Tuesday, December 23, 2008

WHO DISCOVERED THE GOLGI APPARATUS?



A Golgi apparatus is an organelle (a structure within a cell).
Other names for the Golgi apparatus
The Golgi apparatus is also known as the Golgi body, Golgi complex, or (in plants) dictyosome.
Why is it called "Golgi"?
It is named after an Italian microscopist, Camillo Golgi, hence the capital "G". Dr Golgi announced his discovery of this organelle in 1898.
Structure, appearance, and location
There may be a number of Golgi bodies in a cell. They vary in size, but a typical diameter is 2 micrometers. In an electron micrograph, each one can be seen to consist of a stack of flattened sacs, each sac (cisterna) bounded by a membrane. A Golgi apparatus is often found in a perinuclear location, i.e. near the nucleus of the cell. It is also typically close to the rough endoplasmic reticulum.
Receiving, sorting, modifying, and dispatching proteins
There is a continuous movement of vesicles (small, more or less spherical membranes enclosing, in this case, proteins) to one side (the cis surface) of the Golgi apparatus.
These vesicles have budded off the rough endoplasmic reticulum, and contain proteins that were synthesized within the lumen of the rough endoplasmic reticulum.
On reaching the Golgi apparatus, a vesicle fuses with it; its membrane merges with that of the Golgi cisterna. The proteins enter the lumen (cavity) of the cisterna.
Within the Golgi apparatus, the proteins are sorted according to their eventual destination. Some of these proteins may be destined to be secreted (released from the cell to perform a physiological function elsewhere) e.g. as digestive enzymes, others will become proteins of the cell membrane (plasma membrane), and yet others will end up in a lysosome or back in the endoplasmic reticulum.
The Golgi body also modifies proteins (e.g. by adding a polysaccharide chain).
Proteins are moved from stack to stack of the whole structure in vesicles that bud off one layer of the Golgi apparatus and carry the proteins to another.
From the opposite (trans) surface of the Golgi apparatus, vesicles bud off and are moved to their various destinations.
Polysaccharide synthesis
A number of other chemical processes occur in the Golgi body, including polysaccharide synthesis. The Golgi body makes the materials of which new cell walls are made during plant cytokinesis.
Does the Golgi apparatus replicate?
No. It is dismantled at mitosis, and reassembled later.
Analogies
Some people like to make comparisons between parts of the cell and other, more familiar, places or processes. A Golgi apparatus has been likened to a business department that handles receiving and dispatch. The items received must be sorted by destination before they can be dispatched. A postal service is another metaphor.
But what about packing and secretion?
Textbooks sometimes say the function of the Golgi apparatus is packing and secreting proteins.
By "packing" they mean that proteins are surrounded by a membrane in the form of a vesicle before they leave the Golgi apparatus. However, the rough endoplasmic reticulum does this too, before sending proteins to the Golgi apparatus. Endocytosis involves something similar. So there is nothing so very special about the Golgi apparatus here.
Secretion means the release from the cell of a useful substance (as opposed to excretion, which is the release of a waste, potentially toxic substance). Secretion happens when a vesicle reaches the cell membrane and its contents leave the cell by exocytosis. The Golgi apparatus prepares proteins for, but not does perform, secretion.

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chrishaines59 said...
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