Organelle



In cell biology, an organelle is a discrete structure of a cell having specialized functions. There are many types of organelles, particularly in the eukaryotic cells of higher organisms. An organelle is to the cell what an organ is to the body (hence the name organelle, the suffix -elle being a diminutive). Organelles were historically identified through the use of microscopy, and were also identified through the use of cell fractionation.

A few large organelles probably originated from endosymbiont bacteria: Other organelles have had endosymbiotic origins suggested for them (notably flagella; see Evolution of flagella), but these theories are not widely accepted.
 * mitochondria (in almost all eukaryotes)
 * plastids (in plants and algae)
 * chloroplasts, mature forms of etioplasts
 * chromoplasts
 * leucoplasts
 * amyloplasts
 * statoliths
 * elaioplasts
 * proteinoplasts
 * rhodoplasts

Eukaryotic organelles
Eukaryotes are the most structurally complex known cell type, and by definition are in part organized by smaller interior compartments, that are themselves enclosed by lipid membranes that resemble the outermost cell membrane. The larger organelles, such as the nucleus and vacuoles, are easily visible with moderate magnification (although sometimes a clear view requires the application of chemicals that selectively stain parts of the cells); they were among the first biological discoveries made after the invention of the microscope.

Not all eukaryotic cells have all of the organelles listed below, and occasionally, exceptional species of cells are missing organelles which might otherwise be considered universal to eukaryotic cells (such as mitochondria). There are also occasional exceptions to the number of membranes surrounding organelles, listed in the tables below (e.g. some which are listed as double-membraned are sometimes found with single or triple membranes).

Organelles which have double-membranes and their own DNA are believed by many biologists of having originally come from incompletely consumed or invading prokaryotic cells, which were adopted as a part of the invaded cell through endosymbiosis.

Originally, the word organelle referred to large lipid bags within cells; later, as other cell parts were discovered, the meaning was extended to also include smaller parts of cells.

Other related structures:
 * cytosol
 * endomembrane system
 * nucleosome
 * microtubule
 * cell membrane

Prokaryotic organelles
Prokaryotes are not as structurally complex as eukaryotes, and do not have any compartments enclosed by lipid membranes. In the past they were often viewed as having little internal organization, but slowly details are emerging about prokaryotic internal structures. One contributing discovery was that at least some prokaryotes have microcompartments, which are compartments enclosed by proteins.