Golgi apparatus: Difference between revisions
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The '''Golgi apparatus''' (also called the '''Golgi body''', '''Golgi complex''', or '''dictyosome''') is an [[organelle]] found in typical [[eukaryote|eukaryotic]] cells. It was identified in 1898 by the Italian physician [[Camillo Golgi]] and was named after him. The primary function of the Golgi apparatus is to process and package [[macromolecule]]s synthesised by the cell, primarily [[protein]]s and [[lipid]]s. The Golgi apparatus forms a part of the [[endomembrane system]] present in eukaryotic cells. |
The '''Golgi apparatus''' (also called the '''Golgi body''', '''Golgi complex''', or '''dictyosome''') is an [[organelle]] found in typical [[eukaryote|eukaryotic]] cells. It was identified in 1898 by the Italian physician [[Camillo Golgi]] and was named after him. The primary function of the Golgi apparatus is to process and package [[macromolecule]]s synthesised by the cell, primarily [[protein]]s and [[lipid]]s. The Golgi apparatus forms a part of the [[endomembrane system]] present in eukaryotic cells. |
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]]s destined for the [[lysosome]] |
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==Structure== |
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The Golgi is composed of membrane-bound sacs known as [[cisternae]]. Between five and eight are usually present, however as many as fifty-nine have been observed.<ref name="molexpress">{{Cite web|url=http://micro.magnet.fsu.edu/cells/golgi/golgiapparatus.html|title=Molecular Expressions Cell Biology: The Golgi Apparatus|accessdate=2006-11-08}}</ref> Surrounding the main cisternae are a number of spherical [[vesicle]]s which have budded off from the cisternae. |
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The cisternae stack has five functional regions: the cis-Golgi network, cis-Golgi, medial-Golgi, trans-Golgi, and trans-Golgi network. Vesicles from the endoplasmic reticulum (via the [[vesicular-tubular cluster]]) fuse with the cis-Golgi network and subsequently progress through the stack to the trans-Golgi network, where they are packaged and sent to the required destination. Each region contains different enzymes which selectively modify the contents depending on where they are destined to reside.<ref name="lodish">{{cite book | title=Molecular Cell Biology| edition=5th edn |last=Lodish| coauthors=et al.| date=2004| publisher=W.H. Freeman and Company| id=P0-7167-4366-3}}</ref> Please remember that as mentioned below, the precise process of how the structure of the actual lumen, or content of the cisternae of the Golgi aparatus, has not yet been determined. However, it is also supposed to be a multi joined membrane-bound accumulation. |
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===Function=== |
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Cells synthesise a large number of different macromolecules required for life. The Golgi apparatus is integral in modifying, sorting, and packaging these substances for cell secretion ([[exocytosis]]) or for use within the cell. It primarily modifies proteins delivered from the [[rough endoplasmic reticulum]], but is also involved in the transport of [[lipid]]s around the cell, and the creation of [[lysosome]]s. In this respect it can be thought of as similar to a post office; it packages and labels "items" and then sends them to different parts of the cell. |
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Enzymes within the cisternae are able to modify substances by the addition of carbohydrates ([[glycosylation]]) and phosphate ([[phosphorylation]]) to them. In order to do so the Golgi transports substances such as nucleotide sugars into the organelle from the cytosol. Proteins are also labeled with a [[Protein targeting|signal sequence]] of molecules which determine their final destination. For example, the Golgi apparatus adds a [[Mannose|mannose-6-phosphate]] label to proteins destined for [[lysosome]]s. |
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A specific [[example]] of modifying tasks of the Golgi include the important role it plays in the synthesis of [[proteoglycans]], molecules present in the [[extracelluar matrix]] of [[animal]]s. The Golgi is a major site of [[carbohydrate]] synthesis.<ref name="alberts">{{cite book | title=Molecular Biology of the Cell| url=http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowTOC&rid=cell.TOC| last=Alberts| first=Bruce| coauthors=''et al.''| publisher=Garland Publishing}}</ref> |
