Golgi apparatus (GA): sorting centre of the cell
Inside the cell, numerous proteins (enzymes) are synthesised and these proteins should be properly targeted to the respective sites for its activity. The site of DNA replication is nucleus. Numerous enzymes like DNA polymerases, helicases etc are required for the process. Similarly mitochondrion needs enzymes for ATP generation process. Enzymes for post translational modification are required for ER. Lysosome requires digestive enzymes. But all these enzymes are synthesised in the cytoplasm. These proteins should be properly addressed to target it to the correct location, Similar to that of an address in a post cover. And all these proteins should be sorted and properly despatched. The golgi apparatus is the organelle that is responsible for this crucial activity.
Simply golgi apparatus is the post office of the cell.
Definition: Consists of parallely arranged flattened membraneous sacs that lacks ribosomes and can be stained using osmium tetroxide and silver salts. Proteins synthesises in the RER pass through different regions of golgi apparatus (cis, median and trans golgi) where it undergoes further modification.
Dictyosomes: refers to the golgi apparatus of plants and lower invertebrates.
Lipochondria: GA’ other name due to high lipid content.
In 1873, Camillo Golgi, an Italian neurologist developed and discovered a technique for studying Golgi apparatus using silver chromate staining method. He called this internal reticular network of sacs in nerve cells as ‘internal reticular apparatus’. Many scientists at that time raised questions regarding the existence of Golgi apparatus. Golgi apparatus got the status as an organelle of the cell only after electron microscopic studies performed in early 1950.
GA are present in almost all cells except prokaryotic cells and some eukaryotic cells like sperms of bryophytes and pteridophytes and RBC and mature sperm cells of animals. Largest GA are seen in algal cells like Pinularia. Number of GA varies between cells. In animal cells, GA is seen as a localised organelle, where as in plants it (dictyosomes) is seen scattered throughout the cytoplasm.
The simplest unit of GA is called cisternae, a flattened stacked plate like scas.
What is GERL region (Golgi-SER-lysosome) or TGN network (Trans golgi network)?
A system present in the cell that involves GA, ER and lysosomes, mainly involved in endocytosis and exocytosis. GA is a major player in protein targeting. Secretory proteins pass through RER and pinched off and fuse with-cis golgi, then pass through median and trans golgi where it is properly packaged. GERL is the site where acid phosphatase (a typical lysosomal enzyme) makes its appearance.
Function of GERL: is involved in the origin of primary lysosomes and melanin granules
Sorting of secretory proteins
Zone of exclusion?
Around the GA in the cytoplasm, other organelles like mitochondria or ribosomes are scare or absent. This area is called ZOE. Only SER and vesicles are seen in ZOE.
Chemical and Staining properties of GA:
- Chemically GA is rich in lipids (~60%). GA of plant cell contains Phosphatidic acid and phosphatidyl glycerol. In animal cells, phophatidyl choline is present.
- Different regions of GA has specific staining properties enabling specific cytochemical localization
- Osmium tetroxide : selectively stains outer surface of GA which are rich in phospholipids
- Phosphotungstic acid: selectively stains trans face of GA which are rich in polysaccharides and proteins. Enzyme markers in the trans GA are Glycosyl transferase and thiamine pyrophosphtase.
- The enzyme marker for GERL region is Acid phosphatase
Origin: supposed to be originated from SER or de novo or fragmentation of pre-existing stacks.
- GA is the sorting centre of the cell also called as the traffic police’ of the cell”.The cis golgi is the reception site where proteins from the RER as pinched off vesicles gets fused into.In the median golgi, (the finishing and packaging site) further post translational modification takes place .
- In the trans region (the dispatch centre) each protein is properly tagged or addressed in order to be targeted to the final destination (like Mannose -6- phosphate tagging for proteins of lysosomes). Packaged proteins will be pinched off as vesicles from this site.
- In plant cells GA is involved in the deposition of primary and secondary wall materials (pectin, cellulose etc).
- In animal cells, GA is involved in packaging and secretion of variety of substances from the cell like secretion of lactoprotein secretion by mammary glands, secretion of hormones, mucus etc. Other functions include protein sorting, and also involved in the formation of lysosome, acrosome etc.