Is a ribosome an organelle? Depends which biologist you ask
Organelle literally means 'little organ', a term the German zoologist Karl August Mobius proposed in 1884 for the working parts of single cells. Yet biologists still disagree on what qualifies. Some count only membrane-wrapped compartments, some only those descended from ancient bacteria, and others include protein machines like the ribosome that have no membrane at all.
An organelle is a specialised structure inside a cell that performs a particular job, much as organs do in a body. Mobius first used organula, a Latin diminutive, then argued in a published correction that the parts of one-celled organisms deserved the name organella, since they were pieces of a single cell rather than multicellular organs. Decades earlier, in the 1830s, Felix Dujardin had already rejected Christian Ehrenberg's claim that microbes simply carry miniature versions of animal organs.
Definitions come in three strengths. The broadest includes any distinct functional unit. A narrower one keeps only membrane-bound structures, such as the nucleus, endoplasmic reticulum, Golgi apparatus and lysosomes. The strictest counts only endosymbiotic ones, mitochondria and plastids, which were once free-living cells. Almost all eukaryotes have mitochondria and an internal membrane system, while chloroplasts appear only in plants, algae and some protists. Exceptions abound: some organisms lack mitochondria entirely, and certain double-membraned organelles occasionally turn up with one or three membranes.
Membrane-less organelles, sometimes called biomolecular complexes, are large assemblies built mainly of protein, RNA or DNA. Examples include ribosomes, spliceosomes, proteasomes, ATP synthase, nucleosomes and centrioles. How they hold together without a wrapping has been compared to liquid-liquid phase separation, the way oil droplets stay distinct in water. Structures sticking out of the cell, such as cilia, bacterial flagella and archaella, are often called organelles too.
Scientists identify organelles under the microscope and can isolate them by breaking cells apart and separating the fragments. The large ones, the nucleus and vacuoles, were among the first things seen after microscopes were invented. Prokaryotes lack true eukaryotic organelles, but some contain protein-shelled microcompartments considered primitive equivalents. Notably, the cell membrane and cell wall are not organelles, and how many of each organelle a cell contains depends on its job.
Source: Organelle
