Zoology Cell biology Name of the topic: Endoplasmic Reticulum Light microscopists described filamentous structures ergatoplasm, The name endoplasmic reticulum was first used by Porter and Kallman. Endoplasmic reticulum can be seen in all animal and plant cells, except in mature erythrocytes and prokaryotes. Since this network is more concentrated in the endoplasm of the cytoplasm, the name endoplasmic reticulum was proposed. These structures are present as inter connected system of membrane-bound chanalls. Endoplasmic reticulum is called Cytoskeleton of the cell. The occurrence of ER varies from cell to cell. ER is absent in erythrocytes (RBC), egg and embryonic cells. Poorly developed in spermatocytes.
Smooth endoplasmic reticulum is is connected to the nuclear envelope of cells in plants and animals. It's primary function is to facilitate the metabolism of carbohydrates and Steroids. It is abundantly present in adipose tissue, brown fat cells and adrenocortical cells, interstitial cells of testes and cells of corpus luteum of ovaries, sabaceous cells and retinal pigment cells.
Rough endoplasmic reticulum is located around the nucleus in the cells of both plants and animals. It transports proteins and lipids throughout the cell.it is actively engaged in the synthesis of proteins such as acinar cells of pancreas, plasma cells, goblet cells, and cells of some endocrine glands.
Hepatocytes consisting of both smooth and rough endoplasmic reticulum.
The Endoplasmic Reticulum Consists of three components: 1. Cisternae – secretary cells: The cisternae are long, flattened, sac-like, un branched tubules having the diameter of 40 to 50 µm. They remain arranged in bundles or stakes. RER usually exists as cisternae. They have ribosomes on their surface. They are abundant in cells which are active in protein synthesis.
2. Vesicle – pancreatic cells: The vesicles are oval, membrane bound vacuolar structures having the diameter of 25 to 500 µm. They often remain isolated in the cytoplasm and occur in most cells but especially abundant in the SER. Abundant in liver and pancreatic cells.
3. Tubules – Non –secretary cells: The tubules are branched structures forming the reticular system along with the cisternae and vesicle. They usually have the diameter from 50 – 100 µm. Occur almost in all the cells. Often found in SER, present in non-secretary cells like striated muscles.
Endoplasmic reticulum is a continuous membrane system that forms a series of flattened sacs within the cytoplasm of eukaryotic cells and serves multiple functions, being important particularly in the synthesis, folding, modification, and transport of proteins . In animal cells, the ER usually constitutes more than half of the membranous content of the cell. Differences in certain physical and functional characteristics distinguish the two types of ER, known as rough ER and smooth ER. 1. Rough Endoplasmic Reticulum (RER):
RER is named for its rough appearance, which is due to the ribosomes attached to its outer (cytoplasmic) surface. Rough ER lies immediately adjacent to the cell nucleus, and its membrane is continuous with the outer membrane of the nuclear envelope. The ribosomes on rough ER specialize in the synthesis of proteins that possess a signal sequence that directs them specifically to the ER for processing. Proteins synthesized by the rough ER have specific final destinations. Some proteins, for example, remain within the ER, whereas others are sent to the Golgi apparatus, which lies next to the ER. Proteins secreted from the Golgi apparatus are directed to lysosomes or to the cell membrane; still others are destined for secretion to the cell exterior. Proteins targeted for transport to the Golgi apparatus are transferred from ribosomes on rough ER into the rough ER lumen, which serves as the site of protein folding, modification, and assembly.
The proximity of the rough ER to the cell nucleus gives the ER unique control over protein processing. The rough ER is able to rapidly send signals to the nucleus when problems in protein synthesis and folding occur and thereby influences the overall rate of protein translation. When misfolded or unfolded proteins accumulate in the ER lumen, a signaling mechanism known as the unfolded protein response (UPR) is activated. The response is adaptive, such that UPR activation triggers reductions in protein synthesis and enhancements in ER protein-folding capacity and ER-associated protein degradation. If the adaptive response fails, cells are directed to undergo apoptosis (programmed cell death).
