Endoplasmic Reticulum

1. Endoplasmic Reticulum Pankaj R Chavhan,Asst Professor Dept of Zoology Shri S S Science College,Ashti

2. Occurrence- • The endoplasmic reticulum is found in almost all animal and plant cells except in prokaryotes and mature RBCs. It is a network of membrane bound cavities, vesicles and tubules. It is distributed throughout the cytoplasm. It is small and undifferentiated in embryonic cells. It is present n muscles called Sarcoplasmic reticulum. It is the cytoskeleton of the cell. The term endoplasmic -reticulum was introduced by Porter. According to the Porter, endoplasmic reticulum is a complex vacuolar system which extends from nuclear membrane through cytoplasm up to plasma membrane of the cell. It is a network of double membrane which divides the cytoplasm into number of compartments.

3. Origin: • There are several controversial endoplasmic reticulum suggestions about the endoplasmic reticulum. They are as follows: • Invagination of plasma membrane • Evagination of nuclear membrane • Mitochondrial surface membrane origin • Cytoplasmic origin

4. Ultrastructure of Endoplasmic Reticulum • They are structurally variable not only in the different cells but also within the same cell. They are found in the entire cell except mammalian RBCs and bacteria. The development of endoplasmic reticulum within the cell is proportional to their protein synthesis activity. The endoplasmic reticulum composed of cisternae, vesicles and tubules

5. 1. Cisternae: • They are elongated, large flattened, unbranched sac like structure. These form successive layer around the nucleus. They are arranged parallel to each other. They are 40-50μ in thickness. They have ribosomes on their surface. They are found in active secretory cells like liver, pancreas and brain.

6. 2. Vesicles: • Vesicles are rounded, spherical or oval, membrane bound vacuolar structure having the diameter of 25-500μ. They are found in most cell but abundance in pancreatic cells. They are suspended in the cytoplasm near the cisternae. They transfer the substance to other cell organelle

7. Types of Endoplasmic reticulum: • Endoplasmic reticulum is classified into two types on the basis of the presence or absence of ribosomes on it outer surface. They are • A) Granular or Rough Endoplasmic Reticulum (RER) • B) Agranular or Smooth Endoplasmic Reticulum (SER)

8. A) Rough orGranular endoplasmic reticulum: • In some endoplasmic reticulum, ribosomes are attached to the outer surface of the membrane. This type of endoplasmic reticulum is called as rough or granular endoplasmic reticulum. The rough endoplasmic reticulum is involved in the protein synthesis. In RER, ribosomes are often present as polysomes held together by mRNA and are arranged in typical “rosettes” or spirals. RER contains two transmembraneglycoproteins to which are attached the ribosomes by their 60S subunits.

9. B) Smooth or Agranular endoplasmic reticulum: • Ribosomes are not attached with the membrane of this type of endoplasmic reticulum. So the surface of this endoplasmic reticulum is smooth such type of endoplasmic reticulum is known as smooth endoplasmic reticulum. Abundant SER are found in muscle cell, and are called sarcoplasmic reticulum. It is found in cells which are active in synthesis of steroid compound, cholesterol, and glycogen but not participated in protein synthesis. The SER is involved in the synthesis and metabolism of certain small molecules and cellular detoxification. It is often continuous with rough endoplasmic reticulum.

10. A) Rough orGranular endoplasmic reticulum: • In some endoplasmic reticulum, ribosomes are attached to the outer surface of the membrane. This type of endoplasmic reticulum is called as rough or granular endoplasmic reticulum. The rough endoplasmic reticulum is involved in the protein synthesis. In RER, ribosomes are often present as polysomes held together by mRNA and are arranged in typical “rosettes” or spirals. RER contains two transmembraneglycoproteins to which are attached the ribosomes by their 60S subunits.

11. B) Smooth or Agranular endoplasmic reticulum: • Ribosomes are not attached with the membrane of this type of endoplasmic reticulum. So the surface of this endoplasmic reticulum is smooth such type of endoplasmic reticulum is known as smooth endoplasmic reticulum. Abundant SER are found in muscle cell, and are called sarcoplasmic reticulum. It is found in cells which are active in synthesis of steroid compound, cholesterol, and glycogen but not participated in protein synthesis. The SER is involved in the synthesis and metabolism of certain small molecules and cellular detoxification. It is often continuous with rough endoplasmic reticulum.

