Saturday, 4 April 2015

Organelles Of Cell


An organelle is a structurally definable area of ​​a cell with a specific role. The definition is inconsistent: some authors call only structures with membrane and organelles, so for example the nucleus, mitochondria, Golgi apparatus and the endoplasmic reticulum. Others take the concept further and also include other structures, such as centrioles. In unicellular "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 thus not a organelles according to the first definition. However, there are prokaryotic structures that can be seen as organelles in a broader sense.

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 and the plant 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

Chloroplasts in the leaf blade of the moss Plagiomnium affine
Occurring in almost all eukaryotes, mitochondria and specific for algae and higher plants plastids have their own genome and its own machinery for protein synthesis. They are therefore referred to as semi-autonomous organelles'.

According to the endosymbiotic theory is seen with them phylogenetically to descendants of bacteria that were taken by early eukaryotic cells. These bacteria were integrated in the course of evolution in the cell. Due to the presence of mitochondrial precursor was the eukaryotic early, before anaerobic cell for the first time possible to use the much more effective oxygen-dependent energy. The inclusion of cyanobacteria, which evolved into the plastids, the use of sunlight for energy was possible: It emerged eukaryotic algae and thus the precursor of all plants.

Semi Autonomous organelles have a double membrane: the outer is made from the host cell is thus eukaryotic origin. It is derived from the constricted in receiving the organelle predecessor plasma membrane. The inner membrane is of prokaryotic origin. This is modified by the plasma membrane of the symbionts. 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 but were also added to the genome of organelles, eg. As genes for the exchange of proteins and amino acids with the host cell. This left the still existing remains of the one-circular DNA molecule prokaryotic genome and structures that are important for the function of organelles.

Semi Autonomous organelles replicate autonomously 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 semiautonomous organelles only the nucleus has a double membrane, the nuclear envelope. The organelles described in this section are found in all cells of an organism in general. These include plants in the vacuole and in all eukaryotes several components of the endomembrane system: the endoplasmic reticulum, the Golgi apparatus, lysosomes and peroxisomes. A brief description of these organelles found in Article cell at this point. Transport vesicles, which provide material exchange between the different components, are also the endomembrane system. Their inclusion in the definition of an organelle is mixed: Sometimes individual vesicles are called organelles, sometimes not.