Transport Across the Cell Membrane
The cell uses a number of mechanisms to transport material across the membrane. Some types of transport do not require the cell to expend energy and are called passive transport. Other types of transport do require the cell to expend energy and are called active transport. Active transport can be used to move a few molecules at a time or to transport many molecules at one time in bulk transport.
Endocytosis is a form of bulk transport that moves material into a cell from the environment. This transport can move parts of cells, particles, or even whole cells into a cell. The substance needing entry clusters at a location of the cell membrane, often due to the presence of specific receptors. The cell membrane then forms a pocket around the substances. This pocket, or invagination, pinches off from the internal surface of the cell membrane, forming a vesicle. The vesicle travels inside the cell and fuses with a membrane-bound structure such as a lysosome. The fusion of the vesicle opens the pocket and allows for the release of the material from the vesicle.Exocytosis is a form of bulk transport used to move large molecules to the outside of the cell. This can be thought of as a reverse process of endocytosis. Exocytosis involves expelling a substance from the cell into the extracellular fluid that surrounds the cell. This is done first by enclosing the material in an intracellular vesicle that buds from a membrane-bound structure such as the Golgi apparatus. The vesicle travels to the cell membrane and fuses with the interior side of the cell membrane. Fusion opens the vesicle by creating an invagination in the exterior side of the cell membrane. The contents of the vesicle are then released to the environment.