There are two types of endocytosis. If the substances being taken in are particles, such as bacteria, the process is called phagocytosis. If the substances are in solution, such as the end products of digestion, then it is called pinocytosis.
Exocytosis and endocytosis Cells often have to transport large chemical molecules or material in bulk across the plasma membrane. Neither diffusion nor active transport will work here. Instead, cells can release or take in such materials in vesicles, as shown in Figure 1.24. Uptake is called endocytosis and export is exocytosis. Both require energy from ATP. During endocytosis, part of the plasma membrane is pulled inward and surrounds the liquid or solid that is to be moved from the extracellular space into the cell. The material becomes enclosed in a vesicle, which pinches off from the plasma membrane and is drawn into the cell. This is how white blood cells take in bacteria (Figure 1.24).
Figure 1.24 &YBNQMFT PG FOEPDZUPTJT BOE FYPDZUPTJT
Materials for export, such as digestive enzymes, are made in the rER and then transported to the Golgi apparatus to be processed. From here they are enclosed within a membrane-bound package known as a vesicle, and moved to the plasma membrane along microtubules. The arrangement of molecules in the membrane of a vesicle is very similar to that in the plasma membrane. As a vesicle approaches the plasma membrane, it is able to fuse with it and in doing so release its contents to the outside. The flexibility and fluidity of the plasma membrane are essential to enable both endocytosis and exocytosis to happen.Vesicles also help to transfer and organise substances in the cell. They are involved in metabolism, transport and enzyme storage and some chemical reactions also occur inside them. Nerve impulses are able to pass across synapses (the tiny gaps between one nerve cell and the next) due to exocytosis and endocytosis. Neurotransmitters are secreted at the end of a nerve cell fibre by exocytosis. They stimulate the adjacent nerve and are then reabsorbed by endocytosis to be recycled and reused.You can find out more about the transmission of nerve impulses in Subtopic 6.5.
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