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This includes the productions of GAGs, full name [[glycosaminoglycan]]s, long unbranched [[polysaccharide]]s which the Golgi then attaches to a protein synthesized in the ER to form the [[proteoglycan]].<ref> Pyrdz, K. and K.T. Dalan, Synthesis and Sorting of Proteoglycans. Journal of Cell Science, 2000. 113: p. 193-205.</ref>Enzymes in the Golgi will [[polymerize]] several of these GAGs via a [[xylose]] link onto the core protein. Another task of the Golgi involves the [[sulfation]] of certain molecules passing through it’s lumen via sulphotranferases that gain their sulfer molecule from a donor called PAPs. This process occurs on the GAGs of proteoglycans as well as on the core protein. The level of sulfation is very important to the proteoglycans signaling abilities as well as giving the proteoglycan it’s overall negative charge.<ref name="alberts">{{cite book | title=Molecular Biology of the Cell| url=http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowTOC&rid=cell.TOC| last=Alberts| first=Bruce| coauthors=''et al.''| publisher=Garland Publishing}}</ref> |
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The Golgi is also capable of [[phosphorylating]] molecules. To do so it transports [[ATP]] into the [[lumen]].<ref> Capasso, J., et al., Mechanism of phosphorylation in the lumen of the Golgi apparatus. Translocation of adenosine 5'-triphosphate into Golgi vesicles from rat liver and mammary gland. Journal of Biological Chemistry, 1989. 264(9): p. 5233-5240.</ref> The Golgi itself contains resident [[kinase]]s, such as [[casein kinase]]s. One molecule that is phosphorylated in the Golgi is [[Apolipoprotein]], which forms a molecule known as [[VLDL]] that is a constitute of [[blood serum]]. It is thought that the phosphorylation of these molecules is important to help aid in their sorting of [[secretion]] into the blood serum.<ref>Swift, L.L., Role of the Golgi Apparatus in the Phosphorylation of Apolipoprotein B. Journal of Biological Chemistry, 1996. 271(49): p. 31491-31495.</ref> |
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Vesicles which leave the rough endoplasmic reticulum are [[TRAPP complex|transported]] to the ''cis'' face of the Golgi apparatus, where they fuse with the Golgi membrane and empty their contents into the [[Lumen (anatomy)|lumen]]. Once inside they are modified, sorted, and shipped towards their final destination. As such, the Golgi apparatus tends to be more prominent and numerous in cells synthesising and secreting many substances: [[plasma B cell]]s, the [[antibody]]-secreting cells of the immune system, have prominent Golgi complexes. |
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Those proteins destined for areas of the cell other than either the [[endoplasmic reticulum]] or Golgi apparatus are moved towards the ''trans'' face, to a complex network of membranes and associated vesicles known as the ''trans-Golgi network'' (TGN).<ref name="lodish">{{cite book | title=Molecular Cell Biology| edition=5th edn |last=Lodish| coauthors=et al.| date=2004| publisher=W.H. Freeman and Company| id=0-7167-4366-3}}</ref> This area of the Golgi is the point at which proteins are sorted and shipped to their intended destinations by their placement into one of at least three different types of vesicles, depending upon the molecular marker they carry:<ref name="lodish">{{cite book | title=Molecular Cell Biology| edition=5th edn |last=Lodish| coauthors=et al.| date=2004| publisher=W.H. Freeman and Company| id=0-7167-4366-3}}</ref> |
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{| class="wikitable" |
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| width="140" | '''Type''' || '''Description''' || '''Example''' |
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| '''Exocytotic vesicles''' ''(continuous)|| Vesicle contains proteins destined for extracellular release. After packaging the vesicles bud off and immediately move towards the [[plasma membrane]], where they fuse and release the contents into the extracellular space in a process known as ''[[Secretory pathway|constitutive secretion]]''. || [[Antibody]] release by activated [[plasma B cell]]s |
