Organelle

In cell biology, an organelle is a part of a cell that does a specific job.

Organelles usually have a plasma membrane around them.See 'Scope of the term' below. Most of the cell's organelles are in the cytoplasm.Alberts, Bruce et al. 2003. Essential cell biology, 2nd ed. Garland Science.

The name organelle comes from the idea that these structures are to cells what an organ is to the body.

There are many types of organelles in eukaryotic cells. Prokaryotes were once thought not to have organelles, but some examples have now been found.
They are not organized like eukaryote organelles, and are not bounded by plasma membranes. They are called bacterial microcompartments.

 Scope of the term 
The term is now widely used to refer to cell structures surrounded by single or double plasma membranes.Nultsch, Allgemeine Botanik, 11 Aufl. 2001, Thieme VerlagWehner/Gehring: Zoologies. 23. Aufl. 1995, Thieme VerlagBrock, Microbiology, 2. korrigierter Nachdruck 2003. der 1. Aufl. von 2001 However, the older definition of a 'subcellular functional unit' still exists. So, the term is sometimes used for structures which are not membrane-bound.Strasburger's Lehrbuch der Botanik für Hochschulen, 35. Aufl. 2002. S. 42

The plasma membrane is a lipid bilayer with some proteins embedded in it. It keeps the ions and molecules of the organelle from merging with the surroundings.

 Origin of organelles 
Mitochondria and chloroplasts have double-membranes and their DNA. These are believed to come from incompletely digested or invading prokaryotes, which were adopted as a part of the invaded cell. This idea is supported in the endosymbiotic theory.

 Eukaryotic organelles
 Major organelles 
+Major eukaryotic organellesOrganelleMain functionStructureOrganismsNoteschloroplast (plastid)photosynthesisdouble-membrane compartmentplants, protists has some DNA; originally obtained from endosymbiosis.endoplasmic reticulumtranslation and folding of new proteins (rough endoplasmic reticulum), expression of lipids (smooth endoplasmic reticulum)single-membrane compartmentall eukaryotesrough endoplasmic reticulum has many ribosomes, and folds that are flat sacs; smooth endoplasmic reticulum has folds that are tubularflagellumlocomotion, sensory some eukaryotesGolgi apparatussorting and modification of proteinssingle-membrane compartmentall eukaryotesmitochondrionenergy productiondouble-membrane compartmentmost eukaryoteshas some DNA; originally obtained by endosymbiosisnucleusDNA maintenance, RNA transcriptiondouble-membrane compartmentall eukaryotes has bulk of genomevacuolestorage, homeostasissingle-membrane compartment eukaryotes 

 Minor organelles 
+Minor eukaryotic organelles and cell componentsOrganelle/MacromoleculeMain functionStructureOrganismsacrosomehelps spermatoza fuse with ovumsingle-membrane compartmentmany animalsautophagosomevesicle which collects cytoplasmic material and organelles for degradationdouble-membrane compartmentall eukaryotic cellscentrioleanchor for cytoskeletonmicrotubule proteinanimalsciliummovement in or of external medium.  microtubule proteinanimals, protists, few plantscnidocyst stinging coiled hollow tubule cnidarianseyespot apparatusdetects light, allowing phototaxis to take place green algae and other unicellular photosynthetic organisms such as Euglenaglycosomecarries out glycolysissingle-membrane compartmentSome protozoa, such as Trypanosomes.glyoxysomeconversion of fat into sugarssingle-membrane compartmentplantshydrogenosomeenergy & hydrogen productiondouble-membrane compartmenta few unicellular eukaryoteslysosomebreakdown of large molecules (e.g., proteins + polysaccharides)single-membrane compartmentby convention, animal cells; but (wider definition) most eukaryotesmelanosomepigment storagesingle-membrane compartmentanimalsmitosomenot knowndouble-membrane compartmenta few unicellular eukaryotesmyofibrilmuscular contractionbundled filamentsanimalsnucleolusribosome productionprotein-DNA-RNAmost eukaryotesparenthesomenot knownnot knownfungiperoxisomebreakdown of metabolic hydrogen peroxidesingle-membrane compartmentall eukaryotesproteasomedegradation of unneeded or damaged proteins by proteolysisvery large protein complexAll eukaryotes, all archaea, some bacteriaribosometranslation of RNA into proteinsRNA-protein eukaryotes, prokaryotesstress granule  mRNA storage  membraneless (mRNP complexes) Most eukaryotesvesiclematerial transportsingle-membrane compartmentall eukaryotes

 Prokaryotic organelles 
Prokaryotes are not so complex as eukaryotes. They were once thought to have no internal structures inside the lipid membranes.

However, recent research has shown that at least some prokaryotes have microcompartments such as carboxysomes. These subcellular compartments are 100–200 nm in diameter and are enclosed by a shell of proteins.  Even more striking is the description of membrane-bound magnetosomes in bacteria. as well as the nucleus-like structures of the Planctomycetes that are surrounded by lipid membranes.

+Prokaryotic organelles and cell componentsOrganelle/MacromoleculeMain functionStructureOrganismscarboxysomecarbon fixationprotein-shell compartmentsome bacteriachlorosomephotosynthesislight harvesting complexgreen sulfur bacteriaflagellummovement in external mediumprotein filamentsome prokaryotes and eukaryotesmagnetosomemagnetic orientationinorganic crystal, lipid membranemagnetotactic bacterianucleoidDNA maintenance, transcription to RNADNA-proteinprokaryotesplasmidDNA exchangecircular DNAmostly cyanobacteriaribosometranslation of RNA into proteinsRNA-protein eukaryotes, prokaryotesthylakoidphotosynthesisphotosystem proteins and pigmentssome bacteria

 References