Endoplasmic Reticulam<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
ER AS A PART OF ENDOMEMBRANE SYSTEM
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- Endoplasmic reticulum is a part of endomembrane system
- Endomembrane system is dynamic and integrated network through which many of the membranous organelles are connected to each other.
- Besides ER, golgi body, endosomes, lysosomes, microbodies, vacuoles and nuclear membrane are the other components of endomembrane system
- All the components work as a coordinated unit.
- Membrane of each of these has its own distinct structure and function.
- Endomembrane system provides a network through which protein and many other materials move around the cell.
- Endomembrane system also communicates via vesicles with the plasma membrane.
• Materials are shuttled - small transport vesicles which bud from a donor membrane compartment andmove in a directed manner with the help of motor proteins. At their destination they fuse with membrane of acceptor compartment. Repeated cycles of budding and fusion occur
SELF ASSESMENT QUESTIONS
Q.1 Fill in the Blanks
Endomembrane system is --------------- ------------------ through which --------- ---------------- are connected to each other.
Q.2 Components of endomembrane system are -----------, ----------------, ----------------, -------------, -----------------, ------------------- and ----------------.
Q.3 Endomembrane system communicate to plasma membrane via --------------------.
Q.4 Membrane of each of component of endomembrane system has its own distinct -------------- And ---------------.
DISCOVERY OF ER
· In late 19th century, certain regions of some eukaryotic cells (especially involved in secretion) were observed to be stained with basic dye, which was called ERGASTOPLASM.
· In 1945, using electron microscope Keith Porter and his associates observed a fine reticulum in the endosperm of thinly spread cells of chicken fibroplast on drying up preprations fixed with osmium tetraoxide.
· In 1952, Porter and Kellman used the term "Endoplasmic Reticulum" to describe the same Endoplasmic simply mean "Within the plasma of the cell" and RetIculum is the Latin word meaning network.
· With improvement of resolution of electron microscope membrane of ER was observed to be attached with basophilic granules which were identified to be ribosomes.
· In 1974, three cell biologists ALBERT CLAUDE, GEORGE PALADE AND CHRISTIAN DE DUVE isolated and analyzed the sub cellular fractions of cells including microsomes. They were awarded Nobel Prize for that.
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Albert Claude <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />Belgium |
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Christian de Duve Belgium |
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George E. Palade USA |
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In 1974 three cell biologists Albert Claude, George Palade, Christian de Duve were awarded noble prize for their discovery concerning the structural and functional organization of the cell. |
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Keith Porter |
TOOLS AND TECHNIQUES TO STUDY ER
1. Electron Microscopy: K. Porter
Whole mount electron microscopy of cells: a method first used by Porter to obtain the first electron micrograph of a cell. Shown here are cells growing on (A) glass cover slips and (B) EM grids. Cells in A and B are being microinjected (see the tip of the needle in B?). Part C is a low magnification whole mount electron micrograph of a human fibroblast taken at 80 kV. (Images by K. Porter)
2. Microsomes
· When the cells are ruptured by homogenization, the cytoplasmic membranes become fragmented and the ends of the membrane fragments fuse to form spherical vesicles less than 100 µm in diameter. These vesicles are called microsomes.
· Microsomes are mostly fragments of ER and they are non natural structures.
· These microsomes have been used for various studies such as
i) For studying the composition and functions of different types of membranes
ii) For studying various enzymatic activity and
iii) To study the transport of newly synthesized proteins.
Besides microsomes other techniques have also helped in understanding the structure and function of ER, such as autoradiography, using GFP and microchip Proteomics.
SELF ASSESSMENT QUESTIONS
Q.1 who observed Endoplasmic Reticulum for the first time?
Q.2 who gave the term 'Endoplasmic Reticulum' and why was it so called?
Q. 3 what are microsomes? What is the significance of studying microsomes?
Q.4 Name the scientists who isolated the microsomes for the first time.
Q.5 Name the technique used by these scientists for the isolation of microsomes.
FEATURES OF ER
· ER is a most versatile and adaptable organelle in Eukaryotic cells.
· In fact 50-90% of total membrane present in a typical mammalian cell is membrane of ER
· Though in tissue sections ER looks like separate membrane enclosed spaces. ER is composed of interconnected network of membrane channels (as visualized in intact cells).
· ER subdivides the cytoplasm into two main compartments, one enclosed within the membrane (lumenal phase) other situated outside (constituting cytosol).
· Two important agricultural products, vegetable oils and dietry seed proteins are produced by enzymes associated within the ER
· ER exists in two forms.
a) Granular or Rough Endoplasmic Reticulum, which has attached ribosomes.
b) Agranular or Smooth Endoplasmic Reticulum which lacks ribosomes
· ER membranes may assume the forms of cisternae, tubules or vesicles.
§ Cisternae are broad, flat membrane bound spaces arranged parallel to each other too form lamellae and are interconnected with each other.
§ The tubules are 50-100 A in diameter and appear as circles in EM sections.
§ The vesicles or sacs appear as membrane bound isolated globose cavities.
STRUCTURE AND DISTRIBUTION
RER
- The RER is composed of membranous tubules, vesicles and enlarged flattened sheets (cisternae)
- It is present in large amount in those cells which are involved in synthesizing proteins meant for export e.g. Enzyme secreting cells.
SER
- SER consists of an interconnected series of tubules.
- SER is present in those cells which are involved in metabolism of steroid, phospholipids, glycolipids, hormones and detoxification of toxins and drugs e.g. adipose tissue cells, adrenocortical cells, interstitial cells of testis etc.
Differences between RER and SER
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RER |
SER |
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Forms large flattened sheets except transitional elements of RER
Ribosomes are attached on cytosolic side
Present in cells actively engaged in biosynthesis of secretary proteins
RER is involved in glycosylation, folding and assembly of multimeric proteins
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Forms tubular structures
Ribosomes are not present
Present in cells actively involved in producing steroids, hormones
SER is involved in drug detoxification, carbohydrate metabolism and calcium storage
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SELF ASSESSMENT QUESTIONS
Q1. ER subdivides cytoplasm into compartments. Draw the diagram and name them.
Q2. Who showed that ER exists in two forms? Name the two forms and techniques used for their separation.
Q3. RER is different from SER in its structure. How?
Q4. RER and SER are present in cells actively engaged in different functions, specify.
DISTRIBUTION OF ER
- ER is present in eukaryotic cell
- Different types of cells contain different amount of RER and SER
- Cells which secrete large amount of proteins have extensively developed RER e.g. cells of salivary glands and pancreas.
- SER is extensively developed in steroid producing endocrine gland, cells of skeleton muscles, liver and kidney tubules.
FUNCTIONS OF RER
1. PROTEIN SYNTHESIS:-
· Ribosomes attached to cytosolic surface of the RER membrane synthesize proteins which enter into the lumen of ER at the time of synthesis (co-translationally). This includes
a) Secretory Proteins
b) Integral membrane proteins
c) Soluble proteins that reside within compartments of endomembrane system (within ER, golgi bodies, lysosomes, endosomes).
· Protein synthesized on free ribosomes and then transported to lumen of ER (post-translationally) includes :-
a) Peripheral proteins of the inner surface of plasma membrane.
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