Ten questions on membrane biophysics

Biological membranes are among the most complex and functionally versatile structures in living cells. In this Perspective, dedicated to the memory of Joachim Seelig, we explore ten questions that, in our view, define the current frontiers of membrane biophysics. We begin by asking why cells maintain such extraordinary lipid diversity when, as recent work on minimal cells demonstrates, life can survive with just two lipid species. We examine membrane asymmetry and its functional consequences, the nature of protein-lipid interactions, and how the membrane itself can act as an allosteric modulator of protein function through physical mechanisms such as the lateral pressure profile. We consider the controversy surrounding lipid rafts, the consequences of membrane crowding, and what we sacrifice when studying membranes and membrane proteins using mimetic systems. We explore membranes as non-equilibrium systems maintained by continuous free-energy dissipation, the challenges of targeting membranes pharmacologically, and how membranes evolved and develop. Throughout, we emphasize that membranes are not passive barriers but active participants in cellular function, shaped by billions of years of evolution and endowed with a compositional complexity we are only beginning to understand.