3.19: The Golgi apparatus processes products for delivery throughout the body.

Moving farther outward from the nucleus, we encounter another organelle within the endomembrane system: the Golgi (GOHL-jee) apparatus (FIGURE 3-37). The Golgi apparatus processes molecules synthesized in the cell—primarily proteins and lipids—and packages those that are destined for use elsewhere in the body. The Golgi apparatus is also a site of carbohydrate synthesis, including the complex polysaccharides found in many plasma membranes. The Golgi apparatus, which is not connected to the endoplasmic reticulum, is a flattened stack of membranes (each of which is called a Golgi body) that are not interconnected.

After transport vesicles bud from the endoplasmic reticulum, they move through the cytoplasm until they reach the Golgi apparatus. Here, the vesicles fuse with the Golgi apparatus membrane and dump their contents into a Golgi body. The molecules pass through about four successive Golgi body chambers. In each Golgi body, enzymes make slight modifications to the molecules (such as the addition or removal of phosphate groups or sugars). The processing that occurs in the Golgi apparatus often involves tagging molecules (much like adding a postal address or tracking number) to direct them to some other part of the organism. After they are processed, the molecules bud off from the Golgi apparatus in a vesicle, which then moves into the cytosol. If the molecules are destined for delivery and use elsewhere in the body, the transport vesicle eventually fuses with the cell’s plasma membrane and dumps the molecules into the bloodstream via exocytosis.

FIGURE 3-38 shows how the various parts of the endomembrane system work to produce, modify, and package molecules within a cell.