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Biological cell

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 organellesEdit

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)photosynthesisdouble-membrane compartmentplants, protistshas some genes
endoplasmic reticulummodification and folding of new proteins and lipidssingle-membrane compartmentall eukaryotes
Golgi apparatussorting and modification of proteinssingle-membrane compartmentmost eukaryotes
mitochondrionenergy productiondouble-membrane compartmentmost eukaryoteshas some genes
vacuolestorage & homeostasissingle-membrane compartmenteukaryotes
nucleusDNA maintenance & transcription to RNAdouble-membrane compartmentall 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
acrosomehelps spermatoza fuse with ovumsingle-membrane compartmentmany animals
centrioleanchor for cytoskeletonMicrotubule proteinanimals
ciliummovement in or of external mediumMicrotubule proteinanimals, protists, few plants
glyoxysomeconversion of fat into sugarssingle-membrane compartmentplants
hydrogenosomeenergy & hydrogen productiondouble-membrane compartmenta few unicellular eukaryotes
lysosomebreakdown of large moleculessingle-membrane compartmentmost eukaryotes
melanosomepigment storagesingle-membrane compartmentanimals
mitosomenot characterizeddouble-membrane compartmenta few unicellular eukaryotes
myofibrilmuscular contractionbundled filamentsanimals
nucleolusribosome productionprotein-DNA-RNAmost eukaryotes
parenthesomenot characterizednot characterizedfungi
peroxisomeoxidation of proteinsingle-membrane compartmentall eukaryotes
ribosometranslation of RNA into proteinsRNA-protein eukaryotes & prokaryotes
vesiclemiscellaneoussingle-membrane compartmentall eukaryotes

Other related structures:

Prokaryotic organellesEdit

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
carboxysomecarbon fixationprotein-shell compartmentsome bacteria
flagellummovement in external mediumprotein filamentsome prokaryotes and eukaryotes
magnetosomemagnetic orientationinorganic crystal, proteinmagnetotactic bacteria
nucleoidDNA maintenance & transcription to RNADNA-proteinprokaryotes
plasmidDNA exchangecircular DNAsome bacteria
ribosometranslation of RNA into proteinsRNA-protein eukaryotes & prokaryotes

See alsoEdit

ReferencesEdit

  • 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
ar:عضية

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