Key points: Golgi complex, history, introduction, shape, size, structure, function.
In 1890 Camillo Golgi described apparato reticulate interno i.e. internal reticular apparatus in the nerve cells. In 1900 Holmgren described a system of clear canals, which he called trophospongium. Baker (1951,1953) referred to the Golgi apparatus as lipochodria because of the presumed lipid content. Structure similar to golgi complex have been found in plants. Botanists refer to them as dictyosomes.
What are Golgi bodies?
The Golgi complex is generally a single large structure. Nerve cells, liver cells and most of the plant cells have multiple Golgi complexes, there are about 50 in liver cells. Cells with dispersed Golgi complexes may have hundreds scattered throughout the cytoplasm of the cell.
Shape and size of Golgi complex
The shape is variable in different somatic cell types of animals. Even in the same cells there are variation in different functional stages. It varies from a compact mass to disperse filamentous network. The position of Golgi complex is also variable. They are ectodermal in origin and is polarized between the nucleus and periphery. In exocrine cells it lies between nucleus and secretory pole. The size of Golgi complex is large in nerve and gland cells, but small in muscles cells. The size is linked to the functional state.
Structure of Golgi complex under electron microscope
Electron microscope observations of thin sections reveal the presence of three membranous components:
- Flattened sacs or cisternae
- Small tubules and vesicles
- Large vacuoles and filled with amorphous or granular substance.
These membranous structures are characterized by the absence of ribosomes i.e. they are smooth membranes.
Functions of Golgi complex
There are several functions that have been observed attributed to Golgi complex
- Role in protein secretion (migration, transport and packaging)
In pancreatic exocrine cells there are distributed proteins (digestive enzymes). Now proteins are formed at ribosomes which are attached to endoplasmic reticulum. The synthesized proteins are then transferred to ER. From there they move towards Golgi apparatus. In the Golgi complex the proteins are contracted and transformed into zymogen granules. These zymogen granules released from Golgi complex and migrate to the surface of the cell. Here the limiting membrane of the zymogen granule fuses with the plasmalemma thus discharging its contents.
- Secretion of polysaccharides
Precursors enter the goblet cells from the capillaries of the vascular system. The amino acids are synthesized into proteins on the ribosome of ER. The proteins are then transferred to the cisternae of Golgi complex. The simple sugar molecules go directly from blood stream to cisternae where they are complexed with protein to form a glycoprotein.
In many cells the protein released from the ER is combined with the carbohydrate to produce complex carbohydrates like glycoprotein, mucopolysaccharide, glycogen and glycolipids. Addition of carbohydrate components to the protein occurs in the Golgi complex as well as in ER.
After completion of glycosylation the glycoprotein is released into the lumen of Golgi complex cisternae.
Golgi bodies take part in the sulphate metabolism. Compounds containing active Sulphur are formed in two steps process. Sulphate is first activated by ATP in two stages, the process requires two separate enzymes. The process is carried out by enzyme suplotransferases.
- Plasma membrane formation
Secretory granules originating from Golgi complex fuse with the plasma membrane during the process of exocytosis. The membrane of the granules become incorporated with the plasma membrane and contributes to the renewal of plasma membrane components. Golgi complex plays an important part in the synthesis of carbohydrate components in the plasma membrane.
- Lipid packaging and secretion
The epithelial cells produce chylomicrons which contains lipids in the form of lipoproteins. The Golgi complex may also be involved in adding of carbohydrates of chylomicrons. Therefore the overall role of Golgi complex is the concentration and modification of secretory material. These changes convert lipid to chylomicrons. The Golgi complex provides the membrane for the envelopment of lipid so that it can be released from the cell.
- Acrosome formation
In early stages of mammalian development the spermatid of cell has Golgi apparatus which was spherical in shape with parallel flattened cisternae. Later the complex becomes irregular and the cisternae dilate to form sacs. proacrosomic granules appear in the centre of Golgi complex and fuse to form acrosome.
Golgi bodies are ectodermal in origin. They consist of cisternae and large vacuoles. They involve in many processes such as protein synthesis, polysaccharides, lysosomes, acrosome formation etc.
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