What would be the effects of the heat from a volcano on a cell membrane?

Cell membranes are composed of phosopholipid bilayers. 
This means that there is a polar phosphate chain on the exterior of the membrane and
non-polar fatty lipid chains on the interior.  There are also many surface bound and
embedded proteins in the cell membrane that allow chemicals to pass through or attach to
the cell.  Cell membranes are designed to operate most efficiently at standard body
temperatures.  A significant increase in heat will cause the proteins to denature, or
lose their shape.  When proteins lose their shape they lose their biological function as
well.  An increase in temperature will also allow the cell membrane to become less
restrictive and allow more chemicals to enter into the cell, including damaging
chemicals.  Of course at temperatures on the order of fire (or a volcano), carbon based
compounds will simply combust, or burn into elemental carbon (charcoal), thus destroying
life.