Organelle
An organelle is a specialized subunit, within a biological cell, that has a specific function. The name organelle comes from the idea that these structures are parts of cells, as organs are to the body, hence organelle, the suffix -elle being a diminutive. Organelles are either separately enclosed within their own lipid bilayers or are spatially distinct functional units without a surrounding lipid bilayer. Although most organelles are functional units within cells, some functional units that extend outside of cells are often termed organelles, such as cilia, the flagellum and archaellum, and the trichocyst.
Organelles are identified by microscopy, and can also be purified by cell fractionation. There are many types of organelles, particularly in eukaryotic cells. They include structures that make up the endomembrane system, and other structures such as mitochondria and plastids. While prokaryotes do not possess eukaryotic organelles, some do contain protein-shelled bacterial microcompartments, which are thought to act as primitive [|prokaryotic organelles]; and there is also evidence of other membrane-bounded structures. Also, the prokaryotic flagellum which protrudes outside the cell, and its motor, as well as the largely extracellular pilus, are often spoken of as organelles.
History and terminology
In biology, organs are defined as confined functional units within an organism. The analogy of bodily organs to microscopic cellular substructures is obvious, as from even early works, authors of respective textbooks rarely elaborate on the distinction between the two.In the 1830s, Félix Dujardin refuted Ehrenberg's theory that microorganisms have the same organs as multicellular animals, only smaller.
Credited as the first to use a diminutive of organ for cellular structures was German zoologist Karl August Möbius, who used the term organula. In a footnote, which was published as a correction in the next issue of the journal, he justified his suggestion to call organs of unicellular organisms "organella" since they are only differently formed parts of one cell, in contrast to multicellular organs of multicellular organisms.
Types
In the broadest definition, an organelle is any part of the cell that acts as a distinct functional unit. This includes membrane-bound as well as non-membrane-bound, or membrane-less organelles. In a more restrictive definition, only membrane-bound ones are included. In the most restrictive definition, only the endosymbiotic membrane-bound ones are included.The membrane-bound organelles include the endosymbiotic and components formed by the endomembrane system such as the lysosome. An endomembrane system and mitochondria are found in almost all eukaryotes. Plants, algae, and some protists additionally have chloroplasts. A very small minority of bacteria also have a sort-of endomembrane system.
The membrane-less organelles, also called biomolecular complexes, are large assemblies of macromolecules that carry out particular and specialized functions, but are membrane-less. Many of these are referred to as "proteinaceous organelles" as their main structure is made of proteins. MLOs include:
- large RNA and protein complexes: ribosome, spliceosome, vault
- large protein complexes: proteasome, DNA polymerase III holoenzyme, RNA polymerase II holoenzyme, symmetric viral capsids, complex of GroEL and GroES; membrane protein complexes: porosome, photosystem I, ATP synthase
- large DNA and protein complexes: nucleosome
- centriole and microtubule-organizing center
- cytoskeleton
- flagellum
- nucleolus
- stress granule
- germ cell granule
- neuronal transport granule
Eukaryotic organelles
cells are structurally complex, and by definition are organized, in part, by 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. They were among the first biological discoveries made after the invention of the microscope.Not all eukaryotic cells have each of the organelles listed below. Exceptional organisms have cells that do not include some organelles that might otherwise be considered universal to eukaryotes. The several plastids including chloroplasts are distributed among some but not all eukaryotes.
There are also occasional exceptions to the number of membranes surrounding organelles, listed in the tables below. In addition, the number of individual organelles of each type found in a given cell varies depending upon the function of that cell. The cell membrane and cell wall are not organelles.
| Organelle | Main function | Structure | Organisms | Notes |
| chloroplast | photosynthesis, traps energy from sunlight | double-membrane compartment | plants, algae, rare kleptoplastic organisms | has own DNA; theorized to be engulfed by the ancestral archaeplastid cell |
| endoplasmic reticulum | translation and folding of new proteins, expression of lipids | single-membrane compartment | all eukaryotes | rough endoplasmic reticulum is covered with ribosomes, has folds that are flat sacs; smooth endoplasmic reticulum has folds that are tubular |
| flagellum | locomotion, sensory | protein | some eukaryotes | |
| Golgi apparatus | sorting, packaging, processing and modification of proteins | single-membrane compartment | all eukaryotes | cis-face nearest to rough endoplasmic reticulum; trans-face farthest from rough endoplasmic reticulum |
| mitochondrion | energy production from the oxidation of glucose substances and the release of adenosine triphosphate | double-membrane compartment | most eukaryotes | constituting element of the chondriome; has own DNA; theorized to have been engulfed by an ancestral eukaryotic cell |
| nucleus | DNA maintenance, controls all activities of the cell, RNA transcription | double-membrane compartment | all eukaryotes | contains bulk of genome |
| vacuole | storage, transportation, helps maintain homeostasis | single-membrane compartment | all eukaryotes |
Other related structures:
- cytosol
- endomembrane system
- nucleosome
- microtubule
Prokaryotic organelles
However, there is increasing evidence of compartmentalization in at least some prokaryotes. Research has revealed that at least some bacteria 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 magnetotactic bacteria, reported in 2006.
The bacterial phylum Planctomycetota has revealed a number of compartmentalization features. The Planctomycetota cell plan includes intracytoplasmic membranes that separates the cytoplasm into paryphoplasm and pirellulosome. Membrane-bounded anammoxosomes have been discovered in five Planctomycetota "anammox" genera, which perform anaerobic ammonium oxidation. In the Planctomycetota species Gemmata obscuriglobus, a nucleus-like structure surrounded by lipid membranes has been reported.
Compartmentalization is a feature of prokaryotic photosynthetic structures. Purple bacteria have "chromatophores", which are reaction centers found in invaginations of the cell membrane. Green sulfur bacteria have chlorosomes, which are photosynthetic antenna complexes found bonded to cell membranes. Cyanobacteria have internal thylakoid membranes for light-dependent photosynthesis; studies have revealed that the cell membrane and the thylakoid membranes are not continuous with each other.
Advances in synthetic biology have enabled the construction of artificial bacterial organelles that are more reminiscent to eukaryotic ones, including structures formed through liquid-liquid phase separation of "RNA organelle" reported in 2017. These RNA systems termed TEARS is capable of regulating compartmentalize cellular processes, scaffolding and sequestering metabolic pathways. These synthetic organelles can be repurposed as their eukaryotic counterparts, to isolate purify proteins within prokaryotes, enabling a technology termed PandaPure for chromatography-free purification.
| Organelle/macromolecule | Main function | Structure | Organisms |
| nucleoid | DNA maintenance, transcription to RNA | DNA-protein | bacteria and archaea |
| ribosome | translation of RNA into proteins | RNA-protein | bacteria and archaea |
| plasmid | DNA exchange | circular DNA | some bacteria and archaea |
| carboxysome | carbon fixation | protein-shell bacterial microcompartment | some bacteria |
| flagellum | movement in external medium | protein filament | some prokaryotes |
| pilus | Adhesion to other cells for conjugation or to a solid substrate to create motile forces. | a hair-like appendage sticking out the plasma membrane | some prokaryotes |