Organelle

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:

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

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:

• ribosome
• cytoskeleton
• flagellum

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:

• cytosol
• endomembrane system
• nucleosome
• microtubule
• cell membrane

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

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

• Cell
• Endosymbiotic theory

References

• Alberts, Bruce et al. (2002). The Molecular Biology of the Cell, 4th ed., Garland Science, 2002, ISBN 0-8153-3218-1.