Changes

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.

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.