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.

Diagram of a typical eukaryotic 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

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
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 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.

Molecular biology

CellsEukaryoteCytoskeletonMicroscopyCell fractionation