Examples
While most cell biologists consider the term organelle to be synonymous with "cell compartment", other cell biologists choose to limit the term organelle to include only those that are DNA-containing, having originated from formerly autonomous microscopic organisms acquired via endosymbiosis.
Under this definition, there would only be two broad classes of organelles (i.e. those that contain their own DNA, and have originated from endosymbiotic bacteria):
- mitochondria (in almost all eukaryotes)
- plastids (e.g. in plants, algae, and some protists).
Other organelles are also suggested to have endosymbiotic origins, but do not contain their own DNA (notably the flagellum - see evolution of flagella).
Under the more restricted definition of membrane-bound structures, some parts of the cell do not qualify as organelles. Nevertheless, the use of organelle to refer to non-membrane bound structures such as ribosomes is common. This has led some texts to delineate between membrane-bound and non-membrane bound organelles. These structures are large assemblies of macromolecules that carry out particular and specialized functions, but they lack membrane boundaries. Such cell structures include:
- ribosome
- cytoskeleton
- flagellum
- centriole and microtubule-organizing center (MTOC)
- proteasome.
Read more about this topic: Organelle
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