Describe the Golgi apparatus/Golgi complex? Write an essay on Golgi apparatus and secretion? Describe various function of Golgi apparatus in the cells?
Golgi Apparatus/Golgi complex
In this article we will discuss about the Golgi apparatus/Golgi complex:- explanation , distribution,chemical composition, structure, function
The Golgi apparatus occurs in all cells except the prokaryotic cells (viz., mycoplasmas, bacteria and blue green algae) and eukaryotic cells of certain fungi, sperm cells of bryophytes and pteridiophytes, cells of mature sieve tubes of plants and mature sperm and red blood cells of animals. Their number per plant cell can vary from several hundred as in tissues of corn root and algal rhizoids (i.e., more than 25,000 in algal rhizoids, Sievers, 1965), to a single organelle in some algae. Certain algal cells such as Pinularia and Microsterias, contain larg est and most complicated Golgi apparatuses. In higher plants, Golgi apparatuses are particularly common in secretory cells and in young rapidly growing cells.
In animal cells, there usually occurs a single Golgi apparatus, however, its number may vary from
animal to animal and from cell to cell. Thus, paramoeba species has two Golgi apparatus and never cells, liver cells and chordate oocytes have multiple Golgi apparatus, there being about 50 of them in the liver cells.
The famous Italian physician and histologist Camillo Golgi (1898) discovered a small group of interconnecting, membrane-bound structures in the cytoplasm of nerve cells of cat and barn owl. He described these bodies as internal reticular apparatus and named them as Golgi bodies. Later Golgi bodies were described by other names such as idiosomes, dictyosomes, golgisomes, vesicles, lipochondria, Golgi complex and Golgi apparatus. They are found in all eukaryotic cells as stacks of flattened sacs or vesicles or tubules. At present, the names Golgi complex and Golgi apparatus are used in vertebrates. In invertebrate tissues and in plant cells, the material has been named as dictyosome.
Generally they are situated near the nucleus but their position can vary depending upon the physiological state of the cell. They are usually present away from the ribosomes. As a part of endomembrane system, golgi complex are interposed between the ER and the plasma membrane. The number of golgi complex varies from cell to cell and from animal cells to plant cells. Normally there are one or two golgi complex but the number can go up to 50 in the secretory cells of animal tissues and up to several thousands in plant tissues. They are very well developed in cells exhibiting high secretory activity. The shape is constant for a particular cell type but can differ according to the functional state of the cell. Golgi complex are generally large in gland cells, nerve cells but smaller in muscle cells.
In the cells of higher plants, the Golgi bodies or dictyosomes are usually found scattered throughout the cytoplasm and their distribution does not seem to be ordered or localized in any particular manner. In animals cells the Golgi apparatus is a localized organelle. For example, in the cells of ectodermal or endodermal origin, the Golgi apparatus remain polar and occurs in between the nucleus and the periphery (e.g.,thyroid cells, exocrine pancreatic cells and mucus producing goblet cells of intestinal epithelium) and in the nerve cells it occupies a circum nuclear position.
The constituents of golgi complex are made of unit membrane similar to the trilaminar structure in the plasma membrane, Chemically, they are composed of an equal amount of proteins and phospholipids. In addition, they contain several enzymes like nucleotidase, diphosphatase, transferase and other oxidative and lysosomal enzymes.
Electron microscopic studies of golgi complex reveals three distinct morphological components.
1. Cisternae (flattened sacs): The cisternae appear as dense, fluid-filled, overlapping stacks of curved sacs or double membrane lamellae. Each group of lamellae is separated from the other group by a space of about 130 Aº. Each unit membrane of a cisterna is about 60 A to 70 Aº thick and separated by a space of about 70 Aº to 90 Aº from the other unit membranes. A large number of such stacks occur in the acinar cells of the pancreas. The bow-shaped cisternae has a ‘forming face and a ‘maturing face’. The outer, convex, proximal forming face or cis-face faces the ER and nucleus. The inner, concave, distal, maturing face or trans-face is oriented towards the plasma a membrane. Transition vesicles are continuously pinched-off from the nearby ER and join the proximal cisternae. At the maturing face, secretory vesicles break off to transport materials away from the complex. Dictyosomes of plant cells reveal such structural polarity (forming face / maturing face). Each dictyosome consists of 3 to 7 overlapping, membrane-bound, bow-like sacs or cisternae surrounded by tubules, vesicles and vacuoles.
2. Vesicles: These are small, oval, dense, smooth walled transition vesicles located along the convex outer surface of the cisternae and ER. The vesicles are about 400 A° to 500A° in diameter.
3. Large Vacuoles: Several large vacuoles appear at the edge of cisternae. These are formed when the central cisternal spaces are enlarged by widening the cisternal membranes. The vacuoles are irregular in shape and filled with a fluid which reduces metallic salts.
1. Golgi complex are found in large numbers in the secretory cells of salivary glands, liver cells and other gland cells. Hence, they are associated with the process of secretion.
2. They are involved in the concentration of secretory products like lipids, yolk, bile, enzymes etc. in membrane bound vesicles.
3. Golgi hodies are found in close proximity to the contractile vacuoles in Amoeba.
4.The acrosomal cap of mammalian spermatozoon contain products secreted by golgi complex which helps in fertilization.
5.It chemically modifies secretory products. Glycosylation involves joining carbohydrate moeties with proteins and lipids to form glycoproteins and glycolipids.
6.Golgi bodies are associated with the packaging and distribution of secretory material to and from other parts of the cell.
7. In sundews (an insectivorous plant) dictyosomes secrete a stick, slimy, hydrolytic enzyme for trapping insects.
8.Combining sulphates with carbohydrate residues of glycoproteins results in sulphatation in cartilage matrix.
9. Golgi bodies concentrate pectin and cellulose prior to cell plate formation.
10. Golgi bodies are associated with the synthesis of melanin and lipid metabolism.
11.Lysosomal vesicles collect from golgi complex.
12. It is an integral part of endomembrane system for intracellular transport.
13.Cajol (1914) emphasized the secretory role of golgi complex by stating that it is actively engaged in the biosynthesis, transport and release of proteins, peroxisomes and lysosomal enzymes.