Difference between revisions of "Endoplasmic reticulum" - New World Encyclopedia
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| − | The '''endoplasmic reticulum''' or '''ER''' | + | The '''endoplasmic reticulum''', or '''ER''', is an [[organelle]] found in all [[eukaryote|eukaryotic cells]]. It modifies [[protein]]s, makes [[macromolecule]]s, and transfers substances throughout the [[cell]]. The ER is part of the endomembrane system, which is the system of internal membranes within eukaryotic cells that divides the cell into functional and structural compartments, or organelles. Endoplasmic reticulum literally means little net within the cytoplasm (''endoplasmic'' mean "within the cytoplasm" while ''reticulum'' means "little net"). [[prokaryote|Prokaryotic organisms]] do not have membranous organelles and thus do not have an ER. |
| − | It | + | |
| − | [[prokaryote|Prokaryotic organisms]] do not have membranous organelles and thus do not have an ER. | + | The basic structure and composition of the ER is similar to the plasma membrane, although it is actually an extension of the nuclear membrane which encloses genetic material in eukaryotic cells. The ER is the site of the [[protein synthesis|translation]], folding, and transport of proteins that are to become part of the [[cell membrane]] (e.g., [[transmembrane receptor]]s and other integral membrane proteins) as well as proteins that are to be secreted, or "[[exocytosis|exocytosed]]", from the cell (e.g., digestive [[enzyme]]s). |
| − | The basic structure and composition of the ER is similar to the | + | |
| − | == Structure == | + | ==Structure== |
[[Image:nucleus_ER_golgi.jpg|thumb|360px|'''Figure 1 :''' Image of [[cell nucleus|nucleus]], endoplasmic reticulum and Golgi apparatus.<br/> | [[Image:nucleus_ER_golgi.jpg|thumb|360px|'''Figure 1 :''' Image of [[cell nucleus|nucleus]], endoplasmic reticulum and Golgi apparatus.<br/> | ||
(1) Nucleus. | (1) Nucleus. | ||
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The rough and smooth ER differ in both appearance and function but are [[contiguous]] with each other. | The rough and smooth ER differ in both appearance and function but are [[contiguous]] with each other. | ||
| − | === Rough ER === | + | ===Rough ER=== |
The rough ER, manufactures and transports proteins destined for membranes and secretion. | The rough ER, manufactures and transports proteins destined for membranes and secretion. | ||
| − | === Smooth ER === | + | ===Smooth ER=== |
The smooth ER has functions in several metabolic processes, including synthesis of lipids, metabolism of carbohydrates, and detoxification of drugs and poisons. | The smooth ER has functions in several metabolic processes, including synthesis of lipids, metabolism of carbohydrates, and detoxification of drugs and poisons. | ||
| − | === Sarcoplasmic | + | ===Sarcoplasmic Reticulum=== |
In [[striated muscle]] it is specially adapted to surround the [[myofibril]]s, forming triads with invaginations of the [[plasma membrane]] called [[T-tubule]]s. The sarcoplasmic reticulum contains large stores of Calcium, which it releases when the cell become depolarised. This has the effect of triggering muscle contraction. | In [[striated muscle]] it is specially adapted to surround the [[myofibril]]s, forming triads with invaginations of the [[plasma membrane]] called [[T-tubule]]s. The sarcoplasmic reticulum contains large stores of Calcium, which it releases when the cell become depolarised. This has the effect of triggering muscle contraction. | ||
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Only properly-folded proteins are transported from the RER to the [[Golgi complex]]. | Only properly-folded proteins are transported from the RER to the [[Golgi complex]]. | ||
| − | ===Transport of | + | ===Transport of Proteins=== |
Secretory proteins are moved across the ER membrane. Proteins that are transported by the ER and from there throughout the cell are marked with an address tag called a [[protein targeting|signal sequence]]. The N-terminus (one end) of a [[polypeptide]] chain (e.g., a protein) contains a few [[amino acid]]s that work as an address tag, which are removed when the polypeptide reaches its destination. Proteins that are destined for places outside the ER are packed into transport [[vesicle (biology)|vesicle]]s and moved along the [[cytoskeleton]] toward their destination. | Secretory proteins are moved across the ER membrane. Proteins that are transported by the ER and from there throughout the cell are marked with an address tag called a [[protein targeting|signal sequence]]. The N-terminus (one end) of a [[polypeptide]] chain (e.g., a protein) contains a few [[amino acid]]s that work as an address tag, which are removed when the polypeptide reaches its destination. Proteins that are destined for places outside the ER are packed into transport [[vesicle (biology)|vesicle]]s and moved along the [[cytoskeleton]] toward their destination. | ||
The ER is also part of a protein sorting pathway. It is, in essence, the transportation system of the eukaryotic cell. The majority of ER resident proteins are retained in the ER 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. However, variation on KDEL does occur and other sequences can also give rise to ER retention. It is not known if such variation can lead to sub-ER 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. | The ER is also part of a protein sorting pathway. It is, in essence, the transportation system of the eukaryotic cell. The majority of ER resident proteins are retained in the ER 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. However, variation on KDEL does occur and other sequences can also give rise to ER retention. It is not known if such variation can lead to sub-ER 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. | ||
