Here Cornelius Hunter explains how a cell membrane works primarily in poikilothermic organisms (body temp is mediated by environment). Hunter is an IDer but his account of the changes in the cell wall membrane is very clear and instructive. I feel as he does: how did chance mutations set this up as life developed?- http://darwins-god.blogspot.com/2011/06/biological-control-of-cell-membrane.html