3D structural analysis of ER morphology. (A and B) 2D tomograph derived from a 200-nm-thick section shows the NE (orange), pmaER, cecER, tubER, and Golgi (pink; A) and corresponding 3D model (of A) shows all ER domains in a wt yeast cell (bud size = 665 nm; B). The blue shade is the PM. N is the nucleus. Black holes on the NE are nuclear pores. The orange arrow points to a more tubular pmaER structure, whereas the blue arrow points to a fenestrated cisternal pmaER. (C–E) 2D tomograph of cecER (C), tubER (D), and pmaER (with white and black arrows pointing at PM and ER membranes, respectively; E). Note that the black dots are ribosomes (red arrow in C). (F) Range of measured distances between the pmaER and PM membranes (e.g., from black to white arrows in E). (G) The percentage of ribosomes bound to the cytosolic versus PM face of the pmaER demonstrates that the PM face is mostly ribosome excluded. (H) Volume/surface area ratios were calculated from our 3D models for vesicles (30 and 60 nm), tubER, pmaER, and cecER. Brackets show range of measurements, and boxes show SEM. Horizontal lines show means given above the boxes. Bars: (A, C, and D) 200 nm; (B) 100 nm; (E) 50 nm.