Organelle

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

Organelles usually have a plasma membrane around them. Most of the cell's organelles are in the cytoplasm.

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.
They are not organized like eukaryote organelles, and are not bounded by plasma membranes. They are called bacterial microcompartments.

## Scope of the term

The term is now widely used to refer to cell structures surrounded by single or double plasma membranes. However, the older definition of a 'subcellular functional unit' still exists. So, the term is sometimes used for structures which are not membrane-bound.

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.

### Major 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

|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.

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.  Even more striking is the description of membrane-bound magnetosomes in bacteria. as well as the nucleus-like structures of the _Planctomycetes_ that are surrounded by lipid membranes.

|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|mostly cyanobacteria|
|ribosome|translation of RNA into proteins|RNA-protein|eukaryotes, prokaryotes|
|thylakoid|photosynthesis|photosystem proteins and pigments|some bacteria|