Exocytosis is the process by which materials packaged in vesicles are secreted from a cell when the vesicle membrane fuses with the cell membrane (Figure 6.16B). This fusing allows the vesicle contents to be released into the environment, and the vesicle membrane is smoothly incorporated into the cell membrane. In another form of exocytosis, the vesicle touches the cell membrane and a pore forms, releasing the vesicle’s contents. There is no membrane fusion in this process, termed “kiss and run.” We saw an example of exocytosis in describing sweat glands at the start of the chapter.
Table 6.2 summarizes examples of endocytosis and exocytosis.
| Type of process | Example |
|---|---|
| Endocytosis | |
| Receptor- |
Specific uptake of large molecules (e.g., LDL) |
| Pinocytosis | Nonspecific uptake of extracellular fluid (e.g., fluids and dissolved substances from blood) |
| Phagocytosis | Nonspecific uptake of large undissolved particles (e.g., invading bacteria by cells of the immune system) |
| Exocytosis | |
| Release of large molecules | Vesicle fusion with cell membrane (e.g., digestive enzymes in the pancreas) |
| Release of small molecules | Vesicle fusion with cell membrane (e.g., neurotransmitters at the synapse) |