is an increased chance that these molecules will leave the room; and as there are few molecules in the second room there is a slim chance that they will move into the first room.

Ficks Law

 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 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.

Diffusion Through the Lipid Bilayer

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

More Information

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