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Vesicles (from the Latin "vesicula" - blisters) are cell compartments separated from a single or double membrane. As spherical or oval cell organelles, vesicles measure only about one micron in diameter. They are used for transport processes (for example, for protein transport from the Golgi apparatus to the endoplasmic reticulum), or for the course of certain chemical processes that could not take place in the cytoplasm.
A distinction is made between exocytotic vesicles and endocytic vesicles. Exocytotic vesicles contain substances that are to be excreted by the cell. The plasma membrane of the vesicle merges with the cell membrane, whereby the contents of the vesicle are released to the outside. Conversely, in endocytic vesicles, parts of the cell membrane are collapsed together with the material to be ingested and finally cut off, forming a vesicle.
The inner milieu of the vesicles (pH value, enzymatic endowment) varies individually. Depending on the enzyme supply, a distinction is made between a number of specialized vesicles:
lysosomes are formed by the Golgi apparatus and contain hydrolytic enzymes and phosphatases. They serve the digestion of the body's own and foreign substances and thus also play a role in the immune system. After synthesis on the Golgi apparatus, lysosomes in the cytoplasm can fuse with other cell organelles, creating so-called secondary lysosomes. In most of the body cells, heterolysosomes, which result from the fusion of lysosomes and endosomes and generally serve to digest cell-derived material, are predominant. In phagocytes, on the other hand, phagocysosms predominate. These arise from the fusion of lysosomes and phagosomes and serve the digestion of non-cellular material.
peroxisomes are similar in structure to the lysosomes, but are not formed on the Golgi apparatus, but replicate themselves. They contain the enzymatic endowment to form free radicals from oxygen compounds used for defense. Furthermore, the peroxisomes are also important for the energy metabolism in the liver cells.
microsomes are the smallest vesicles (about 0.1 microns in diameter). They consist of fragments of the endoplasmic reticulum and contain the essential cytochrome P450 enzyme for metabolism.