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Bulk Transport

Bulk transport is not used for selective transport of particular molecules but rather, as its name implies, for transporting large amounts of various molecules, and, in some cases, even whole cells. There are two main types of bulk transport (see Figure 11.3):
  • ExocytosisExocytosis is used to transport molecules out of the cell. In this process, cytoplasmic vesicles (see Chapter 10) move to the plasma membrane and fuse with it. The membrane from the vesicles become incorporated in the plasma membrane, and the contents of the vesicles are released to the external surroundings of the cell.
  • Endocytosis: The converse of exocytosis is endocytosis. In this process, the plasma membrane “sinks” inward and forms a vesicle around particles and fluids on the extracellular side of the membrane. The newly formed vesicle pinches off from the membrane. The contents are transported to various places into the cytoplasm and may be stored for later use. There are three types of endocytosis:
  • EinocytosisThis type of endocytosis refers to the flow of liquid droplets.
  • Receptor mediated endocytosisThis type of transport involves molecules binding to specific receptors clustered at the cell surface. This triggers endocytosis of the molecules. As seen in electron micrographs, the receptors bunch up in the membrane as it sinks into the cytoplasm. Often these structures are called coated pits.
  • PhagocytosisPhagocytosis is usually found in free living cells, such as amoebas. However, it also occurs in multicellular organisms: the best example of this is white blood cells, such as macrophages, in the human immune system (see Chapter 15). In phagocytosis, the cell membrane wraps around an extracellular object, forming a large vesicle that usually fuses with lysosomes in the cytoplasm. The contents of the vesicle are then digested. Phagocytosis is distinguished from other types of endocytosis by the size and amount of material taken in, and the drastic movement and changes that occur in the cell to engulf the object.

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