The Golgi apparatus (also called the Golgi body, Golgi complex, or dictyosome) is an organelle found in most eukaryotic cells. In Cell biology, an organelle (pronunciation /ɔː(rgəˡnɛl/ is a specialized subunit within a cell that has a specific function and is usually separately enclosed Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called It was identified in 1898 by the Italian physician Camillo Golgi and was named after him. Year 1898 ( MDCCCXCVIII) was a Common year starting on Saturday (link will display the full calendar of the Gregorian calendar (or a Common Camillo Golgi ( July 7, 1843 &ndash January 21, 1926) was an Italian Physician and Scientist. The primary function of the Golgi apparatus is to process and package the macromolecules such as proteins and lipids that are synthesized by the cell. The term macromolecule by definition implies "large Molecule " Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Lipids are broadly defined as any fat- Soluble ( lipophilic) naturally-occurring Molecule, such as fats oils waxes cholesterol sterols fat-soluble It is particularly important in the processing of proteins for secretion. Secretion is the process of segregating elaborating and releasing chemicals from a cell, or a secreted Chemical substance or amount of substance The Golgi apparatus forms a part of the endomembrane system of eukaryotic cells. The endomembrane system is composed of the different membranes that are suspended in the Cytoplasm within a Eukaryotic cell.
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Structure
The Golgi is composed of membrane-bound stacks known as cisternae. A cisterna (plural cisternae) comprises a flattened membrane disk that makes up the Golgi apparatus. Between five and eight are usually present; however, as many as sixty have been observed. [1]
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. A vesicular-tubular cluster (VTC also referred to as the ER-Golgi intermediate compartment (or ERGIC is an Organelle in eukaryotic cells Each region contains different enzymes which selectively modify the contents depending on where they are destined to reside. [2]
The trans face of the trans-Golgi network is the face from which vesicles leave the Golgi. These vesicles then proceed to later compartments such as the cell surface (or plasma membrane), secretory vesicles or late endosomes. The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer Secretion is the process of segregating elaborating and releasing chemicals from a cell, or a secreted Chemical substance or amount of substance
A cisterna (plural cisternae) comprises a flattened membrane disk that makes up the Golgi apparatus. A typical Golgi has anywhere from 3 to 7 cisternae stacked upon each other like a stack of dinner plates, but there are usually around 6. The cisternae carry Golgi enzymes to help or to modify cargo proteins traveling through them destined for other parts of the cell.
The cisternae also carry structural proteins important for its maintenance as a flattened membrane and its stacking upon each other.
The earliest cisternae are called the cis-cisternae, followed by the medial cisternae, then the trans-cisternae (as they move away from the endoplasmic reticulum). The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle
The formation of new cisternae is often called the cis-Golgi network and at the end of the Golgi where transport to other parts of the cell occurs is called the trans-Golgi network. Both are thought to be specialized cisternae leading in and out of the Golgi apparatus.
Cisternae may also refer to flattened regions of the rough endoplasmic reticulum. The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle
Function
Cells synthesize 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 lipids around the cell, and the creation of lysosomes. The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle Lipids are broadly defined as any fat- Soluble ( lipophilic) naturally-occurring Molecule, such as fats oils waxes cholesterol sterols fat-soluble Lysosomes are Organelles that contain Digestive enzymes (acid Hydrolases. 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.
Enzymes within the cisternae are able to modify substances by the addition of carbohydrates (glycosylation) and phosphates (phosphorylation). A cisterna (plural cisternae) comprises a flattened membrane disk that makes up the Golgi apparatus. Glycosylation is the enzymatic process that links Saccharides to produce glycans, either free or attached to Proteins and Lipids This enzymatic Phosphorylation is the addition of a Phosphate (PO4 group to a Protein molecule or a small molecule 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 signal sequence of molecules which determine their final destination. This article deals with protein targeting in Eukaryotes except where noted For example, the Golgi apparatus adds a mannose-6-phosphate label to proteins destined for lysosomes. Mannose is a Sugar Monomer of the Hexose series of Carbohydrates Metabolism Mannose enters the carbohydrate Metabolism Lysosomes are Organelles that contain Digestive enzymes (acid Hydrolases.
