Organelles

Organelle

An organelle is a specialized subunit within a cell, having a specific function, suspended in the cytoplasm of a eukaryotic cell.

Organelles were historically identified through the use of microscopy, and were also identified through the use of cell fractionation.

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

Eukaryotic organelles

Eukaryotes are the most structurally complex 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 the light microscope (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 every one of the organelles listed below. Exceptional species of cells do not have some organelles which might otherwise be considered universal to eukaryotes (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.

**Major eukaryotic organelles**
Organelle	Main function	Structure	Organisms	Notes
chloroplast (plastid)	photosynthesis	double-membrane compartment	plants, protists	has some genes
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 which 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 DNA
vacuole	storage, homeostasis	single-membrane compartment	eukaryotes
nucleus	DNA maintenance, RNA transcription	double-membrane compartment	all eukaryotes	has bulk of genome

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.

Originally, the word organelle referred to large lipid-encased formerly autonomous endosymbiont within cells^{[citation
needed]}. As other intracellular compartments were discovered, the meaning was generalized to include any lipid-encased intracellular component with a specialized biochemical function.

**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 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	breakdown of metabolic hydrogen peroxide	single-membrane compartment	all eukaryotes
ribosome	translation of RNA into proteins	RNA-protein	eukaryotes, prokaryotes
vesicle	material transport	single-membrane compartment	all eukaryotes

Other related structures:

A few large organelles probably originated from endosymbiont bacteria:

Other plastids, such as leucoplasts, amyloplasts, Etioplasts, Elaioplast, rhodoplasts, and chromoplasts.
mitochondrion

Other related structures:

The content of this section is licensed under the GNU Free Documentation License (local copy). It uses material from the Wikipedia article "Organelle" modified April 22, 2006 with previous authors listed in its history.