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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.

A few large organelles probably originated from endosymbiont bacteria:

Other organelles have had endosymbiotic origins suggested for them (notably flagella; see Evolution of flagella), but these theories are not widely accepted.

Eukaryotic organelles[edit | edit source]

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).

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 and lipids single-membrane compartment all eukaryotes
Golgi apparatus sorting and modification of proteins single-membrane compartment most eukaryotes
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 bags within cells; later, as other cell parts were discovered, the meaning was extended to also include smaller parts of cells.

Other eukaryotic organelles and cell components
Organelle Main function Structure Organisms
acrosome helps spermatoza fuse with ovum single-membrane compartment many animals
centriole anchor for cytoskeleton Microtubule protein animals
cilium movement in or of external medium Microtubule protein animals, protists, few plants
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 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 organelles[edit | edit source]

Prokaryotes are not as structurally complex as eukaryotes, and do 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.

Prokaryotic organelles and cell components
Organelle Main function Structure Organisms
carboxysome carbon fixation protein-shell compartment some bacteria
flagellum movement in external medium protein filament some prokaryotes and eukaryotes
magnetosome magnetic orientation inorganic crystal, protein 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

See also[edit | edit source]

References[edit | edit source]

  • 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).
Organelles of the cell
Acrosome | Chloroplast | Cilium/Flagellum | Centriole | Endoplasmic reticulum | Golgi apparatus | Lysosome | Melanosome | Mitochondrion | Myofibril | Nucleus | Parenthesome | Peroxisome | Plastid | Ribosome | Vacuole | Vesicle

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