Organelle

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


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.

A typical animal cell. Within the cytoplasm, the major organelles and cellular structures include: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (6) Golgi apparatus (7) cytoskeleton (8) smooth endoplasmic reticulum (9) mitochondria (10) vacuole (11) cytosol (12) lysosome (13) centriole.

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:

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:

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)

Major eukaryotic organelles
Organelle Main function Structure Organisms Notes
chloroplast (plastid) photosynthesis double-membrane compartment plants, protists has some genes
endoplasmic reticulum modification and folding of new proteins (rough endoplasmic reticulum) and lipids (smooth endoplasmic reticulum) single-membrane compartment all eukaryotes rough endoplasmic reticulum is devoid with ribosomes, folds are flat sacs; smooth endoplasmic reticulum has folds which are tubular
Golgi apparatus sorting and modification of proteins single-membrane compartment all eukaryotes cis face (convex) nearest to rough endoplasmic reticum; trans face (concave) farthest to rough endoplasmic reticulum
mitochondrion energy production double-membrane compartment most eukaryotes has some genes
vacuole storage & homeostasis single-membrane compartment eukaryotes
nucleus DNA maintenance & transcription to RNA double-membrane compartment all eukaryotes has bulk of genome

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 eukaryotic organelles and cell components
Organelle/Macromolecule Main function Structure Organisms
acrosome helps spermatoza fuse with ovum single-membrane compartment many animals
autophagosome vesicle which sequesters cytoplasmic material and organelles for degradation double-membrane compartment all eukaryotic cells
centriole anchor for cytoskeleton Microtubule protein animals
cilium movement in or of external medium Microtubule protein animals, protists, few plants
glycosome carries out glycolysis single-membrane compartment Some protozoa, such as Trypanosomes.
glyoxysome conversion of fat into sugars single-membrane compartment plants
hydrogenosome energy & hydrogen production double-membrane compartment a few unicellular eukaryotes
lysosome breakdown of large molecules (e.g. proteins + polysaccharides) single-membrane compartment most eukaryotes
melanosome pigment storage single-membrane compartment animals
mitosome not characterized double-membrane compartment a few unicellular eukaryotes
myofibril muscular contraction bundled filaments animals
nucleolus ribosome production protein-DNA-RNA most eukaryotes
parenthesome not characterized not characterized fungi
peroxisome oxidation of protein single-membrane compartment all eukaryotes
ribosome translation of RNA into proteins RNA-protein eukaryotes & prokaryotes
vesicle miscellaneous single-membrane compartment all eukaryotes

Other related structures:

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.[1] Even more striking is the description of prokaryotic organelles, such as magnetosomes,[2][3] as well as the nucleus-like organelles of the Planctomycetes that are surrounded by lipid membranes.[4]

Prokaryotic organelles and cell components
Organelle/Macromolecule Main function Structure Organisms
carboxysome carbon fixation protein-shell compartment some bacteria
chlorosome photosynthesis light harvesting complex green sulfur bacteria
flagellum movement in external medium protein filament some prokaryotes and eukaryotes
magnetosome magnetic orientation inorganic crystal, lipid membrane magnetotactic bacteria
nucleoid DNA maintenance & transcription to RNA DNA-protein prokaryotes
plasmid DNA exchange circular DNA some bacteria
ribosome translation of RNA into proteins RNA-protein eukaryotes & prokaryotes
thylakoid photosynthesis photosystem proteins and pigments mostly cyanobacteria

See also

References

  1. Kerfeld CA, Sawaya MR, Tanaka S, et al (2005). "Protein structures forming the shell of primitive bacterial organelles". Science 309 (5736): 936-8. PMID 16081736.
  2. Komeili A, Li Z, Newman DK, Jensen GJ (2006). "Magnetosomes are cell membrane invaginations organized by the actin-like protein MamK". Science 311 (5758): 242-5. PMID 16373532.
  3. Scheffel A, Gruska M, Faivre D, Linaroudis A, Plitzko JM, Schüler D (2006). "An acidic protein aligns magnetosomes along a filamentous structure in magnetotactic bacteria". Nature 440 (7080): 110-4. PMID 16299495.
  4. Fuerst JA (2005). "Intracellular compartmentation in planctomycetes". Annu. Rev. Microbiol. 59: 299-328. PMID 15910279.
  • Alberts, Bruce et al. (2002). The Molecular Biology of the Cell, 4th ed., Garland Science, 2002, ISBN 0-8153-3218-1.
ar:عضية

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