Membranes and evolution.

Biological membranes are thin amphiphilic sheaths, only a few nanometres thick, that define both the boundaries of all cells as well as the diversity of the internal compartments in eukaryotes. The plasma membrane of a typical prokaryote houses about 20-30% of the cell's expressed proteins, and its lipids account for approximately 10% of the cell's dry mass. The numbers for eukaryotic cells are comparable - the difference in surface area to volume ratio is overall compensated by the eukaryotic endomembrane system. Roughly a fourth of the protein encoded by the human genome carries at least one stretch of sequence predicted to serve as a transmembrane domain. Membranes host substrate exchange, sensing and communication, and life-giving energy conservation via chemiosmotic ATP synthesis.