Organelle

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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.

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

Organelles typically have their own plasma membrane round them.[1] Most of the cell's organelles are in the cytoplasm.[2][3]

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.[4] They are not organised like eukaryote organelles, and are not bounded by plasma membranes. They are called bacterial microcompartments.[5]

Scope of the term[change | edit source]

The term is now widely used to refer to cellular structures surrounded by single or double plasma membranes.[6][7][8][9] However, the older definition of a 'subcellular functional unit' still coexists. That is, the term is sometimes used for structures which are not membrane-bound.[10][11]

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[change | edit source]

Mitochondria and chloroplasts, which have double-membranes and their own DNA, are believed to have originated from incompletely consumed or invading prokaryotic organisms, which were adopted as a part of the invaded cell. This idea is supported in the endosymbiotic theory.

Eukaryotic organelles[change | edit source]

Major organelles[change | edit source]

Major eukaryotic organelles
Organelle Main function Structure Organisms Notes
chloroplast (plastid) photosynthesis double-membrane compartment plants, protists (rare kleptoplastic organisms) has some DNA; originated by endosymbiosis.
endoplasmic reticulum translation and folding of new proteins (rough endoplasmic reticulum), expression of lipids (smooth endoplasmic reticulum) single-membrane compartment all eukaryotes rough endoplasmic reticulum is covered with ribosomes, has folds that are flat sacs; smooth endoplasmic reticulum has folds that are tubular
Golgi apparatus sorting and modification of proteins single-membrane compartment all eukaryotes
mitochondrion energy production double-membrane compartment most eukaryotes has some DNA; originated by endosymbiosis
vacuole storage, homeostasis single-membrane compartment eukaryotes
nucleus DNA maintenance, RNA transcription double-membrane compartment all eukaryotes has bulk of genome

Minor organelles[change | edit source]

Minor 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 collects 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
eyespot apparatus detects light, allowing phototaxis to take place green algae and other unicellular photosynthetic organisms such as Euglena
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 known double-membrane compartment a few unicellular eukaryotes
myofibril muscular contraction bundled filaments animals
nucleolus ribosome production protein-DNA-RNA most eukaryotes
parenthesome not known not known fungi
peroxisome breakdown of metabolic hydrogen peroxide single-membrane compartment all eukaryotes
proteasome degradation of unneeded or damaged proteins by proteolysis very large protein complex All eukaryotes, all archaea, some bacteria
ribosome translation of RNA into proteins RNA-protein eukaryotes, prokaryotes
vesicle material transport single-membrane compartment all eukaryotes

Prokaryotic organelles[change | edit source]

Prokaryotes are not so complex as eukaryotes. They were thought to have no internal structures enclosed by lipid membranes.[12]

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.[4] Even more striking is the description of membrane-bound magnetosomes in bacteria.[13][14] as well as the nucleus-like structures of the Planctomycetes that are surrounded by lipid membranes.[15]

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

References[change | edit source]

  1. See 'Scope of the term' below.
  2. Alberts, Bruce et al. 2003. Essential cell biology, 2nd ed. Garland Science.
  3. National Research Council (U.S.). Committee on Research Opportunities in Biology (1989). Opportunities in biology. p. 104. ISBN 9780309039277. http://books.google.com/books?id=Tz8rAAAAYAAJ&source=gbs_navlinks_s. Retrieved 23 December 2010.
  4. 4.0 4.1 Kerfeld C. et al (2005). "Protein structures forming the shell of primitive bacterial organelles.". Science 309 (5736): 936–8. doi:10.1126/science.1113397. PMID 16081736.
  5. Bobik T.A. (2007). "Bacterial microcompartments" (PDF). Microbe (Am Soc Microbiol) 2: 25–31. http://www.asm.org/ASM/files/ccLibraryFiles/Filename/000000002765/znw00107000025.pdf.
  6. Nultsch, Allgemeine Botanik, 11. Aufl. 2001, Thieme Verlag
  7. Wehner/Gehring, Zoologies, 23. Aufl. 1995, Thieme Verlag
  8. Alberts, Bruce et al. 2002. The molecular biology of the cell, 4th ed, Garland Science. ISBN 0-8153-3218-1. online via "NCBI-Bookshelf"
  9. Brock, Mikrobiologie, 2. korrigierter Nachdruck 2003. der 1. Aufl. von 2001
  10. Strasburger's Lehrbuch der Botanik für Hochschulen, 35. Aufl. 2002. S. 42
  11. Alliegro MC, Alliegro MA, Palazzo RE (2006). "Centrosome-associated RNA in surf clam oocytes". Proc. Nat. Acad. Sci. USA 103 (24): 9037–9038. doi:10.1073/pnas.0602859103. PMC 1482561. PMID 16754862.
  12. Ryter A (1988). "Contribution of new cryomethods to a better knowledge of bacterial anatomy". Ann. Inst. Pasteur Microbiol. 139 (1): 33–44. doi:10.1016/0769-2609(88)90095-6. PMID 3289587.
  13. Komeili A. et al (2006). "Magnetosomes are cell membrane invaginations organized by the actin-like protein MamK". Science 311 (5758): 242–5. doi:10.1126/science.1123231. PMID 16373532.
  14. Scheffel A. et al (2006). "An acidic protein aligns magnetosomes along a filamentous structure in magnetotactic bacteria". Nature 440 (7080): 110–4. doi:10.1038/nature04382. PMID 16299495.
  15. Fuerst JA (2005). "Intracellular compartmentation in planctomycetes". Annu. Rev. Microbiol. 59: 299–328. doi:10.1146/annurev.micro.59.030804.121258. PMID 15910279.