Describe the structure and function of the Golgi apparatus. Explain where the Golgi apparatus fits into the directional flow of membrane that exists in the cell.
The Golgi apparatus, like the ER, is also a network of membrane-bound fluid-filled spaces that resemble stacks of flattened sacs. Indeed, the Golgi apparatus is connected at points to the ER and forms another part of the internal communications system of a cell. The Golgi apparatus functions in aiding secretion from the cell; the apparatus collects, prepares, packages, and, finally, releases secretory materials to the surface of the cell in membrane-encased secretory vesicles. In pancreatic cells, for example, the Golgi apparatus packages and releases the hormone insulin to the blood. The Golgi apparatus also produces lysosomes, vesicles that contain powerful hydrolytic enzymes for digesting particles taken into the cell during endocytosis. After one of the Golgi's secretory vesicles has ruptured and expelled its secretory materials from the cell, its membrane often becomes part of the cell membrane itself. This transformation represents the last stage in the directional flow of membrane that exists in cells, whereby the nuclear membrane differentiates into rough ER, then into smooth ER, then into Golgi apparatus, then into secretory vesicles, and ultimately into cell membrane.
