So size difference is, like many generalizations, only a useful yardstick, not an absolute truth. Basic Structures of Prokaryotic Cells. Only water, lipid-soluble materials pass freely. But cells have many specific protein carriers in membrane, carry out selective transport. View interactive pdb file of phospholipid .
Phospholipids assemble into a lipid bilayer .
But hopanoids present, probably stabilize membranes. The latter molecule produces lipid monolayers instead of bilayers . View physiology of Archaea . Membrane proteins contribute stability and many functions, such as transport. Length is about 1000x longer than cell.