Lipid osmosis, membrane tension, and other mechanochemical driving forces of lipid flow.

Nonvesicular lipid transport among different membranes or membrane domains plays crucial roles in processes such as lipid homeostasis and organelle biogenesis. However, the forces that drive such lipid transport are not well understood. We propose that lipids tend to flow towards the membrane area with a higher membrane protein density in a process termed lipid osmosis . This process lowers the membrane tension in the area when the lipid flow reaches equilibrium. We examine the thermodynamic basis and experimental evidence of lipid osmosis and the corresponding osmotic membrane tension . We predict that lipid osmosis can drive lipid flows between different membrane regions through lipid transfer proteins, scramblases, or other similar barriers that selectively pass lipids but not membrane proteins. We also speculate on the biological functions of lipid osmosis. Finally, we explore other driving forces for lipid transfer and describe potential methods and systems to further test our theory.