Constituents of cells, which can be clearly separated from the rest of the cell, characterized by specific cellular functions. (Mitochondria for example are responsible for metabolic pathways resulting in the production of ATP, chloroplasts are responsible for photosynthesis.)
A number of organelles have evolved from formerly independent
organisms. These organisms have been integrated into the eukaryotic cells to such an extent
that they can hardly be recognized as independent units
(
endosymbioses).