79-568: GGT may refer to: Biology and medicine [ edit ] Gamma-glutamyltransferase , an enzyme that catalyzes the reaction between a peptide and an amino acid Glutathione hydrolase , an enzyme that hydrolyzes glutathione A codon for the amino acid Glycine Germline gene therapy , to treat genetic diseases Language [ edit ] Gitua language , spoken on New Guinea Transport [ edit ] Exuma International Airport , The Bahamas (IATA: GGT ) Golden Gate Transit ,
158-410: A peptide or water (forming glutamate ). GGT plays a key role in the gamma-glutamyl cycle , a pathway for the synthesis and degradation of glutathione as well as drug and xenobiotic detoxification. Other lines of evidence indicate that GGT can also exert a pro-oxidant role, with regulatory effects at various levels in cellular signal transduction and cellular pathophysiology. This transferase
237-419: A fraction of the lipid in direct contact with integral membrane proteins, which is tightly bound to the protein surface is called annular lipid shell ; it behaves as a part of protein complex. Cholesterol is normally found dispersed in varying degrees throughout cell membranes, in the irregular spaces between the hydrophobic tails of the membrane lipids, where it confers a stiffening and strengthening effect on
316-495: A host target cell, and thus such blebs may work as virulence organelles. Bacterial cells provide numerous examples of the diverse ways in which prokaryotic cell membranes are adapted with structures that suit the organism's niche. For example, proteins on the surface of certain bacterial cells aid in their gliding motion. Many gram-negative bacteria have cell membranes which contain ATP-driven protein exporting systems. According to
395-444: A large quantity of proteins, which provide more structure. Examples of such structures are protein-protein complexes, pickets and fences formed by the actin-based cytoskeleton , and potentially lipid rafts . Lipid bilayers form through the process of self-assembly . The cell membrane consists primarily of a thin layer of amphipathic phospholipids that spontaneously arrange so that the hydrophobic "tail" regions are isolated from
474-479: A large variety of protein receptors and identification proteins, such as antigens , are present on the surface of the membrane. Functions of membrane proteins can also include cell–cell contact, surface recognition, cytoskeleton contact, signaling, enzymatic activity, or transporting substances across the membrane. Most membrane proteins must be inserted in some way into the membrane. For this to occur, an N-terminus "signal sequence" of amino acids directs proteins to
553-405: A limited variety of chemical substances, often limited to a single substance. Another example of a transmembrane protein is a cell-surface receptor, which allow cell signaling molecules to communicate between cells. 3. Endocytosis : Endocytosis is the process in which cells absorb molecules by engulfing them. The plasma membrane creates a small deformation inward, called an invagination, in which
632-452: A lipid bilayer. In 1925 it was determined by Fricke that the thickness of erythrocyte and yeast cell membranes ranged between 3.3 and 4 nm, a thickness compatible with a lipid monolayer. The choice of the dielectric constant used in these studies was called into question but future tests could not disprove the results of the initial experiment. Independently, the leptoscope was invented in order to measure very thin membranes by comparing
711-471: A membrane is the rate of passive diffusion of molecules through the membrane. These molecules are known as permeant molecules. Permeability depends mainly on the electric charge and polarity of the molecule and to a lesser extent the molar mass of the molecule. Due to the cell membrane's hydrophobic nature, small electrically neutral molecules pass through the membrane more easily than charged, large ones. The inability of charged molecules to pass through
790-427: A minute amount of about 2% and sterols make up the rest. In red blood cell studies, 30% of the plasma membrane is lipid. However, for the majority of eukaryotic cells, the composition of plasma membranes is about half lipids and half proteins by weight. The fatty chains in phospholipids and glycolipids usually contain an even number of carbon atoms, typically between 16 and 20. The 16- and 18-carbon fatty acids are
869-402: A plasma membrane and an outer membrane separated by periplasm ; however, other prokaryotes have only a plasma membrane. These two membranes differ in many aspects. The outer membrane of the gram-negative bacteria differs from other prokaryotes due to phospholipids forming the exterior of the bilayer, and lipoproteins and phospholipids forming the interior. The outer membrane typically has
