Golgi network cis face cis cisterna trans cisterna
transfer vesicle trans face
Figure 10.8. The Golgi apparatus.
transfer vesicle cis cisterna trans cisterna trans-Golgi network secetory vesicle trans face
morphology of the membranes from which it is formed. The cis cisternae are made of membranes 5.5 nm thick, like those of the rough endoplasmic reticulum, while the trans cisternae are 10 nm thick, like the plasma membrane. Each cisterna is characterized by a central flattened region where the luminal space, as well as the gap between adjacent cisternae, is uniform. The margin of each cisterna is often dilated and is often fenestrated (i.e., has holes through it) as well.
Small, spherical vesicles are always found in association with the Golgi apparatus, especially with the edges of the cis face. These are referred to as transfer vesicles; some of them carry proteins from the rough endoplasmic reticulum to the Golgi stacks, others transfer proteins between the stacks, that is, from cis to middle and from middle to trans. As proteins move through the Golgi apparatus, the oligosaccharides already attached to them are modified, and additional oligosaccharides can be added. As well as having important functions once the protein has reached its final destination, glycosylations play an important role in sorting decisions at the trans-Golgi network.
Most of the single-membrane organelles of the eukaryotic cell pass material between themselves by vesicular traffic, in vesicles that bud off from one compartment to fuse with another (Fig. 10.9). In this way the cargo proteins are never in contact with the cytosol. Two main directions of traffic can be identified (Fig. 10.1). The exocytotic pathway runs from the endoplasmic reticulum through the Golgi apparatus to the plasma membrane. The endocytotic pathway runs from the plasma membrane to the lysosome. This is the route by which extracellular macromolecules can be taken up and processed. If vesicles are to be moved over long distances, they are transported along cytoskeletal highways (page 389).
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