Size scales of cellular lipid assemblies. Lipids (average length≈2 nm and cross section≈0.7 nm²) can form high-order structures of different sizes, such as micelles (≈5 nm diameter), transport vesicles (50–200 nm diameter), or long lipid bilayers that define the boundaries of entire organelles and of the whole cell (up to several μm). Molecular schemes: yellow, hydrophobic moieties; grey, polar moieties. Electron microscopy images of HeLa cells courtesy of Bruno Mesmin and Sandra Lacas-Gervais.