Fig. 1. Electron micrographs of `studs', the globular headpieces of the V<sub>1</sub> complex of the V-type H<sup>+</sup> ATPase. (a) Stereoelectron micrograph of a vesicle from the apical region of a mitochondria-rich cell after rapid freezing and freeze-drying of apical membrane segments of toad urinary bladder (courtesy of D. Brown, Boston). (b) Negative stained electron micrograph of a vesicle after purification of goblet cell apical membranes from the midgut of the tobacco hornworm (courtesy of M. Huss and H. Wieczorek, Osnabrück). The diameter of `studs' (portasomes) is approximately 10 nm. The average density is about 16 800 studs µm^-2 of membrane in the toad urinary bladder (Brown et al., 1987) and about 5000 µm^-2 in the tobacco hornworm.