Organelle

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Schematic of typical animal cell, showing subcellular components. Organelles: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) Cytoskeleton (8) smooth ER (9) mitochondria (10) vacuole (11) cytoplasm (12) lysosome (13) centrioles

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.

Although the term "organelle" is considered in some cell biology spheres to be synonymous with "cell compartment", some 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 probably having originated from endosymbiont bacteria:

• mitochondria (in almost all eukaryotes)
• plastids (in plants and algae)
  • chloroplasts (mature forms of etioplasts)
  • chromoplasts
  • leucoplasts
    • amyloplasts
      • statoliths
    • elaioplasts
    • proteinoplasts
  • rhodoplasts (in red algae)
  • apicoplasts (in the malaria parasite, Plasmodium falciparum)

Further organelles are suggested to have had endosymbiotic origins (notably the flagellum; see evolution of flagella), but these theories are neither widely accepted nor phylogenetically verified.

[edit] 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-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

[edit] Prokaryotic organelles

Prokaryotes are not as structurally complex as eukaryotes, and were thought 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. Even more striking is the description of prokaryotic organelles, such as magnetosomes (see Komeili et al. Science, 2006 and Scheffel et al. Nature, 2006) and the nucleus-like organelles of the Planctomycetes (see Fuerst Annual Review of Microbiology, 2005), surrounded by lipid membranes.

[edit] See also

• Cell
• Endosymbiotic theory

[edit] References

• Alberts, Bruce et al. (2002). The Molecular Biology of the Cell, 4th ed., Garland Science, 2002, ISBN 0-8153-3218-1.
• Kerfeld, Cheryl A et al., Protein Structures Forming the Shell of Primitive Bacterial Organelles, Science 309:936-938 (5 August 2005) Pubmed.