12. Annulate lamellae: • Usually the membranes of endoplasmic reticulum are continuous without Many pores and pore complexes, which are very common in nuclear membrane. But in some cells, some membranes of ER possess pores and pore complexes. These represent annulate lamellae. These are found in the cells of invertebrates, in the immature oocytes and spermatocytes of vertebrates and the embryonic and foetal cells which have a high rate of metabolism. Annulate lamellae are double membrane sheets. About 2-12 lamellae are arranged to form stacks. Structurally, these are similar to the nuclear membrane. It is evidenced that annulate lamellae arise from the nuclear membrane. The outer membrane of the nuclear envelope forms finger-like processes which are pinched off into the cytoplasm in the form of vesicles called blebs. This process is known as blebbing. These vesicles migrate towards the periphery of the cells in rows and fuse to form cisternae. The material present in the surrounding matrix becomes associated with the pore of cisternae forming annuli. This results in the formation of annulate lamellae.

13. Function of Endoplasmic Reticulum • The endoplasmic reticulum has the following functions:- • 1. Mechanical support: • • It provides additional mechanical support for the colloidal matrix of the cytoplasm. Hence it is known as the cytoskeleton of the cell. • 2. Protein synthesis: • • Granular endoplasmic reticulum is concerned with protein synthesis. • • The amount of granular endoplasmic reticulum depends upon the biosynthetic activity of cell. • • In cell synthesizing small amount of protein, endoplas • • In actively synthesizing cell, endoplasmic reticulum is well developed and consists of numerous parallel cisternae.

14. 3. Transport: • • It acts as a kind of cellular circulatory system. • • Proteins, enzymes, lipids etc are transported to the various part of the cell through endoplasmic reticulum. Fig- 3.7Smooth endoplasmic reticulum Fig-3.8 Rough endoplasmic reticulum • • The endoplasmic reticulum may also store the material thus act as a store house.

15. 4. Intra-cellular impulse conduction: • In skeletal muscle the endoplasmic reticulum is associated with the conduction of intracellular impulses from surface membrane to the deep region of muscle fibres.

16. 5. Synthesis of cholesterol and steroid hormones: • In testes, ovary and adrenal gland, the smooth endoplasmic reticulum has a role in the synthesis of steroid compound • 6. Enzymatic activity: • Numerous enzymes are associated with the endoplasmic reticulum membrane. Thus endoplasmic reticulum membrane provides larger surface for different metabolic activities such as lipid metabolism and glycogen synthesis. • 7. Glycosylation: • The carbohydrate is covalently bonded to protein in ER. • The addition and subsequent processing of carbohydrates (glycosylation) attached to the protein is done by specialized molecule built into ER membrane and Golgi complex.

17. 8. Drug detoxification: • • Enzymes in the smooth ER of the liver also modify or detoxify hydrophobic chemicals such as Fig- 3.9 Ultrastructure of rough endoplasmic reticulum. pesticides and carcinogens by chemically converting them into more water-soluble, conjugated products that can be excreted from the body. • 9. Cell division: • Endoplasmic reticulum is associated with the cell division i.e. during cell division it helps in the formation of nuclear membrane. • It also helps in the formation of cell plate during cell division.

18. 10. Formation of other cytomembranes: • Other membranes structure of the cell like the nuclear membrane and Golgi complex differentiated from ER. During cell division, the nuclear membrane disintegrates into small vesicles. The vesicles move towards the poles of the spindle in metaphase. Later on, in telophase the fragmented vesicle and the elements of ER migrate and then accumulate around the chromosome group. The elements thus fuse forming a complete nuclear membrane. The bits of ER participate in the formation of cell-plate also. • 11. In intestinal epithelium, smooth endoplasmic reticulum has a role in lipid transport.

19. 12. In oxyntic cells of stomach, it is concerned with chlorides ion secretion. • 13. Fats are made up of three fatty acids molecules and one glycerol molecule. The coupling of fatty acid with glycerol occurs with endoplasmic reticulum. • 14. ER membranes are site of ATP synthesis which is the source of energy for all metabolic and transport of material.