The Golgi also plays an important role in the synthesis of proteoglycans, molecules present in the extracellular matrix of animals, and it is a major site of carbohydrate synthesis. Proteoglycans represent a special class of Glycoproteins that are heavily glycosylated. In Biology, the extracellular matrix ( ECM) is the Extracellular part of animal tissue that usually provides structural support to the cells Carbohydrates (from ' Hydrates of Carbon ' or saccharides ( Greek σάκχαρον meaning " Sugar " are the most [3]
This includes the productions of glycosaminoglycans or GAGs, long unbranched polysaccharides which the Golgi then attaches to a protein synthesized in the endoplasmic reticulum to form the proteoglycan. Glycosaminoglycans (GAGs or mucopolysaccharides are long unbranched Polysaccharides consisting of a repeating Disaccharide unit Polysaccharides are relatively complex Carbohydrates They are Polymers made up of many Monosaccharides joined together by Glycosidic bonds Proteoglycans represent a special class of Glycoproteins that are heavily glycosylated. [4]Enzymes in the Golgi will polymerize several of these GAGs via a xylose link onto the core protein. In Polymer chemistry, polymerization is a process of reacting Monomer Molecules together in a Chemical reaction to form three-dimensional networks Xylose or wood sugar is an Aldopentose &mdash a Monosaccharide containing five Carbon Atoms and including an Aldehyde Functional Another task of the Golgi involves the sulfation of certain molecules passing through its lumen via sulphotranferases that gain their sulphur molecule from a donor called PAPs. Sulfation refers to the process whereby a Lead-acid battery (such as a Car battery) loses its ability to hold a charge after it is kept in a discharged state too long 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' signalling abilities as well as giving the proteoglycan its overall negative charge. [3]
The Golgi is also capable of phosphorylating molecules. Phosphorylation is the addition of a Phosphate (PO4 group to a Protein molecule or a small molecule To do so it transports ATP into the lumen. Adenosine-5'-triphosphate ( ATP) is a multifunctional Nucleotide that is most important as a " molecular currency" of intracellular Energy A lumen (Lat lūmen, an opening or light (pl lumina is the inside space or lining of a tubular structure such as an artery or intestine [5] The Golgi itself contains resident kinases, such as casein kinase 1 and casein kinase 2. In Chemistry and Biochemistry, a kinase, alternatively known as a phosphotransferase, is a type of Enzyme that transfers Phosphate The Casein kinase 1 family ( of Protein kinases are Serine / Threonine -selective enzymes that function as regulators of Signal transduction pathways The Casein kinase 2 ( is a Serine / Threonine -selective Protein kinase that is a tetramer of two alpha subunits and two beta subunits One molecule that is phosphorylated in the Golgi is Apolipoprotein, which forms a molecule known as VLDL that is a constitute of blood serum. Blood plasma is the Liquid component of Blood, in which the Blood cells are suspended It is thought that the phosphorylation of these molecules is important to help aid in their sorting for secretion into the blood serum. Secretion is the process of segregating elaborating and releasing chemicals from a cell, or a secreted Chemical substance or amount of substance [6]
The Golgi also has a putative role in apoptosis, with several Bcl-2 family members localised there, as well as to the mitochondria. In addition a newly characterised anti-apoptotic protein, GAAP (Golgi anti-apoptotic protein), which almost exclusively resides in the Golgi, protects cells from apoptosis by an as-yet undefined mechanism (Gubser et al. , 2007).
