Concluding (closing?) remarks

This paper summarizes a series of ideas and/or concepts, most of which were the object of the <i>Faraday Discussion</i> on 'Structural and functional asymmetry of plasma membranes'. A historical review is provided of the early symmetrical and asymmetrical models for membrane structure. Membrane asymmetry in the framework of evolution is suggested as a possible field of study. Functional membrane asymmetry in experimental models is briefly discussed, and the hypothesis that functional asymmetry preceded structural asymmetry in evolution is proposed. Lipid asymmetry and lipid scrambling in bilayers are presented as two complementary aspects of the same process. The use of sphingomyelinases in experimental studies of asymmetry is criticized, due to the lipid scrambling properties of the sphingomyelinase end-product ceramide. The paper ends with a note on the (apparently?) cyclical nature of scientific research.