Organelle

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

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.

Organelles usually have a plasma membrane around 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 organized like eukaryote organelles, and are not bounded by plasma membranes. They are called bacterial microcompartments.^[5]

Scope of the term

The term is now widely used to refer to cell structures surrounded by single or double plasma membranes.^[6][7][8][9] However, the older definition of a 'subcellular functional unit' still exists. So, 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

Mitochondria and chloroplasts have double-membranes and their DNA. These are believed to come from incompletely digested or invading prokaryotes, which were adopted as a part of the invaded cell. This idea is supported in the endosymbiotic theory.

Eukaryotic organelles

Major organelles

Major eukaryotic organelles

Organelle	Main function	Structure	Organisms	Notes
chloroplast (plastid)	photosynthesis	double-membrane compartment	plants, protists	has some DNA; originally obtained from 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 has many ribosomes, and folds that are flat sacs; smooth endoplasmic reticulum has folds that are tubular
flagellum	locomotion, sensory		some eukaryotes
Golgi apparatus	sorting and modification of proteins	single-membrane compartment	all eukaryotes
mitochondrion	energy production	double-membrane compartment	most eukaryotes	has some DNA; originally obtained by endosymbiosis
nucleus	DNA maintenance, RNA transcription	double-membrane compartment	all eukaryotes	has bulk of genome
vacuole	storage, homeostasis	single-membrane compartment	eukaryotes

Minor organelles

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
cnidocyst	stinging	coiled hollow tubule	cnidarians
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	by convention, animal cells; but (wider definition) 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
stress granule	mRNA storage	membraneless (mRNP complexes)	Most eukaryotes
vesicle	material transport	single-membrane compartment	all eukaryotes

Prokaryotic organelles

Prokaryotes are not so complex as eukaryotes. They were once thought to have no internal structures inside the 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.^[13] Even more striking is the description of membrane-bound magnetosomes in bacteria.^[14][15] as well as the nucleus-like structures of the Planctomycetes that are surrounded by lipid membranes.^[16]

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

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. National Academies. p. 104. ISBN 978-0-309-03927-7. Retrieved 23 December 2010.
4. Kerfeld C.; et al. (2005). "Protein structures forming the shell of primitive bacterial organelles". Science. 309 (5736): 936–8. Bibcode:2005Sci...309..936K. doi:10.1126/science.1113397. PMID 16081736. S2CID 24561197.
5. Bobik T.A. (2007). "Bacterial microcompartments" (PDF). Microbe. 2. Am Soc Microbiol: 25–31. Archived from the original (PDF) on 2008-08-02. Retrieved 2011-01-06.
6. Nultsch, Allgemeine Botanik, 11 Aufl. 2001, Thieme Verlag
7. Wehner/Gehring: Zoologies. 23. Aufl. 1995, Thieme Verlag
8. Alberts, Bruce (2004). Molecular Biology of the Cell. ISBN 978-0-8153-3218-3.
9. Brock, Microbiology, 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. Bibcode:2006PNAS..103.9034A. doi:10.1073/pnas.0602859103. PMC 1482561. PMID 16754862.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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. Kerfeld CA; et al. (2005). "Protein structures forming the shell of primitive bacterial organelles". Science. 309 (5736): 936–8. Bibcode:2005Sci...309..936K. doi:10.1126/science.1113397. PMID 16081736. S2CID 24561197.
14. Komeili A.; et al. (2006). "Magnetosomes are cell membrane invaginations organized by the actin-like protein MamK". Science. 311 (5758): 242–5. Bibcode:2006Sci...311..242K. doi:10.1126/science.1123231. PMID 16373532. S2CID 36909813.
15. Scheffel A.; et al. (2006). "An acidic protein aligns magnetosomes along a filamentous structure in magnetotactic bacteria". Nature. 440 (7080): 110–4. Bibcode:2006Natur.440..110S. doi:10.1038/nature04382. PMID 16299495. S2CID 4372846.
16. Fuerst JA (2005). "Intracellular compartmentation in planctomycetes". Annu. Rev. Microbiol. 59: 299–328. doi:10.1146/annurev.micro.59.030804.121258. PMID 15910279.