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#1733086199505948-438: A polarized cell is the surface of the plasma membrane that forms its basal and lateral surfaces. It faces outwards, towards the interstitium , and away from the lumen. Basolateral membrane is a compound phrase referring to the terms "basal (base) membrane" and "lateral (side) membrane", which, especially in epithelial cells, are identical in composition and activity. Proteins (such as ion channels and pumps ) are free to move from
1027-403: A porous quality due to its presence of membrane proteins, such as gram-negative porins , which are pore-forming proteins. The inner plasma membrane is also generally symmetric whereas the outer membrane is asymmetric because of proteins such as the aforementioned. Also, for the prokaryotic membranes, there are multiple things that can affect the fluidity. One of the major factors that can affect
1106-644: A public transport system in California, U.S. See also [ edit ] Galactolipid galactosyltransferase (GGGT), an enzyme in glycerolipid metabolism Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title GGT . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=GGT&oldid=1145672324 " Category : Disambiguation pages Hidden categories: Short description
1185-494: A significant role in the proper autocatalytic cleavage and proper folding of GGT. Single site mutations at asparagine residues were shown to result in a functionally active yet slightly less thermally stable version of the enzyme in vitro, while knockout of all asparagine residues resulted in an accumulation of the uncleaved, propeptide form of the enzyme. GGT is predominantly used as a diagnostic marker for liver disease . Elevated serum GGT activity can be found in diseases of
1264-453: A universal mechanism for cell protection and development. By the second half of the 19th century, microscopy was still not advanced enough to make a distinction between cell membranes and cell walls. However, some microscopists correctly identified at this time that while invisible, it could be inferred that cell membranes existed in animal cells due to intracellular movement of components internally but not externally and that membranes were not
1343-474: A variety of cellular processes such as cell adhesion , ion conductivity , and cell signalling and serve as the attachment surface for several extracellular structures, including the cell wall and the carbohydrate layer called the glycocalyx , as well as the intracellular network of protein fibers called the cytoskeleton . In the field of synthetic biology, cell membranes can be artificially reassembled . Robert Hooke 's discovery of cells in 1665 led to
1422-430: Is a pathway for internalizing solid particles ("cell eating" or phagocytosis ), small molecules and ions ("cell drinking" or pinocytosis ), and macromolecules. Endocytosis requires energy and is thus a form of active transport. 4. Exocytosis : Just as material can be brought into the cell by invagination and formation of a vesicle, the membrane of a vesicle can be fused with the plasma membrane, extruding its contents to
1501-424: Is a single polypeptide chain that crosses the lipid bilayer seven times responding to signal molecules (i.e. hormones and neurotransmitters). G-protein coupled receptors are used in processes such as cell to cell signaling, the regulation of the production of cAMP, and the regulation of ion channels. The cell membrane, being exposed to the outside environment, is an important site of cell–cell communication. As such,
1580-585: Is an important feature in all cells, especially epithelia with microvilli. Recent data suggest the glycocalyx participates in cell adhesion, lymphocyte homing , and many others. The penultimate sugar is galactose and the terminal sugar is sialic acid , as the sugar backbone is modified in the Golgi apparatus . Sialic acid carries a negative charge, providing an external barrier to charged particles. The cell membrane has large content of proteins, typically around 50% of membrane volume These proteins are important for
1659-455: Is different from Wikidata All article disambiguation pages All disambiguation pages Gamma-glutamyltransferase Gamma-glutamyltransferase (also γ-glutamyltransferase , GGT , gamma-GT , gamma-glutamyl transpeptidase ; EC 2.3.2.2 ) is a transferase (a type of enzyme ) that catalyzes the transfer of gamma- glutamyl functional groups from molecules such as glutathione to an acceptor that may be an amino acid ,
