Fig. 8. Schematic diagram of fracture planes through ER membranes and lipid droplets. (A) In the electron microscope lipid droplets (LD) are frequently seen cupped by the ER. (B) One ER membrane lies adjacent to the droplet whereas the other membrane is farther removed. At the molecular level both membranes are comprised of cytoplasmic and endoplasmic leaflets of phospholipids, and the lumen of the ER intervenes between the two membranes. The lipid droplet core is enveloped by a phospholipid monolayer. (C) Upon fracturing between membrane leaflets as indicated by dashed lines, P-faces of the inner ER membrane, E-faces of outer ER membrane and the E-face equivalent of the lipid droplet core are revealed together with convexly fractured droplets. (D) Concave (E-face_eq) fractures of droplets, by contrast, are accompanied by E-faces of the inner ER membrane, P-faces of the outer ER membrane and the P-face of the lipid droplet monolayer. Phospholipid molecules (and immunoreactive integral membrane proteins, not shown) remaining on the replicas after SDS washing are depicted in blue (P-face) and purple (E-face). Gray areas represent residual membrane monolayers removed by SDS washing. Asterisks represent the physical locations of gold particles in the replicas marking specific membrane proteins following immunogold labeling. Arrows indicate the direction of viewing of the replicas in the electron microscope.