Golgi Apparatus | Definition, Function, Location & Structure
Golgi apparatus was first observed by Camillo Golgi in 1898. He described it as a densely stained reticular structures present near the nucleus of the cell. Therefore, these were given the name Golgi bodies, after his name. It is present in eukaryotic cells, except in mature sieve tubes of plants, mature RBCS of mammals, sperm cells of bryophytes and pteridophytes, etc. It is also absent in prokaryotic cells. In plants, it is called dictyosomes as Golgi apparatus is made up of unconnected units.
Structure of Golgi Bodies
There are four parts of Golgi complex, viz, cisternae, tubules, vesicles, golgian vacuoles.
- These are flattened sac-like structures stacked on one another. There are usually 4-8 cisternae present in a stack. These cisternae ręsemble with smooth endoplasmic reticulum.
- Cisternae form an extensive network and are arranged near the nucleus in a concentric pattern.
- The shape, size and number of cisternae may vary in different cells but have a similar organisation in one type of cells. Their size may range from 0.5 um to 1.0 um in diameter.
- The Golgi cisternae are concentrically arranged near the nucleus with distinct convex cis or the forming face and concave trans or the maturing face. The cis and the trans faces of the organelles are entirely different, but interconnected.
(b) Tubules :
These are small, flat, interconnecting structures arising from the periphery of cisternae due to fenestrations.
These are large rounded sacs present at the edges of cisternae in clusters. These are pinched off from the tubules.
These are of two types:
- Smooth vesicles: These are smooth surfaced secretory vesicles and contain secretory granules.
- Coated vesicles : These are rough surfaced, spherical protuberances arising from the tubules of cisternae.
(d) Golgian Vacuoles:
These are large, spherical vacuoles produced at maturing face. These are filed with some granular or amorphous substances. Some of them function as lysosomes.
Functions of Golgi Apparatus
- The important function of Golgi apparatus is to process, package and transport the materials for secretions. The packaged material is delivered either to the intracellular targets i.e., within the cell or secreted to extracellular targets i.e., outside the cell. The material to be secreted moves from ER to the Golgi apparatus in the form of transitional vesicles. These vesicles then fuse with the cis face and move towards the maturing face of the golgi apparatus. Therefore, Golgi apparatus is closely associated with ER in structural as well as functional aspects.
- A number of proteins synthesised by ribosomes present on the ER are transferred to golgi apparatus. These proteins are then modified in the cisternae of Golgi apparatus before they are released from its transface.
- Golgi apparatus is the important site of formation of glycoproteins (glycosylation of proteins) and glycolipids (glycosidation of lipids).
- Root cap cells are rich in Golgi bodies which secrete mucilage for the lubrication of root tip.
- Acrosome of the sperm is modified Golgi apparatus.
- Formation of plasma membrane during cytokinesis.