Figure 13–25 Retrieval of soluble ER resident proteins. er resident proteins that escape from the er are returned by vesicle transport. (a) The kdel receptor present in both vesicular tubular clusters and the golgi apparatus captures the soluble er resident proteins and carries them in copI-coated transport vesicles back to the er. (recall that the copIcoated vesicles shed their coats as soon as they are formed.) upon binding its ligands in the tubular cluster or golgi, the kdel receptor may change conformation, so as to facilitate its recruitment into budding copI-coated vesicles. (b) The retrieval of er proteins begins in vesicular tubular clusters and continues from later parts of the golgi apparatus. In the environment of the er, the er resident proteins dissociate from the kdel receptor, which is then returned to the golgi apparatus for reuse. we discuss the different compartments of the golgi apparatus shortly.
The golgi apparatus consists of an ordered series of compartments
Because it could be selectively visualized by silver stains, the Golgi apparatus was one of the first organelles described by early light microscopists. It consists of a collection of flattened, membrane-enclosed compartments called cisternae, that somewhat resemble a stack of pita breads. Each Golgi stack typically consists of four to six cisternae (Figure 13–26), although some unicellular flagellates can have more than 20. In animal cells, tubular connections between corresponding cisternae link many stacks, thus forming a single complex, which is usually located near the cell nucleus and close to the centrosome (Figure 13–27A). This localization depends on microtubules. If microtubules are experimentally depolymerized, the Golgi apparatus reorganizes into individual stacks that are found throughout the cytoplasm, adjacent to ER exit sites. Some cells, including most plant cells, have hundreds of individual Golgi stacks dispersed throughout the cytoplasm where they are typically found adjacent to ER exit sites (Figure 13–27B).
Figure 13–26 The Golgi apparatus.
Three-dimensional reconstruction from electron micrographs of the golgi apparatus in a secretory animal cell. The  cis face of the golgi stack is that closest to the er. (b) a thin-section electron micrograph of an animal cell. In plant  cells, the golgi apparatus is generally  more distinct and more clearly separated from other intracellular membranes than  in animal cells. (a, redrawn from a.
rambourg and y. clermont, Eur. J. Cell Biol. 51:189–200, 1990. with permission from wissenschaftliche verlagsgesellschaft; b, courtesy of brij J. gupta.)
Figure 13–27 Localization of the Golgi apparatus in animal and plant cells.
The golgi apparatus in a cultured fibroblast stained with a fluorescent antibody that recognizes a golgi resident protein (bright orange). The golgi apparatus is polarized, facing the direction in which the cell was crawling before fixation.
The golgi apparatus in a plant cell that is expressing a fusion protein consisting of a resident golgi enzyme fused to green fluorescent protein. (a, courtesy of John henley and Mark Mcniven; b, courtesy of chris hawes.)