The cell membrane comprises numerous protein and lipid molecules capable of asymmetric organization between leaflets and liquid-liquid phase separation. We use single supported lipid bilayers (SLBs) to model cell membranes, and study how cholesterol and asymmetrically oriented ganglioside receptor G(M1) affect membrane structure using synchrotron x-ray reflectivity. Using mixtures of cholesterol, sphingomyelin, and 1,2-dioleoyl-sn-glycero-3-phosphocholine, we characterize the structure of liquid-ordered and liquid-disordered SLBs in terms of acyl-chain density, headgroup size, and leaflet thickness. SLBs modeling the liquid-ordered phase are 10 A thicker and have a higher acyl-chain electron density (rho(chain) = 0.33 e(-)/A(3)) compared to SLBs modeling the liquid-disordered phase, or pure phosphatidylcholine SLBs (rho(chain) = 0.28 e(-)/A(3)). Incorporating G(M1) into the distal bilayer leaflet results in membrane asymmetry and thickening of the leaflet of 4-9 A. The structural effect of G(M1) is more complex in SLBs of cholesterol/sphingomyelin/1,2-dioleoyl-sn-glycero-3-phosphocholine, where the distal chains show a high electron density (rho(chain) = 0.33 e(-)/A(3)) and the lipid diffusion constant is reduced by approximately 50%, as measured by fluorescence microscopy. These results give quantitative information about the leaflet asymmetry and electron density changes induced by receptor molecules that penetrate a single lipid bilayer.