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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.
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.