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| '''Secretory vesicles''' ''(regulated)''|| Vesicle contains proteins destined for extracellular release. After packaging the vesicles bud off and are stored in the cell until a signal is given for their release. When the appropriate signal is received they move towards the membrane and fuse to release their contents. This process is known as ''[[Secretory pathway|regulated secretion]]''. || [[Neurotransmitter]] release from [[neuron]]s |
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| '''Lysosomal vesicles''' || Vesicle contains proteins destined for the [[lysosome]], an organelle of degredation containing many acid [[hydrolase]]s, or to lysosome-like storage organelles. These proteins include both digestive enzymes and membrane proteins. The vesicle first fuses with the [[endosome|late endosome]], and the contents are then transferred to the lysosome via unknown mechanisms. || Digestive [[protease]]s destined for the [[lysosome]] |
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Revision as of 19:32, 6 March 2007
The Golgi apparatus (also called the Golgi body, Golgi complex, or dictyosome) is an organelle found in typical eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi and was named after him. The primary function of the Golgi apparatus is to process and package macromolecules synthesised by the cell, primarily proteins and lipids. The Golgi apparatus forms a part of the endomembrane system present in eukaryotic cells.
]]s destined for the lysosome |}
Transport mechanism
The transport mechanism which proteins use to progress through the Golgi apparatus is not yet clear; however a number of hypotheses currently exist. Until recently, the vesicular transport mechanism was favoured but now more evidence is coming to light to support cisternal maturation. The two proposed models may actually work in conjunction with each other, rather than being mutually exclusive.[1]
- Cisternal maturation model: the cisternae of the Golgi apparatus move by being built at the cis face and destroyed at the trans face. Vesicles from the endoplasmic reticulum fuse with each other to form a cisterna at the cis face, consequently this cisterna would appear to move through the Golgi stack when a new cisterna is formed at the cis face. This model is supported by the fact that structures larger than the transport vesicles, such as collagen rods, were observed microscopically to progress through the Golgi apparatus.[1] This was initially a popular hypothesis, but lost favour in the 1980s. Recently it has made a comeback, as laboratories at the University of Chicago and the University of Tokyo have been able to use new technology to directly observe Golgi compartments maturing.[2]. Additional evidence comes from the fact that COP1 vesicles move in the retrograde direction,. transporting ER proteins back to where they belong by recognizing a signal peptide.[3]
- Vesicular transport model: Vesicular transport views the Golgi as a very stable organelle, divided into compartments is the cis to trans direction. Membrane bound carriers transported material between the ER and Golgi and the different compartments of the Golgi.[4] Experimental evidence inlcudes the abundance of small vesicles (known technically as shuttle vesicles) in proximity to the Golgi apparatus. Directionality is achieved by packaging proteins into either forward-moving or backward-moving (retrograde) transport vesicles, or alternatively this directionality may not be necessary as the constant input of proteins from the endoplasmic reticulum on the cis face of the Golgi would ensure flow. Irrespectively, it is likely that the transport vesicles are connected to a membrane via actin filaments to ensure that they fuse with the correct compartment.[1]
It has also been proposed that the cisternae are interconnected, and the transport of cargo molecules within the Golgi is due to diffusion, while the localisation of Golgi-resident proteins is achieved by an unknown mechanism.[citation needed]
References
- ^ a b c Alberts, Bruce. Molecular Biology of the Cell. Garland Publishing.
{{cite book}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Glick, B.S. and Malhotra, V. (1998). "The curious status of the Golgi apparatus". Cell. 95: 883–889.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Pelham, H.R.B. and J.E. Rothman, The Debate about Transport in the Golgi - Two Sides of the Same Coin? Cell, 2000. 102: p. 713-719.
- ^ Glick, B.S., Organisation of the Golgi apparatus. Current Opinion in Cell Biology, 2000. 12: p. 450-456.
General references