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.