The basic structural component of cellular membranes are lipid molecules, of which a typical cell contains about a thousand different types. This is an amazingly large number, considering that a single species of lipid suffices to make a perfectly nice membrane. Over the past two decades biologists and biophysicists have come to appreciate that membranes are more than just fluid elastic sheets that compartmentalize cells and hold a host of embedded proteins in place. Rather, their fluidity,...