2. Smooth Endoplasmic Reticulum (SER):
Smooth endoplasmic reticulum by contrast, is not associated with ribosomes, and its functions differ. The smooth ER is involved in the synthesis of lipids, including cholesterol and phospholipids, which are used in the production of new cellular membrane. In certain cell types, smooth ER plays an important role in the synthesis of steroid hormones from cholesterol. In cells of the liver, it contributes to the detoxification of drugs and harmful chemicals. The sarcoplasmic reticulum is a specialized type of smooth ER that regulates the calcium ion concentration in the cytoplasm of striated muscle cells. SER The highly convoluted and labyrinthine structure of the ER led to its description in 1945 as a “lace-like reticulum” by cell biologists Keith Porter, Albert Claude, and Ernest Fullman, who produced the first electron micrograph of a cell. In the late 1940s and early 1950s, Porter and colleagues Helen P. Thompson and Frances Kallman introduced the term endoplasmic reticulum to describe the organelle. Porter later worked with Romanian-born American cell biologist George E. Palade to elucidate key characteristics of the ER.
Functions of smooth ER include lipid metabolism (both catabolism and anabolism; they synthesize a variety of phospholipids, cholesterol, and steroids). Glycogenolysis (degradation of glycogen; glycogen being polymerized in the cytosol). Drug detoxification (by the help of the cytochrome P-450). The endoplasmic reticulum provides an ultrastructural skeletal framework to the cell and gives mechanical support to the colloidal cytoplasmic matrix. The exchange of molecules by the process of osmosis, diffusion and active transport occurs through the membranes of the endoplasmic reticulum. The endoplasmic reticulum is the main component of the endomembrane system, also called the cytoplasmic vacuolar system or cytocavity network. The endoplasmic membranes contain many enzymes that perform various synthetic and metabolic activities. Further, the endoplasmic reticulum provides an increased surface for various enzymatic reactions. The endoplasmic reticulum acts as an intracellular circulatory or transporting system. As a growing secretory polypeptide emerges from the ribosome, it passes through the RER membrane and gets accumulated in the lumen of RER.
Here, the polypeptide chains undergo tailoring, maturation, and molecular folding to form functional secondary or tertiary protein molecules. RER pinches off certain tiny protein-filled vesicles which ultimately get fused to cis Golgi. The ER membranes are found to conduct intra- cellular impulses. For example, the sarcoplasmic reticulum transmits impulses from the surface membrane into the deep region of the muscle fibers. The ER membranes form the new nuclear envelope after each nuclear division.
The SER contains several key enzymes that catalyze the synthesis of cholesterol which is also a precursor substance for the biosynthesis of two types of compounds— the steroid hormones and bile acids. RER also synthesize membrane proteins and glycoproteins which are cotranslationally inserted into the rough ER membranes. Thus, the endoplasmic reticulum is the site of the biogenesis of cellular membranes. The sarcoplasmic reticulum (SR) is a variation of the SER. It is able to store many ions in solution that the cell will need at a later time. When a cell needs to do something immediately, it doesn’t make sense to search the environment for extra ions that may or may not be floating around. It is easier to have them stored in a pack for easy use. For example, when you are running around and your muscle cells are active, they need calcium (Ca) ions. The SR can release those ions immediately. When you are resting, they are able to store them for later use. Origin of Endoplasmic Reticulum: Palade stated that – it may be originated as the infoldings of plasma membrane. ER may be formed from the evagination of nuclear membrane, through the formation of annulate lamellae (Gay 1955, Rebhun 1956).
Chemical composition Mainly composed of lipoproteins, lipids - lecithin, cephalin, sphingomyelin, Nucleotide diphosphatase, NADH – Cytochrome C reductase,Mg++ activated ATP-ases, Sucrases, Several carrier enzymes – transport substances
Functions of Endoplasmic Reticulum 1. Mechanical Support 2. Permeability 3. Transport 4. Protein synthesis 5. Synthesis of Cholesterol and Steroid Hormones 6. Detoxification 7. Lipid synthesis 8. Glycogenolysis 9. Storage of Ca++ ions 10. Cell plate formation 11. Cytovacuolar system
WEB RESOURCES:
1. http://www.biologydiscussion.com/ 2. http://www.iaszoology.com/ 3. https://www.slideshare.net/ 4. https://en.wikipedia.org/wiki/Endoplasmic_reticulum
Book references: 1. cell and molecular biology,vE.D.P.De Robertis;E.M.F.De Roberties,jr.8th edition(2004) 2. Text book of zoology-Invertebrates and cell bilogy-Telugu Academy (2013)
Prepared by: Dr. Y. Savithri, Lecturer in Zoology, Govt. College for Men(A), Kadapa.