Receptors and lipids. Illustration of a cell membrane, showing the role played by lipids (orange) in membrane biochemistry. Here, two receptors (one yellow, one red) are shown floating in lipid 'rafts', areas with higher concentrations of cholesterol compared to the surrounding phospholipid bilayer that makes up the normal membrane structure. The phospholipids consist of a phosphate head (blue) and a fatty acid tail (orange). Membrane receptors are specialised proteins that receive chemicals as part of signalling between cells and the surrounding environment.