In terms of cell biology these sections, working from the rough endoplasmic reticulum (RER) outwards, are as follows:
1) Cis Golgi network (Goods inwards)
Also called the cis Golgi reticulum it is the entry area to the Golgi apparatus. It follows the 'transitional elements' which are smooth areas of the RER that are also known as the 'endoplasmic reticulum Golgi intermediate compartments' (ERGIC).
2) Golgi stack (Main processing area)
This section is composed of a variable number, typically 3-6, of flattened sacs called cisternae (sing. cisterna). The cisternae of the Golgi stack are divided into three working areas: cis cisternae, medial cisternae and trans cisternae.
3) trans Golgi network (Goods outwards)
This section is directly connected to the trans cisternae and it is here that final reactions and sorting takes place. The concentrated biochemicals are packed into sealed droplets or vesicles that form by budding off from the trans Golgi surface. The vesicles are then transported away for use in the cell and beyond.
Golgi apparatus - what does it do?
The Golgi apparatus is rather like a food supermarket with an in store bakery. It takes in products from the Rough Endoplasmic Reticulum (RER) in what is called 'bulk flow' (the equivalent of a bulk delivery to the supermarket). These chemical products are transported to the Golgi apparatus in sealed droplets or sacs called vesicles and move to a 'deliveries only' part of the Golgi apparatus.
In the Golgi apparatus the vesicles are delivered into the 'unloading bay' of the cis Golgi network. Here the 'goods received' are checked over. Any goods that have been wrongly delivered, including chemicals that should have stayed in the RER, are sent back, packed in vesicles to the rough endoplasmic reticulum.
The proteins and lipids that have been correctly delivered are then passed into the cisternae of the Golgi stack and processed and sorted in an orderly sequence according to any 'labels' they bear. Some of the items from the rough endoplasmic reticulum go to the equivalent of the supermarket in store bakery and are converted into other products and re-labelled. In plants for example as much as 80% of biochemical activity in the Golgi cisternae can be devoted to producing chemicals such as pectin and polysaccharides used in making cell walls.
The correct 'labelling' of products is critical. Inclusion cell (or I cell) disease, an inherited lysosome storage disorder in humans, is caused by a metabolic labelling error. The error causes chemicals to be despatched to the cell surface and secreted whereas the correct labelling would have despatched them to lysosomes. The lysosomes then accumulate material that should have been broken down. This accumulation causes the disorder.
Moving through Golgi or Golgi moving?
The way in which chemicals move through the Golgi apparatus from cisterna to cisterna is not fully resolved. One idea is that a new cisterna forms at the cis end (the end nearest the rough endoplasmic reticulum) and then changes as it moves away from the RER becoming in time the trans end. A more accepted idea is that chemicals being processed in the Golgi apparatus travel from one cisterna to another in transport vesicles or possibly along microtubules. Whatever the transport method, what is clear is that different chemical reactions take place in specially designated parts of the Golgi apparatus.
Golgi biochemicals. Where do they go? How do they get there?
There are three main destinations for biochemicals released from the trans Golgi network: (1) inside the cell to the lysosomes; (2) the plasma membrane and (3) outside of the cell. In each case the destination is clearly linked to function.
Using the food supermarket analogy, all the biochemicals transported away from the trans Golgi network have labels and barcodes built into them. They are all packed in vesicles and the construction of the vesicle or vessel is largely related to the vesicle contents, its destination and end use.
Destination 1: inside the cell, 'the lysosome line'
About 40-50 different biochemicals despatched from the Golgi apparatus in vesicles are destined for delivery to the lysosomes. Animal cells contain many lysosomes and it is in these structures that some life expired organelles and other materials are digested (see item CU9 about lysosomes).
Destination 2: the plasma membrane, 'the continuous secretion line'.
Vesicles containing biochemicals for continuous secretion flow to and fuse with the plasma membrane. This group of secretions will contribute to the biochemicals of the extracellular matrix, act as chemical signals to other cells, and provide proteins for the repair and replacement of the plasma membrane. This constitutive (or continuous) secretory pathway is also the default pathway. Products from the Golgi apparatus not labelled for other routes use this line.
Destination 3: outside the cell, 'the regulated secretion line'
Vesicles and chemicals of this group are produced in specialist secretory cells. They move from the trans Golgi network (TGN) towards the plasma
membrane but accumulate in number before reaching the membrane.
Certain triggers will make the vesicles fuse with the plasma membrane and release their contents in regulated bursts from the cell surface. Insulin release is an example of this when it is triggered by a rise in blood glucose level. Food intake is similar in that it triggers the release of mucus and digestive enzymes into the alimentary canal.
Golgi and 'clones'
When a cell divides the Golgi apparatus, like the RER, breaks up into small fragments. These fragments are divided more or less evenly between the daughter cells. A new Golgi apparatus can only grow from a fragment of Golgi apparatus from the previous cell, so there is therefore the potential for a new Golgi apparatus to grow from each small fragment. However, if there are no fragments there will be no Golgi apparatus. Without a Golgi apparatus the cell will not function.
Summary
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The Golgi apparatus is a critical member of the biochemical manufacturing and supply chain inside a cell. It receives biochemicals in a 'bulk flow' from the rough endoplasmic reticulum (RER). It is the only organelle in the cell that receives, sorts, modifies, concentrates, packs and despatches biochemicals for use inside and outside the cell.
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In specialist secretory cells the Golgi complex is responsible for the sorting and packing of such well-known items as insulin, digestive enzymes and pectin.
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The Golgi apparatus produces specialist vesicles or vessels for the transport of its products. Some of these have special wrappings or coatings that help identify the contents. Some vesicles are recyclable.
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Products from the Golgi apparatus go to three main destinations:
(1) inside the cell to lysosomes (2) the plasma membrane (3) outside the cell.
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