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Cellular compartments in cell biology comprise all of the closed parts within the cytosol of a eukaryotic cell, usually surrounded by a single or double lipid layer membrane. These compartments are often, but not always, defined as membrane-bound organelles. The formation of cellular compartments is called compartmentalization.
Both organelles, the mitochondria and chloroplasts (in photosynthetic organisms), are compartments that are believed to be of endosymbiotic origin. Other compartments such as peroxisomes, lysosomes, the endoplasmic reticulum, the cell nucleus or the Golgi apparatus are not of endosymbiotic origin. Smaller elements like vesicles, and sometimes even microtubules can also be counted as compartments.
It was thought that compartmentalization is not found in prokaryotic cells.,[1] but the discovery of carboxysomes and many other metabolosomes revealed that prokaryotic cells are capable of making compartmentalized structures, albeit these are in most cases not surrounded by a lipid bilayer, but of pure proteinaceous built.[2][3][4]