Cholesterol exists in abundance in the plasma membranes of animal cells (making up >35% of lipids in Torpedo postsynaptic membrane), and is distributed unevenly within the lipid bilayer because of its association with embedded proteins. Many membrane proteins (e.g. nicotinic acetylcholine receptors) require cholesterol to achieve their physiological function.
Cryo-EM enables identification of cholesterol-rich patches within the lipid bilayer because the cholesterol molecule exposes only a hydroxl and contributes no mass at the level of the phospholipid headgroups (asterisk, top figure).
Patches enriched in cholesterol therefore form gaps in the 'tram-track' densities made by the large phospholipid headgroups when the bilayer is viewed edge-on (as in middle figure). These gaps in the tram-track densities locate the patches, and tangential views encompassing just the phospholipid headgroup region (bottom figure) reveal how the patches are organised.
Unwin, N. Segregation of lipids near acetylcholine-receptor channels imaged by cryo-EM. IUCrJ. 4, 393-399 (2017). (pdf)