Organelle

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organelle, any of the specialized structures within a cell that perform a specific function (e.g., mitochondria, ribosomes, endoplasmic reticulum). Organelles in unicellular organisms are the equivalent of organs in multicellular organisms. The contractile vacuole of protozoans, for example, extracts fluid wastes from the cell and eliminates them from the organism, as does the kidney in larger organisms.