Fig. 1: Phospholipids in bilayer membranes.

Phospholipid molecules are composed of a hydrophilic polar phosphate “head” containing the phosphate and glycerol units and two hydrophobic non-polar fatty acid “tails”. Phospholipids are organized into a bilayer sheet, with two layers of molecules arranged tail-to-tail. Within this structure, the hydrophobic tails cluster together away from water, forming the membrane’s interior, while the hydrophilic heads face the aqueous environments inside and outside the vesicle. Photoswitchable phospholipids contain a photoswitchable unit in the hydrophobic tail that can be switched between two conformations upon light exposure.