Cells have functions for secreting various substances synthesized in the cell to the external environment. In prokaryotic cells, synthesized proteins are directly secreted outside the cell through the cell membrane; however, in animal and plant cells, proteins are secreted outside the cell through vesicles called secretory vesicles. This type of secretion is called exocytosis. For example, various substances such as digestive enzymes, serum albumin, hormones, and neurotransmitters are secreted outside the animal cell as far as to ECM, which plays an important role in tissue architecture. These are sorted by the trans Golgi network in the Golgi body, loaded into a secretory vesicle, and transported in the direction of the cell membrane. The secretory vesicle then secretes inner substances outside the cell by fusing with the cell membrane (see Fig. 12-9).

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