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
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.
|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. As other intracellular compartments were discovered, the meaning was generalized to include any lipid-encased intracellular component with a specialized biochemical function.
|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:
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.