Diffusion - Physiology

()Map TRANSPORT ACROSS MEMBRANES (Map)

Introduction

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.

Ficks Law

 Q = DA((C1 - C2) ÷ L)
 Q = the transport rate of a substance by diffusion 
 D = The diffusion coefficent 
 A = The cross sectional area which the substance is to diffuse accross 
 C1 C2 = The concentrations in the two areas  
 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.

Diffusion as a Transport Mechanism

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.

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.

Diffusion Through Water Filled Protein Channels

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.