Vesicular transport
The vesicles that leave the rough endoplasmic reticulum are transported to the cis face of the Golgi apparatus, where they fuse with the Golgi membrane and empty their contents into the lumen. TRAPP (TRAnsport Protein Particle is a Protein involved in particle transport between Organelles Protein folding and the Endoplasmic Reticulum (ER A lumen (Lat lūmen, an opening or light (pl lumina is the inside space or lining of a tubular structure such as an artery or intestine 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 cells, the antibody-secreting cells of the immune system, have prominent Golgi complexes. Plasma cells (also called plasma B cells or plasmocytes) are cells of the Immune system that secrete large amounts of antibodies. Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily
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). The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle [2] 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:[2]
| Type | Description | Example |
| 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 constitutive secretion. The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer The secretory pathway is a series of steps a cell uses to move Proteins out of the cell a process known as secretion. | Antibody release by activated plasma B cells |
| Secretory vesicles (regulated) | Vesicle contains proteins destined for extracellular release. Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily Plasma cells (also called plasma B cells or plasmocytes) are cells of the Immune system that secrete large amounts of antibodies. 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 regulated secretion. The secretory pathway is a series of steps a cell uses to move Proteins out of the cell a process known as secretion. | Neurotransmitter release from neurons |
| Lysosomal vesicles | Vesicle contains proteins destined for the lysosome, an organelle of degradation containing many acid hydrolases, or to lysosome-like storage organelles. See Chemical synapse for an introduction to concepts and terminology used in this article Neurons (ˈnjuːɹɒn also known as neurones and nerve cells) are responsive cells in the Nervous system that process and transmit information Lysosomes are Organelles that contain Digestive enzymes (acid Hydrolases. In Biochemistry, a hydrolase is an Enzyme that catalyzes the Hydrolysis of a Chemical bond. These proteins include both digestive enzymes and membrane proteins. The vesicle first fuses with the late endosome, and the contents are then transferred to the lysosome via unknown mechanisms. In Biology, an endosome is a membrane-bound compartment inside cells roughly 300-400 nm in diameter when fully mature | Digestive proteases 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. A protease is any Enzyme that conducts Proteolysis, that is begins protein Catabolism by Hydrolysis of the Peptide bonds that link Lysosomes are Organelles that contain Digestive enzymes (acid Hydrolases. TRAPP (TRAnsport Protein Particle is a Protein involved in particle transport between Organelles Protein folding and the Endoplasmic Reticulum (ER 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. This is sometimes referred to as the combined model. [3]
- 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. Collagen is the main Protein of Connective tissue in Animals and the most abundant protein in Mammals making up about 50% of the whole-body protein [3] 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. The University of Chicago is a Private university located principally in the Hyde Park neighborhood of Chicago. The, abbreviated as, is a major Research university located in Tokyo, Japan. [7] Additional evidence comes from the fact that COPI vesicles move in the retrograde direction, transporting ER proteins back to where they belong by recognizing a signal peptide. COPI is a protein that coats vesicles that transports proteins from the cis end of the Golgi complex to the rough Endoplasmic reticulum (ER [8]
- Vesicular transport model: Vesicular transport views the Golgi as a very stable organelle, divided into compartments is the cis to trans direction. In Cell biology, an organelle (pronunciation /ɔː(rgəˡnɛl/ is a specialized subunit within a cell that has a specific function and is usually separately enclosed Membrane bound carriers transported material between the ER and Golgi and the different compartments of the Golgi. [9] Experimental evidence includes the abundance of small vesicles (known technically as shuttle vesicles) in proximity to the Golgi apparatus. Directionality is achieved by packaging proteins are connected to a membrane via actin filaments to ensure that they fuse with the correct compartment. Actin is a globular roughly 42-kDa Protein found in all eukaryotic cells (except for Nematode sperm where it may be present at concentrations of [3]
Links
References
- ^ Molecular Expressions Cell Biology: The Golgi Apparatus. Retrieved on 2006-11-08. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1519 - Hernán Cortés enters Tenochtitlán and Aztec ruler Moctezuma welcomes him with great a Celebration
- ^ a b c Lodish; et al. (2004). Molecular Cell Biology, 5th edn, W. H. Freeman and Company. P0-7167-4366-3.
- ^ a b c d e Alberts, Bruce; et al. . Molecular Biology of the Cell. Garland Publishing.
- ^ Pyrdz, K. and K. T. Dalan, Synthesis and Sorting of Proteoglycans. Journal of Cell Science, 2000. 113: p. 193-205.
- ^ 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.
- ^ Swift, L. L. , Role of the Golgi Apparatus in the Phosphorylation of Apolipoprotein B. Journal of Biological Chemistry, 1996. 271(49): p. 31491-31495.
- ^ Glick, B. S. and Malhotra, V. (1998). "The curious status of the Golgi apparatus". Cell 95: 883-889.
- ^ 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.
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