noun, plural of organelle
Literally, the term means "little organs". As the body is composed of various organs, the cell, too, has "little organs" that perform special functions. They are membrane-bound compartments or structures of a cell.
A eukaryotic cell contains many organelles, for example, the nucleus, endoplasmic reticulum, golgi apparatus, mitochondria, and chloroplast (plastid). However, not all these organelles are found in only one cell or in an organism. The chloroplast, for instance, is abundant in plant cells but not in animal cells.
Prokaryotes, which were believed to have no organelles, have been recently described to possess “organelles”. Examples are carboxysome (a protein-shell compartment for carbon fixation in some bacteria), chlorosome (a light harvesting complex in green sulfur bacteria), magnetosome (found in magnetotactic bacteria), and thylakoid (in some cyanobacteria).