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external image endoplasmicreticulumfigure1.jpgEndoplasmic Reticulum - The endoplasmic reticulum is a network of sacs that manufactures, processes, and transports chemical compounds for use inside and outside of the cell. It is connected to the double-layered nuclear envelope, providing a pipeline between the nucleus and the cytoplasm. In plants, the endoplasmic reticulum also connects between cells via the plasmodesmata.

Endoplasmic Reticulum

Endoplasmic reticulum is a network of tubules, vesicles and sacs that are interconnected. They may serve specialized functions in the cell including protein synthesis, sequestration of calcium, production of steroids, storage and production of glycogen, and insertion of membrane proteins. The first part of this presentation will focus on rough endoplasmic reticulum which gets its name from the presence of ribosomes on its surface.
http://cellbio.utmb.edu/cellbio/RER1.htm

The endoplasmic reticulum (ER) is repsonible for the production of the protein and lipid components of most of the cell's organelles.
http://library.thinkquest.org/C004535/endoplasmic_reticulum.html

Another organelle in the cell is the endoplasmic reticulum (ER). While the function of the nucleus is to act as the cell brain, the ER functions as a packaging system. It does not work alone. The ER works closely with the Golgi apparatus, ribososmes, RNA, mRNA, and tRNA. It creates a network of membranes found through the whole cell. The ER may also look different from cell to cell, depending on the cell's function.
The ER contains a great amount of folds - but the membrane forms a single sheet enclosing a single closed sac. This internal space is called the ER lumen. The ER is additionally responsible for moving proteins and other carbohydrates to the Golgi apparatus, to the plasma membrane, to the lysosomes, or wherever else needed.

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Rough Endoplasmic Reticulum
Rough Endoplasmic Reticulum
smooth: Throughout the eukaryotic cell, especially those responsible for the production of hormones and other secretory products, is a vast network of membrane-bound vesicles and tubules called the endoplasmic reticulum, or ER for short. The ER is a continuation of the outer nuclear membrane and its varied functions suggest the complexity of the eukaryotic cell.
The smooth endoplasmic reticulum is so named because it appears smooth by electron microscopy. Smooth ER plays different functions depending on the specific cell type including lipid and steroid hormone synthesis, breakdown of lipid-soluble toxins in liver cells, and control of calcium release in muscle cell contraction.

Question:

What is the function of the endoplasmic reticulum


Answer: It is a network of passageways that carries materials from one part of the cell to the other.

Rough: Rough endoplasmic reticulum appears "pebbled" by electron microscopy due to the presence of numerous ribosomes on its surface. Proteins synthesized on these ribosomes collect in the endoplasmic reticulum for transport throughout the cell.

Rough and Smooth

As you learn more about cells you will discover two types of ER. There are rough ER and smooth ER. They both have the same types of membranes but they have different shapes and rough ER has ribosomes attached. Rough ER looks like sheets of bumpy membranes while smooth ER looks more like tubes. Sometimes the ER looks like a flat balloon. Sacs of the ER called cisternae store the complex molecules.

Smooth Endoplasmic Reticulum
Smooth Endoplasmic Reticulum
Smooth ER has its purpose in the cell. It acts as a storage organelle. It is important in the creation and storage of steroids. It also stores ions in solution that the cell may need at a later time. Steroids are a type of ringed organic molecule used for many purposes in an organism. They are not always about building muscle mass like a weight lifter. The ion storage is important because sometimes a cell needs ions fast. It might not want to search the environment for ions, so it is easier to have them stored in a pack for easy use.

Rough ER was mentioned in the section on ribosomes. They are very important in the synthesis and packaging of proteins. Some of those proteins might be used in the cell and some are sent out. The ribosomes are attached to the membrane of the ER. As the ribosome builds the amino acid chain, the chain is pushed into the ER. When the protein is complete, the rough ER pinches off a vesicle. That vesicle, a small membrane bubble, can move to the cell membrane or the Golgi apparatus.

http://www.biology4kids.com/files/cell_er.html

Endoplasmic reticulum
Endoplasmic reticulum

Endoplasmic reticulum
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- Tubular network fused to nuclear membrane
- Goes through cytoplasm onto cell membrane
- Stores, separates, and serves as cell's transport system
- Smooth type: lacks ribosomes
- Rough type (pictured): ribosomes embedded in surface
http://library.thinkquest.org/12413/structures.html

The endoplasmic reticulum (ER), called "the mother of all membranes," is spotlighted in this timely new book. The work presented here is especially exciting since GFP-technology has provided new ways of looking at the dynamics of the ER and its relationship to other organelles, particularly the Golgi apparatus and peroxisomes. This book provides in-depth knowledge of the ER and the diverse roles it plays, for instance, in protein trafficking, homeostasis of cellular calcium, plant hormone and lipid synthesis. These manifold activities are reflected in the great plasticity of the ER as a structure, with more than a dozen specialized domains recognized for this organelle.


