The endoplasmic reticulum or ER is an organelle found in all eukaryotic cells that is an interconnected network of tubules, vesicles and cisternae that is 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.^[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. Schematic of typical animal cell, showing subcellular components. ... Kingdoms Animalia - Animals Fungi Plantae - Plants Protista Alternative Phylogeny Unikonta    Opisthokonta    Amoebozoa Bikonta    Apusozoa    Cabozoa       Rhizaria       Excavata    Corticata       Archaeplastida       Chromalveolata Animals, plants, fungi, and protists are eukaryotes (IPA: ), organisms with a complex cell or cells, where the genetic material is organized into a membrane-bound nucleus or nuclei. ... A tubule is a very small tube or fistular structure. ... This article or section is in need of attention from an expert on the subject. ... A cisterna (plural cisternae) comprises a flattened membrane disk which makes up the Golgi apparatus. ... Biological and artificial methods for creation of proteins differ significantly. ... // The cell membrane (also called the plasma membrane or plasmalemma) is a semipermeable lipid bilayer common to all living cells. ... Transmembrane receptors are integral membrane proteins, which reside and operate typically within a cells plasma membrane, but also in the membranes of some subcellular compartments and organelles. ... An Integral Membrane Protein (IMP) is a protein molecule (or assembly of proteins) that is permanently attached to the biological membrane. ... This page is currently under construction. ... Ribbon diagram of the enzyme TIM, surrounded by the space-filling model of the protein. ... General Name, Symbol, Number calcium, Ca, 20 Chemical series alkaline earth metals Group, Period, Block 2, 4, s Appearance silvery white Standard atomic weight 40. ... Electron micrograph of a section of a liver cell showing glycogen deposits as accumulations of electron dense particles (arrows). ... Bold text Steroid skeleton of lanosterol. ... A macromolecule is a molecule composed of a very large number of atoms. ... The endomembrane system is the system of internal membranes within eukaryotic cells that divide the cell into functional and structural compartments, or organelles. ... Drawing of a cell membrane A component of every biological cell, the cell membrane (or plasma membrane) is a thin and structured bilayer of phospholipid and protein molecules that envelopes the cell. ...

Structure

Figure 1: Nucleus, endoplasmic reticulum and Golgi apparatus.
1 Nucleus     2 Nuclear pore     3 Rough endoplasmic reticulum (rER)     4 Smooth endoplasmic reticulum (sER)     5 Ribosome on the rough ER     6 Proteins that are transported     7 Transport vesicle     8 Golgi apparatus     9 Cis face of the Golgi apparatus     10 Trans face of the Golgi apparatus     11 Cisternae of the Golgi apparatus

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. Yours, Magnus Manske Source: [1]. See also User:Magnus Manske This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... Yours, Magnus Manske Source: [1]. See also User:Magnus Manske This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... The eukaryotic cell nucleus. ... Diagram of the endomembrane system in a typical eukaryote cell Micrograph of Golgi apparatus, visible as a stack of semicircular black rings near the bottom. ... The eukaryotic cell nucleus. ... Nuclear pore. ... Figure 1: Ribosome structure indicating small subunit (A) and large subunit (B). ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... In cell biology, a vesicle is a relatively small and enclosed compartment, separated from the cytosol by at least one lipid bilayer. ... Diagram of the endomembrane system in a typical eukaryote cell Micrograph of Golgi apparatus, visible as a stack of semicircular black rings near the bottom. ... A cisterna (plural cisternae) comprises a flattened membrane disk which makes up the Golgi apparatus. ... The eukaryotic cytoskeleton. ... The cytosol (cf. ...

Rough endoplasmic reticulum

The surface of the rough endoplasmic reticulum is studded with protein-manufacturing ribosomes giving it a "rough" appearance (hence its name).^[2] But it should be noted that these ribosomes are not resident of the endoplasmic reticulum incessantly. The ribosomes only bind to the ER once it begins to synthesize a protein destined for sorting. ^[3] The membrane of the rough endoplasmic reticulum is continuous with the outer layer of the nuclear envelope. Although there is no continuous membrane between the rough ER and the Golgi apparatus, membrane bound vesicles shuttle proteins between these two compartments.^[4] The rough endoplasmic reticulum works in concert with the Golgi complex to target new proteins to their proper destinations. Figure 1: Ribosome structure indicating small subunit (A) and large subunit (B). ... The nuclear envelope (also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is the double membrane of the nucleus that encloses genetic material in eukaryotic cells. ... Diagram of the endomembrane system in a typical eukaryote cell Micrograph of Golgi apparatus, visible as a stack of semicircular black rings near the bottom. ... Protein targeting a. ...

Smooth endoplasmic reticulum

The smooth endoplasmic reticulum has functions in several metabolic processes, including synthesis of lipids, metabolism of carbohydrates and calcium concentration, and attachment of receptors on cell membrane proteins. 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. It consists of tubules and vesicles that branch forming a network. In some cells there are dilated areas like the sacs of rough endoplasmic reticulum. The network of smooth endoplasmic reticulum allows increased surface area for the action or storage of key enzymes and the products of these enzymes. The smooth endoplasmic reticulum is known for its storage of calcium ions in muscle cells.

Sarcoplasmic reticulum

The sarcoplasmic reticulum is a special type of smooth ER found in striated muscle. The only structural difference between this organelle and the smooth endoplasmic reticulum is the medley of protein they have, both bound to their membranes and drifting within the confines of their lumens. This fundamental difference is indicative of their functions: the smooth ER built to synthesize molecules and the sarcoplasmic reticulum built to store and pump calcium ions. ... Structure of a skeletal muscle Skeletal muscle is a type of striated muscle, attached to the skeleton. ...

