Organelle

In cell biology, an organelle is a discrete structure of a cell having specialized functions, and is separately enclosed in its own lipid membrane. There are many types of organelles, particularly in the eukaryotic cells of higher organisms. Prokaryotes were once thought not to have any organelles, but some examples have now been identified, although these are not widespread.



The name organelle comes from the idea that these structures are to cells what an organ is to the body (hence the name organelle, the suffix -elle being a diminutive). Organelles are identified through the use of microscopy, and can also be identified by cell fractionation.

Examples and disputes
Some cell biologists consider the term organelle to be synonymous with "cell compartment", other cell biologists strictly limit the term's definition to DNA-containing, formerly autonomous organisms acquired via primary, secondary, or tertiary endosymbiosis. A few of such large organelles having originated from endosymbiont bacteria:
 * mitochondria (in almost all eukaryotes)
 * plastids (in plants, algae and protists)
 * chloroplasts (mature forms of etioplasts)
 * chromoplasts
 * leucoplasts
 * amyloplasts
 * statoliths
 * elaioplasts
 * proteinoplasts
 * rhodoplasts (in red algae)
 * apicoplasts (in the malaria parasite, Plasmodium falciparum)

Other organelles are also suggested to have endosymbiotic origins (notably the flagellum; see evolution of flagella), but these hypotheses are not widely accepted nor phylogenetically verified.

Not all parts of the cell qualify as organelles, and the use of the term to refer to some structures is disputed. These structures are large assemblies of macromolecules that carry out particular and specialised functions, but they lack membranes boundaries. Such cell structures, which are likely not organelles, include:
 * ribosome
 * cytoskeleton
 * flagellum

Eukaryotic Cells
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. Eukaryotic cells include animal, plants, fungi, etc. They have membrane-bound organelles. 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). In addition to this, the amount of the individual organelles varies depending upon the function of the specific cell to which it is found (example, muscle cells have more smooth endoplasmic reticulum which helps in muscle contraction)

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-encased formerly autonomous endosymbiont within cells. As other intracellular compartments were discovered, the meaning was generalized (in the United States, mainly) to include any lipid-encased intracellular component with a specialized biochemical function.

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

Prokaryotic Cells
Prokaryotes are not as structurally complex as eukaryotes, and were thought not to have any compartments enclosed by lipid membranes. All bacterias are prokaryotic cells. They do not have membrane-bound organelles. 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. Even more striking is the description of prokaryotic organelles, such as magnetosomes, as well as the nucleus-like organelles of the Planctomycetes that are surrounded by lipid membranes.