Thursday, September 16, 2010

Golgi Apparatus

Camillo Golgi in 1898 while studying the eukaryotic cells observed a system of tightly packed smooth surfaced vesicles lying near the nucleus. This structure was named as Golgi apparatus, Golgi body, and Golgi complex in animals and in plants as dictyosome. Golgi complex is an organelle with diverse shape and number. In most of animals’ cells there is present only one Golgi apparatus but are abundantly found in cells that secrete chemical substances like pancreatic cells which secrete digestive enzymes and neve cells that produce neurotransmitters. In certain plant cells the number may be in hundreds. Golgi apparatus is formed flattered sacs or cisternae but some tubules and vesicles may also participate in the formation of Golgi complex. Number of fluid filled flattered sacs may range from 3 – 7 in most of animals out lower organisms have up to 30 flattered cells. These flattered cells are arranged in a concentric fashion, the convex sacs lie closer to the nuclear membrane and are termed as cis Golgi or forming face. Farthest concave sacs are named as trans-Golgi or maturing face. Proteins or material enter Golgi body through forming face and after modification are released from maturing face.
Golgi body is chemically made up of lipoprotein and a number of other organic molecules that are present or transported through it. Golgi complex is continuous with endoplasmic reticulum canals on one side and to secretary vesicles leading the cell membrane on the other.

Mechanism of secretion of Golgi complex: Following six steps of secretion are involved in pancreas and other zymogen secreting glands.
(1)        Ribosome stage: It explains synthesis of protein molecules protein molecules by ribosomes attached with ribosomal endoplasmic reticulum.
(2)        Cistarnae stage: This stage involves the flow of protein (forward form ribosomes) through ER tubes called cisternae towards dictyosomes.
(3)        Intracellular transport: Secreted proteins are pinched off as transitional vesicles and tubules from ER and they flow in the cytoplasm towards dictyosomes where they fuse to form large condensing vacuole at the forming face of Golgi.
(4)        Concentration of Secretion: By process of concentration the condensing vacuole is converted into zymogen granules.
(5)        Intracellular stage: Zymogen granules are now changed by Golgi into secretary granules and are stored in the cell and are released in response to proper stimulus (a hormone or neurotransmitter) that acts on the cell.
(6)        Exocytosis: The discharge of secretary granules is effected by exocytosis.
Function of Golgi bodies:
(1)        Secretion: It secrets many secretary granules like lysosomes, peroxisomes.
(2)        Exocytosis: Proteins packed in secretary vesicles are released into cytoplasm close to the plasma membrane when the vesicles reach the plasma membrane, they fuse with it and release their contents to the outside of cell by exocytosis.
(3)        Storage of proteins: Proteins synthesized by Ribosomes are sealed off in little packets called transfer vesicles which pass from the endoplasmic reticulum to Golgi apparatus and fuse with it. In Golgi apparatus proteins are concentrated and chemically modified and can be used with cell or can be exported out of the cell.
(4)        Formation of Glycolipid and Glycoprotein: Carbohydrates, lipids and proteins synthesized by endoplasmic reticulum are modified as glycolipid and glycoprotein in within Golgi complex.
(5)        Cell wall formation: Golgi bodies are also involved in the formation of new plant cell wall.

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