external image rer1.jpgEndoplasmic reticulum is a network of tubules, vesicles and sacs that are interconnected. They may serve specialized functions in the cell including protein synthesis, sequestration of calcium, production of steroids, storage and production of glycogen, and insertion of membrane proteins. The first part of this presentation will focus on rough endoplasmic reticulum which gets its name from the presence of ribosomes on its surface.http://cellbio.utmb.edu/cellbio/RER1.htm
The endoplasmic reticulum (ER) is a eukaryotic organelle that forms an interconnected network of tubules, vesicles, and cisternae within cells. The lacey membranes of the endoplasmic reticulum were first seen by Keith R. Porter, Albert Claude, and Ernest F. Fullam in 1945.[1[[http://en.wikipedia.org/wiki/Endoplasmic_reticulum#cite_note-FirstER-0|]]]
These structures are responsible for several specialized functions: protein translation, folding and transport of proteins to be used in the cell membrane (e.g. transmembrane receptors and other integral membrane proteins), or to be secreted (exocytosed) from the cell (e.g. digestive enzymes); sequestration of calcium; and production and storage of glycogen, steroids, and other macromolecules.[2[[http://en.wikipedia.org/wiki/Endoplasmic_reticulum#cite_note-1|]]] The endoplasmic reticulum is part of the endomembrane system. The basic structure and composition of the ER membrane is similar to the plasma membrane.



Another organelle in the cell is the endoplasmic reticulum (ER). While the function of the nucleus is to act as the cell brain, the ER functions as a packaging system. It does not work alone. The ER works closely with the Golgi apparatus, ribososmes, RNA, mRNA, and tRNA. It creates a network of membranes found through the whole cell. The ER may also look different from cell to cell, depending on the cell's function.

Rough Endoplasmic Reticulum
Rough Endoplasmic Reticulum

Rough and Smooth

As you learn more about cells you will discover two types of ER. There are rough ER and smooth ER. They both have the same types of membranes but they have different shapes and rough ER has ribosomes attached. Rough ER looks like sheets of bumpy membranes while smooth ER looks more like tubes. Sometimes the ER looks like a flat balloon. Sacs of the ER called cisternae store the complex molecules.

Smooth Endoplasmic Reticulum
Smooth Endoplasmic Reticulum
Smooth ER has its purpose in the cell. It acts as a storage organelle. It is important in the creation and storage of steroids. It also stores ions in solution that the cell may need at a later time. Steroids are a type of ringed organic molecule used for many purposes in an organism. They are not always about building muscle mass like a weight lifter. The ion storage is important because sometimes a cell needs ions fast. It might not want to search the environment for ions, so it is easier to have them stored in a pack for easy use.

Rough ER was mentioned in the section on ribosomes. They are very important in the synthesis and packaging of proteins. Some of those proteins might be used in the cell and some are sent out. The ribosomes are attached to the membrane of the ER. As the ribosome builds the amino acid chain, the chain is pushed into the ER. When the protein is complete, the rough ER pinches off a vesicle. That vesicle, a small membrane bubble, can move to the cell membrane or the Golgi apparatus.

Process of protein synthesis
Process of protein synthesis



external image endoplasmic_reticulums.gif
The general structure of the endoplasmic reticulum is an extensive membrane network of cisternae (sac-like structures) held together by the cytoskeleton. The phospholipid membrane encloses a space, the cisternal space (or lumen), from the cytosol. The functions of the endoplasmic reticulum vary greatly depending on the exact type of endoplasmic reticulum and the type of cell in which it resides. The three varieties are called rough endoplasmic reticulum, smooth endoplasmic reticulum and sarcoplasmic reticulum.
The quantity of RER and SER in a cell can quickly interchange from one type to the other, depending on changing metabolic needs: one type will undergo numerous changes including new proteins embedded in the membranes in order to transform. Also, massive changes in the protein content can occur without any noticeable structural changes, depending on the enzymatic needs of the cell (as per the functions listed below).

