Organelle
An organelle or organelle ( diminutive for organ, in the literal sense, " Orgänchen ") is a structurally definable area of a cell with a specific function. The exact definition of the term is defined ( see also below): Some authors only describe structures with membrane as organelles, ie for example the nucleus, mitochondria, the Golgi apparatus and the endoplasmic reticulum. Other grasp on the concept and also include other structures, such as centrioles. In single-celled " organelle " in this sense is used as a term for complex structures such as Scourge and eyespot.
Single-celled living organisms without a nucleus ( prokaryotes ) usually have no membranes inside the cell and therefore no organelles according to the first definition. However, there are prokaryotic structures which may be regarded as organelles in the broad sense.
- 2.3.1 Cell type - specific organelles of multicellular organisms
- 2.3.2 taxon specific organelles
History of concepts and definitions
As an organ of a defined functional unit within a living organism is called in biology. The analogy with the microscopic structures within a cell is apparently so obvious for Authors appropriate textbooks that they will not be discussed in detail. The first, a diminutive of the word, organ ' used for corresponding cellular structures, presumably was the German zoologist Karl August Möbius ( 1884):
Organulum (plural: Organula ) is the diminutive of the Latin organum. In a footnote, which appeared as a correction in the next issue of the magazine, said Mobius:
The most original definition of the term therefore limited exclusively to cell components of individual learning. Some later published works call Möbius name as author.
It took several years until the expression Organulum or newer organelle generally interspersed and in a wider sense also included components of cells of multicellular organisms. Books and textbooks in 1900, Valentin hackers, Edmund Wilson and Oscar Hertwig, still spoke of the organs of the cell. Later, both names were probably used for a time side by side: Bengt Lidforss wrote in 1915: "A formation of these organs or organelles can not find at least in higher plants instead ".
Around 1920, the term organelle was used for the propulsion structures ( "motor organelle complex", flagella and their anchoring ) and other structures of unicellular organisms. Alfred Kühn wrote in 1920 of the centrioles as division organelles, however, for what is true in the Vahlkampfien that "the alternative: organelle or product of structure formation " had not yet been decided - but without explaining what the difference between the two alternatives would be.
Max Hartmann used the term in 1953 in a textbook for extracellular ( pellicle, shells, cell walls) and intracellular skeletons of single-celled organisms.
Only later, the now widely used definition made out to be viewed by the surrounded only by a membrane cell components as organelles. Sometimes this is even more restricted and only mitochondria and plastids, which have their own genome, are called organelles. But also the more original definition of subcellular functional units in general is still in use.
The origin of the term organelle in the German language seems to have been forgotten. Albert Frey -Wyss Ling wrote in 1978 by the " English term, the organelle ' ", which is often wrong with that organelle ' instead, the organelle would translate '. Frey- Wyssling suggested that all energy-consuming structural elements of the cell and only this as organelles should be referred to, so for example, centrosomes, ribosomes and nucleoli. This energy -dependent definition, however, is not enforced.
Contrary to the name organelle, which always refers to a single object (such as a mitochondrion ), the name compartment for the sum of all similar cellular spaces will be used. A cell can thus have many mitochondria, but only a mitochondrial compartment. Also, the cytoplasm is a compartment, but no organelle.
Limited membrane organelles
Mitochondria, the nucleus and plastids ( chloroplasts and their relatives) are surrounded by a double membrane. Other membrane-bounded organelles have a simple membrane. These include the components of the endomembrane system in plants and the vacuole. In addition, there are some specific membrane-bounded organelles that only in certain cell types or certain eukaryotic species groups, mostly unicellular, occur.
Semi Autonomous organelles
The occurring in almost all eukaryotes, mitochondria and specific to algae and higher plant plastids have their own genome and its own machinery for protein biosynthesis. They are therefore referred to as semi- autonomous organelles '.
According to the endosymbiont theory is seen with them phylogenetically to descendants of bacteria that have been recorded by early eukaryotic cells. These bacteria have been integrated in the course of evolution in the cell. Due to the presence of mitochondrial precursor was early eukaryotic previously anaerobic cell for the first time possible to make use of the much more effective oxygen-dependent energy. The inclusion of cyanobacteria, which evolved into the plastids, the use of sunlight to produce energy was possible: It originated eukaryotic algae and thus the precursor of all plants.
Semi Autonomous organelles have a double membrane: the outer is made of the host cell, so it is of eukaryotic origin. It is derived from the constricted in receiving the organelle predecessor plasma membrane. The inner membrane is of prokaryotic origin. Here is the modified plasma membrane of the symbiont. It constitutes a diffusion barrier for the exchange of molecules and electrons
Redundant structures of the bacterial cells were lost, most of the genes were transferred into the nucleus of the host cell or were also lost. Some genes were also added to the genome of the organelle, e.g., the genes for the exchange of proteins and amino acids to the host cell. This left the still existing remains of consisting of a circular DNA molecule prokaryotic genome and structures that are important for the function of the organelle.
Semi Autonomous organelles proliferate independently by division. In the division of the host cell, they are distributed to the daughter cells.
Other common membrane-bounded organelles
In addition to the semi -autonomous organelles only the nucleus has a double membrane, the nuclear envelope. The organelles in this section are found in all cells of an organism generally. These include, in plants, the vacuole and in all eukaryotes, various components of the endomembrane system: the endoplasmic reticulum, the Golgi apparatus, lysosomes and peroxisomes. A brief description of these organelles found in the cell item at this location. Transport vesicles, which provide for exchange of material between the other components are also included in the endomembrane system. Their inclusion in the definition of an organelle is inconsistent: sometimes individual vesicles are called organelles, sometimes not.
Animal cells
- Endosomes
- Secretory vesicles
- ...
Plant cells
In plant cells endosomes missing. But they have plastids and a vacuole. A plant cell has at least one of the Plastidtypen chloroplast, chromoplast and bandaids. During differentiation, a Plastidtyp can convert it into another.
Special membrane-bounded organelles
Cell type - specific organelles of multicellular organisms
Most of the listed organelles not only occur in some cell types of certain multicellular beings, in other cell types of the same creatures but.
Taxon-specific organelles
Here organelles are shown that occur in eukaryotic unicellular or multicellular in certain species in all cells.
Eukaryotic organelle without a membrane
As already described above, requires a newer definition of the term organelle that a surrounding membrane is present. The older meaning of the term, which is also still used, however, knows no such requirement. According to this meaning, all cellular structures, which are regarded as an organ -like, referred to as organelle. The delineation of such defined organelles to larger molecular complexes is difficult. For example, ribosomes are called organelles, then why not spliceosomes or the large enzyme complexes of DNA replication and transcription? Accordingly, the allocation of small structures on the organelles is not uniform. With an allocation of ribosomes or nucleoli, from organelles would result that organelles, namely mitochondria and plastids or the nucleus itself may have organelles.
The other organelle - concept also allows the inclusion of extracellular structures such as the plant cell walls or shells of single-celled organisms.
With the wide variety of intra-and extracellular structures that could be considered as organelles, there is among these structures no general structural or functional similarities. The following table shows some examples of incomplete.
Prokaryotic organelles
Prokaryotes usually have no internal membranes and therefore no organelles by the narrower definition. Exceptions are magnetosomes of magnetotactic bacteria and thylakoids of cyanobacteria. After the further definition of the term, however, numerous organelle structures can be described in a manner known to the following table indicates a few. Mesosomen, invaginations of the plasma membrane of bacteria were maintained for a period of organelles. However, it turned out that it was artifacts.