The sarcoplasmic reticulum contains large stores of calcium, which it sequesters and then releases when the cell is depolarised.^[5] This has the effect of triggering muscle contraction.

Functions

The endoplasmic reticulum serves many general functions, including the facilitation of protein folding and the transport of synthesized proteins in sacs called cisternae. This article relates to cell biology. ...

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. In biology, chaperones are proteins whose function is to assist other proteins in achieving proper folding. ... Protein disulfide isomerase or PDI (EC 5. ... Hsp70 is a family of heat shock proteins including HSP70 (also known as Hsp72), Bip and the prokaryotic protein DnaK with an approximate molecular weight of 70 kDa. ... a type of protein found in the smooth endoplasmic reticulum ... Calreticulin is a protein that binds Ca2+ ions (a second messenger molecule in signal transduction), rendering it inactive. ... In cell biology, the Golgi apparatus, Golgi body, Golgi complex, or dictyosome is an organelle found in nearly all eukaryotic cells. ...

Transport of proteins

Secretory proteins, mostly glycoproteins, are moved across the endoplasmic reticulum membrane. Proteins that are transported by the endoplasmic reticulum and from there throughout the cell are marked with an address tag called a signal sequence. The N-terminus (one end) of a polypeptide chain (i.e., a protein) contains a few amino acids that work as an address tag, which are removed when the polypeptide reaches its destination. Proteins that are destined for places outside the endoplasmic reticulum are packed into transport vesicles and moved along the cytoskeleton toward their destination. A signal peptide is a short (15-60 amino acids long) peptide chain that directs the post transrational transport of a protein. ... Peptides are the family of molecules formed from the linking, in a defined order, of various amino acids. ... Phenylalanine is one of the standard amino acids. ... This article or section is in need of attention from an expert on the subject. ... The eukaryotic cytoskeleton. ...

The endoplasmic reticulum is also part of a protein sorting pathway. It is, in essence, the transportation system of the eukaryotic cell. The majority of endoplasmic reticulum resident proteins are retained in the endoplasmic reticulum through a retention motif. This motif is composed of four amino acids at the end of the protein sequence. The most common retention sequence is KDEL (lys-asp-glu-leu). However, variation on KDEL does occur and other sequences can also give rise to endoplasmic reticulum retention. It is not known if such variation can lead to sub-endoplasmic reticulum localizations. There are three KDEL receptors in mammalian cells, and they have a very high degree of sequence identity. The functional differences between these receptors remain to be established. Phenylalanine is one of the standard amino acids. ...

Other functions

• Insertion of proteins into the endoplasmic reticulum membrane: Integral proteins must be inserted into the endoplasmic reticulum membrane after they are synthesized. Insertion into the endoplasmic reticulum membrane requires the correct topogenic sequences.
• Glycosylation: Glycosylation involves the attachment of oligosaccharides.
• Disulfide bond formation and rearrangement: Disulfide bonds stabilize the tertiary and quaternary structure of many proteins.

Integral membrane protein of the transmembrane type An Integral Membrane Protein (IMP) is a protein molecule (or assembly of proteins) that in most cases spans the biological membrane with which it is associated (especially the plasma membrane) or which, in any case, is sufficiently embedded in the membrane to remain... A topogenic sequence is a segment of a protein that ensures a protein acquires the proper orientation during its insertion into the endoplasmic reticulum. ... Glycosylation is the process or result of addition of saccharides to proteins and lipids. ... An oligosaccharide is a saccharide polymer containing a small number (typically three to six) of component sugars, also known as simple sugars. ...

History

The lacey membranes of the endoplasmic reticulum were first seen by Keith R. Porter, Albert Claude, and Ernest F. Fullam in 1945.^[6] KeithPorter (1912-1997) was an American cell biologist. ... Albert Claude (August 24, 1899 – May 22, 1983) was a Belgian biologist who won the Nobel Prize in Physiology or Medicine in 1974. ...

See also

• Protein targeting
• Secretory pathway

Protein targeting a. ... A secretory pathway is a term used to describe different methods that cells use to transport material to the outside, usually from the endoplasmic reticulum via the Golgi apparatus. ...

References

1. ^ Spurger, L. (2002). Endoplasmic reticulum: Structure and function. University of Texas Medical Branch. Retrieved September 13, 2006, from http://cellbio.utmb.edu/cellbio/rer1.htm
2. ^ Campbell, Neil A. (1996) Biology Fourth Edition. Benjamin/Cummings Publishing, pp. 120-121 ISBN 0-8053-1940-9
3. ^ Lodish, Harvey, et al. (2003) Molecular Cell Biology 5th Edition. W. H. Freeman, pp. 659-666 ISBN 0716743663
4. ^ Endoplasmic reticulum. (n.d.). McGraw-Hill Encyclopedia of Science and Technology. Retrieved September 13, 2006, from Answers.com Web site: http://www.answers.com/topic/endoplasmic-reticulum
5. ^ Toyoshima C, Nakasako M, Nomura H, Ogawa H (2000). "Crystal structure of the calcium pump of sarcoplasmic reticulum at 2.6 A resolution". Nature 405 (6787): 647-55. PMID 10864315. 
6. ^ KR. Porter, A. Claude, and EF. Fullam. (March, 1945) A study of tissue culture cells by electron microscopy. J Exp Med 81:233-246. Retrieved on Jan 06, 2007. (http://www.jem.org/cgi/reprint/81/3/233)

External links

• UMich Orientation of Proteins in Membranes localization/localization-Endoplasmic%20reticulum%20membrane
• Animations of the various cell functions referenced here