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Thursday, September 16, 2010

Golgi Apparatus

Thursday, September 16, 2010 - 0 Comments

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.

Endoplasmic Reticulum of the Cell

Endoplasmic reticulum (ER) is a complex, membrane bound labyrinth of flattered sheets, sacs and tubules that branches and spreads throughout the cytoplasm. The number and shape of endoplasmic reticulum may vary from one cell type to another, also in different physiological and developmental stages of the same cell type. The enclosed shapes called cisternae contain certain chemical substances that may vary from cell to cell. Endoplasmic reticulum has communication with the exterior, with the nuclear envelope as well as to the Golgi apparatus. Endoplasmic reticulum is continuous from the nuclear envelope to the plasma membrane in the form of a series of channels that help various material to circulate throughout the cytoplasm. It is also a storage unit for enzymes and other proteins and a point of attachment for ribosomes.
Types of endoplasmic reticulum: On the basis of appearances, endoplasmic reticulum is classified as:
(1) Rough or granular endoplasmic reticulum.
(2) Smooth or agranular endoplasmic reticulum.

(1)        Rough or granular endoplasmic reticulum:
This is the type of endoplasmic reticulum that bears on its cytoplasmic face large number of small granular structures, the ribosomes. Due to the presence of these ribosomes endoplasmic reticulum gives rough appearance so named as rough or granular endoplasmic reticulum. Ribosomes present on ER help in protein synthesis that is then transported to different parts of the cell including Golgi bodies through cisternae.

(2)        Smooth or agranular endoplasmic reticulum:
This is the type of endoplasmic reticulum without ribosomes. They have their own enzyme system and perform certain important functions.
Most cells contain both types of endoplasmic reticulum although relative proportion varies among cells.

Functions of Endoplasmic reticulum:

It plays an important role in the activity of cell. The functions are:
(1) Mechanical support: Due to flexible nature of plasma membrane and ability to extend into the cytoplasm, it has connections with nuclear envelope and Golgi apparatus which help to provide mechanical support to the cell.
(2) Transportation of material: As the endoplasmic reticulum has direct or indirect convection with the important organelles of the cell as well as with the cytoplasm and exterior, it acts as a transporter for the transportation of different material within the cell and from surrounding cells.
(3) Synthesis and transportation of proteins: The rough endoplasmic reticulum is involved in the synthesis and transportation of cellular proteins.
(4) Detoxificaiton of harmful substances: Smooth endoplasmic reticulum due to its own enzyme system metabolizes or destroys the toxic substances like steroids, carcinogens and toxins.
(5) Synthesis of lipids: Smooth endoplasmic reticulum synthesis different types of lipids that are used for the formation of plasma membrane and steroid hormones like testosterone and estrogens. Glycogen and glycolipids are also synthesized here.
(6) Site of new membranes: They are site for synthesis of proteins and lipids and are also considered to be primary site of new membranes.
(7) Storage of calcium ions: Smooth endoplasmic reticulum sores calcium ions in muscle cells which are required for muscle contraction.

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