The Golgi Apparatus and Lysosomes

Golgi Apparatus

Recall that that vesicles can bud from the ER and transport their contents elsewhere, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles still need sorting, packaging, and tagging so that they wind up in the right place. Sorting, tagging, packaging, and distribution of lipids and proteins takes place in the Golgi apparatus. We also refer to this organelle as the Golgi body, a series of flattened membranes (see image below).

The Golgi apparatus in this white blood cell is visible as a stack of semicircular, flattened rings. Notice this in the lower portion of the image. See the several vesicles near the Golgi apparatus. Image Attribution: modification of work by Louisa Howard

How the Golgi Apparatus Functions

We refer to the receiving side of the Golgi apparatus as the cis face. Conversely, we refer to the opposite side as the trans face. The transport vesicles that formed from the ER travel to the cis face, fuse with it. Then, they empty their contents into the lumen of the Golgi apparatus. As the proteins and lipids travel through the Golgi, they undergo further modifications that allow for their sorting. The most frequent modification is the addition of short chains of sugar molecules. These newly modified proteins and lipids are then tagged with phosphate groups or other small molecules. This happens so that they can be routed to their proper destinations.

Finally, the modified and tagged proteins are packaged into secretory vesicles that bud from the trans face of the Golgi. While some of these vesicles deposit their contents into other parts of the cell where they are useful, other secretory vesicles fuse with the plasma membrane and release their contents outside the cell.

In another example of form following function, cells that engage in a great deal of secretory activity (such as cells of the salivary glands that secrete digestive enzymes or cells of the immune system that secrete antibodies) have an abundance of Golgi.

In plant cells, the Golgi apparatus has the additional role of synthesizing polysaccharides. Some of these polysaccharides are incorporated into the cell wall while some are used in other parts of the cell.

Lysosomes

Lysosomes play the role of digestive component and organelle-recycling facility in animal cells. In addition, we consider them to be parts of the endomembrane system. Lysosomes also use their hydrolytic enzymes to destroy pathogens (disease-causing organisms) that might enter the cell.

A good example of this occurs in a group of white blood cells we refer to as macrophages. Macrophages are part of your body’s immune system. In a process known as phagocytosis or endocytosis, a section of the plasma membrane of the macrophage invaginates (folds in) and engulfs a pathogen. The invaginated section, with the pathogen inside, then pinches itself off from the plasma membrane and becomes a vesicle. The vesicle fuses with a lysosome. The lysosome’s hydrolytic enzymes then destroy the pathogen (see image below).

A macrophage has engulfed (phagocytized) a potentially pathogenic bacterium. Then it fuses with a lysosome within the cell to destroy the pathogen. Other organelles are present in the cell but we do not show them for simplicity. Image Attribution: OpenStax Biology