Quick look: Golgi apparatus(or complex, or body, or ‘the ‘Golgi’) is found in all plant and animal cells and is the term given to groups of flattened disc-like structures located close to the endoplasmic reticulum.
The number of ‘Golgi apparatus’ within a cell is variable. Animal cells tend to have fewer and larger Golgi apparatus. Plant cells can contain as many as several hundred smaller versions.
The Golgi apparatus receives proteins and lipids (fats) from the rough endoplasmic reticulum. It modifies some of them and sorts, concentrates and packs them into sealed droplets called vesicles. Depending on the contents these are despatched to one of three destinations:
Destination 1: within the cell, to organelles called lysosomes.
Destination 2: the plasma membrane of the cell
Destination 3: outside of the cell.
The name behind the apparatus
The Golgi apparatus is the only cell organelle to be named after a scientist. The visible characteristics of the organelle were first reported by Camillo Golgi (1843-1926) at a meeting of the Medical Society of Pavia on 19 April 1898 when he named it the ‘internal reticular apparatus’.
Debate about the existence of the apparatus continued even after 1913 when the term ‘Golgi apparatus’ was officially given to the ‘internal reticular apparatus’. It was not until 1954 that work in electron microscopy finally put the seal of approval on the existence of the organelle and the eponym ‘the Golgi’, was fully accepted.
Going for Golgi. Where is the Golgi apparatus and what is it?
Where is it?
Golgi apparatus is present in eucaryotic cells as one or more groups of flattened, membrane-bounded compartments or sacs. They are located very near the rough endoplasmic reticulum and hence near the nucleus.
What is it?
The compartments of the Golgi apparatus look rather like a pile of Pitta breads with the one at the top and bottom not being smooth but having broken open outermost surfaces. The number of compartments in any one Golgi apparatus is usually between 3 and 8. The number of sets of Golgi apparatus in a cell can be as few as 1, as in many animal cells, or many hundreds as in some plant cells. Specialised secretory cells contain more sets of Golgi apparatus than do other cells.
The Golgi apparatus is part of a manufacturing and supply chain
In non-biological terms the Golgi apparatus can be divided into three main sections:
1) Goods inwards
2) Main processing area
3) Goods outwards
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.
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.
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.
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.
Products from the Golgi apparatus go to three main destinations:
(1) inside the cell to lysosomes (2) the plasma membrane (3) outside the cell.