| − | ===Other | + | ===Other Functions=== |
*'''Insertion of proteins into the ER membrane''': [[Integral protein]]s must be inserted into the ER membrane after they are synthesized. Insertion into the ER membrane requires the correct [[topogenic sequence]]s. | *'''Insertion of proteins into the ER membrane''': [[Integral protein]]s must be inserted into the ER membrane after they are synthesized. Insertion into the ER membrane requires the correct [[topogenic sequence]]s. | ||
*'''Glycosylation''': [[Glycosylation]] involves the attachment of [[oligosaccharide]]s. | *'''Glycosylation''': [[Glycosylation]] involves the attachment of [[oligosaccharide]]s. | ||
Revision as of 17:27, 25 July 2006
The endoplasmic reticulum, or ER, is an organelle found in all eukaryotic cells. It modifies proteins, makes macromolecules, and transfers substances throughout the cell. The ER is part of the endomembrane system, which is the system of internal membranes within eukaryotic cells that divides the cell into functional and structural compartments, or organelles. Endoplasmic reticulum literally means little net within the cytoplasm (endoplasmic mean "within the cytoplasm" while reticulum means "little net"). Prokaryotic organisms do not have membranous organelles and thus do not have an ER.
The basic structure and composition of the ER is similar to the plasma membrane, although it is actually an extension of the nuclear membrane which encloses genetic material in eukaryotic cells. The ER is the site of the translation, folding, and transport of proteins that are to become part of the cell membrane (e.g., transmembrane receptors and other integral membrane proteins) as well as proteins that are to be secreted, or "exocytosed", from the cell (e.g., digestive enzymes).
Structure
(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 ER consists of an extensive membrane network of tubes and cisternae (sac-like structures) held together by the cytoskeleton. The membrane encloses a space, the cisternal space (or internal lumen) from the cytosol. This space is acting as a gateway. Parts of the ER membrane are continuous with the outer membrane of the nuclear envelope, and the cisternal space of the ER is continuous with the space between the two layers of the nuclear envelope (the intermembrane space). Parts of the ER are covered with ribosomes (which assemble amino acids into proteins based on instructions from the nucleus). Their rough appearance under electron microscopy led to their being called rough ER (rER), other parts are free of ribosomes and are called smooth ER (sER). The ribosomes on the surface of the rough ER insert the freshly produced proteins directly into the ER, which processes them and then passes them on to the Golgi apparatus (Fig. 1). The rough and smooth ER differ in both appearance and function but are contiguous with each other.
Rough ER
The rough ER, manufactures and transports proteins destined for membranes and secretion.
Smooth ER
The smooth ER has functions in several metabolic processes, including synthesis of lipids, metabolism of carbohydrates, and detoxification of drugs and poisons.
Sarcoplasmic Reticulum
In striated muscle it is specially adapted to surround the myofibrils, forming triads with invaginations of the plasma membrane called T-tubules. The sarcoplasmic reticulum contains large stores of Calcium, which it releases when the cell become depolarised. 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 proteins. Correct folding of newly-made proteins is made possible by several ER proteins including: Protein Disulfide Isomerase (PDI), Hsc70 family, calnexin, calreticulin, and the peptidylpropyl isomerase family. Only properly-folded proteins are transported from the RER to the Golgi complex.
Transport of Proteins
Secretory proteins are moved across the ER membrane. Proteins that are transported by the ER 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 (e.g., 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 ER are packed into transport vesicles and moved along the cytoskeleton toward their destination. The ER is also part of a protein sorting pathway. It is, in essence, the transportation system of the eukaryotic cell. The majority of ER resident proteins are retained in the ER 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. However, variation on KDEL does occur and other sequences can also give rise to ER retention. It is not known if such variation can lead to sub-ER 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.
Other Functions
- Insertion of proteins into the ER membrane: Integral proteins must be inserted into the ER membrane after they are synthesized. Insertion into the ER 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.
| Organelles of the cell |
|---|
| Acrosome | Chloroplast | Cilium/Flagellum | Centriole | Endoplasmic reticulum | Golgi apparatus | Lysosome | Melanosome | Mitochondrion | Myofibril | Nucleus | Parenthesome | Peroxisome | Plastid | Ribosome | Vacuole | Vesicle |
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