Rough endoplasmic reticulum

The surface of the rough endoplasmic reticulum (RER) is studded with protein-manufacturing ribosomes giving it a "rough" appearance (hence its name).[3[[http://en.wikipedia.org/wiki/Endoplasmic_reticulum#cite_note-campbell-2|]]] The RER is not always studded with these ribosomes, however; initially, the RER appears smooth due to a lack of resident ribosomes. The ribosomes only bind to the ER once it begins to synthesize a protein destined for sorting.[4[[http://en.wikipedia.org/wiki/Endoplasmic_reticulum#cite_note-lodish-3|]]] The free ribosome begins synthesizing a protein until a cytosolic signal recognition particle recognizes the pre-piece of 5-15 hydrophobic amino acids preceded by a positively charged amino acid. This allows the complex to loop the sequence through the hydrophobic RER membrane. The pre-piece is then cleaved off within the lumen of the RER.
The membrane of the RER is continuous with the outer layer of the nuclear envelope. Although there is no continuous membrane between the RER and the Golgi apparatus, membrane-bound vesicles shuttle proteins between these two compartments.[5[[http://en.wikipedia.org/wiki/Endoplasmic_reticulum#cite_note-4|]]] Vesicles are surrounded by coating proteins called COPI and COPII. COPII targets vesicles to the golgi and COPI marks them to be brought back to the RER. The RER works in concert with the Golgi complex to target new proteins to their proper destinations.
The RER is key in multiple functions:
  • lysosomal enzymes with a mannose-6-phosphate marker added in the cis-Golgi network
  • Secreted proteins, either secreted constitutively with no tag, or regulated secretion involving clathrin and paired basic amino acids in the signal peptide.
  • integral membrane proteins that stay imbedded in the membrane as vesicles exit and bind to new membranes. Rab proteins are key in targeting the membrane, SNAP and SNARE proteins are key in the fusion event.
  • initial glycosylation as assembly continues. This is either N-linked or O-linked (O-linked may likely occur in the golgi).
    • N-linked glycosylation: if the protein is properly folded, glycosyltransferase recognizes the AA sequence NXS or NXT (with the S/T residue phosphorylated) and adds a 14 sugar backbone (2 N-acetylglucosamine, 9 branching mannose, and 3 glucose at the end) to the side chain nitrogen of Asn.

Smooth endoplasmic reticulum

The smooth endoplasmic reticulum (SER) has functions in several metabolic processes, including synthesis of lipids and steroids, metabolism of carbohydrates, regulation of calcium concentration, drug detoxification, attachment of receptors on cell membrane proteins, and steroid metabolism.[6[[http://en.wikipedia.org/wiki/Endoplasmic_reticulum#cite_note-5|]]] It is connected to the nuclear envelope. Smooth endoplasmic reticulum is found in a variety of cell types (both animal and plant) and it serves different functions in each. The Smooth ER also contains the enzyme glucose-6-phosphatase which converts glucose-6-phosphate to glucose, a step in gluconeogenesis. The SER consists of tubules and vesicles that branch forming a network. In some cells there are dilated areas like the sacs of RER. The network of SER allows increased surface area for the action or storage of key enzymes and the products of these enzymes.

The endoplasmic reticulum serves many general functions, including the facilitation of protein folding and the transport of synthesized proteins in sacs called cisternae.
Correct folding of newly-made proteins is made possible by several endoplasmic reticulum chaperone proteins, including protein disulfide isomerase (PDI), ERp29, the Hsp70 family member Grp78, calnexin, calreticulin, and the peptidylpropyl isomerase family. Only properly-folded proteins are transported from the rough ER to the Golgi complex.
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Endoplasmic Reticulum

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[Electron Micrograph of a plant cell's endoplasmic reticulum. Copyright Daniel Kunkel]

The endoplasmic reticulum (ER) is repsonible for the production of the protein and lipid components of most of the cell's organelles. The ER contains a great amount of folds - but the membrane forms a single sheet enclosing a single closed sac. This internal space is called the ER lumen. The ER is additionally responsible for moving proteins and other carbohydrates to the Golgi apparatus, to the plasma membrane, to the lysosomes, or wherever else needed.
There are two types of ER - rough, which is coated with ribosomes, and smooth, which isn't. Rough ER is the site of protein synthesis. The smooth ER is where the vesicles carrying newly synthesized proteins (from the rough ER) are budded off.
external image endoplasmic_reticulums.gif


Throughout the eucaryotic cell, especially those responsible for the production of hormones and other secretory products, is a vast amount of membrane called the endoplasmic reticulum, or ER for short. The ER membrane is a continuation of the outer nuclear membrane and its function suggests just how complex and organized the eucaryotic cell really is.
Endoplasmic Reticulum
Endoplasmic Reticulum

When viewed by electron microscopy, some areas of the endoplasmic reticulum look "smooth" (smooth ER) and some appear "rough" (rough ER). The rough ER appears rough due to the presence of ribosomes on the membrane surface. Smooth and Rough ER also have different functions. Smooth ER is important in the synthesis of lipids and membrane proteins. Rough ER is important in the synthesis of other proteins.
Information coded in DNA sequences in the nucleus is transcribed as messenger RNA. Messenger RNA exits the nucleus through small pores to enter the cytoplasm. At the ribosomes on the rough ER, the messenger RNA is translated into proteins. These proteins are then transferred to the Golgi in "transport vesicles" where they are further processed and packaged into lysosomes, peroxisomes, or secretory vesicles.