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#17330861995051738-469: Is expressed in high levels in many different tumors. It is known to accelerate tumor growth and to increase resistance to cisplatin in tumors. Human proteins that belong to this family include GGT1 , GGT2 , GGT6 , GGTL3 , GGTL4 , GGTLA1 and GGTLA4 . (See Template:Leucine metabolism in humans – this diagram does not include the pathway for β-leucine synthesis via leucine 2,3-aminomutase) Cell membrane The cell membrane (also known as
1817-531: Is first moved by cytoskeleton from the interior of the cell to the surface. The vesicle membrane comes in contact with the plasma membrane. The lipid molecules of the two bilayers rearrange themselves and the two membranes are, thus, fused. A passage is formed in the fused membrane and the vesicles discharges its contents outside the cell. Prokaryotes are divided into two different groups, Archaea and Bacteria , with bacteria dividing further into gram-positive and gram-negative . Gram-negative bacteria have both
1896-630: Is found in many tissues, the most notable one being the liver , and has significance in medicine as a diagnostic marker. The name γ-glutamyltransferase is preferred by the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology . The Expert Panel on Enzymes of the International Federation of Clinical Chemistry also used this name. The older name is gamma-glutamyl transpeptidase (GGTP). GGT
1975-462: Is found underlying the cell membrane in the cytoplasm and provides a scaffolding for membrane proteins to anchor to, as well as forming organelles that extend from the cell. Indeed, cytoskeletal elements interact extensively and intimately with the cell membrane. Anchoring proteins restricts them to a particular cell surface — for example, the apical surface of epithelial cells that line the vertebrate gut — and limits how far they may diffuse within
2054-414: Is incorporated into the membrane, or deleted from it, by a variety of mechanisms: The cell membrane consists of three classes of amphipathic lipids: phospholipids , glycolipids , and sterols . The amount of each depends upon the type of cell, but in the majority of cases phospholipids are the most abundant, often contributing for over 50% of all lipids in plasma membranes. Glycolipids only account for
2133-549: Is not. GGT is elevated by ingestion of large quantities of alcohol (needs reference) However, determination of high levels of total serum GGT activity is not specific to alcohol intoxication, and the measurement of selected serum forms of the enzyme offer more specific information. Isolated elevation or disproportionate elevation compared to other liver enzymes (such as ALT or alanine transaminase ) can indicate harmful alcohol use or alcoholic liver disease , and can indicate excess alcohol consumption up to 3 or 4 weeks prior to
2212-465: Is present in the cell membranes of many tissues, including the kidneys , bile duct , pancreas , gallbladder , spleen , heart , brain , and seminal vesicles . It is involved in the transfer of amino acids across the cellular membrane and leukotriene metabolism. It is also involved in glutathione metabolism by transferring the glutamyl moiety to a variety of acceptor molecules including water, certain L -amino acids, and peptides, leaving
2291-426: Is similar to alkaline phosphatase (ALP) in detecting disease of the biliary tract . Indeed, the two markers correlate well, though there are conflicting data about whether GGT has better sensitivity . In general, ALP is still the first test for biliary disease . The main value of GGT is in verifying that ALP elevations are, in fact, due to biliary disease; ALP can also be increased in certain bone diseases, but GGT
2370-477: Is under active investigation as a cardiovascular risk marker. GGT in fact accumulates in atherosclerotic plaques , suggesting a potential role in pathogenesis of cardiovascular diseases, and circulates in blood in the form of distinct protein aggregates, some of which appear to be related to specific pathologies such as metabolic syndrome , alcohol addiction and chronic liver disease . Elevated levels of GGT can also be due to congestive heart failure . GGT
2449-403: The cysteine product to preserve intracellular homeostasis of oxidative stress . This general reaction is: In prokaryotes and eukaryotes, GGT consists of two polypeptide chains, a heavy and a light subunit, processed from a single chain precursor by an autocatalytic cleavage. The active site of GGT is known to be located in the light subunit. Co-translational N -glycosylation serves
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2528-414: The cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix and other cells to hold them together to form tissues . Fungi , bacteria , most archaea , and plants also have a cell wall , which provides a mechanical support to the cell and precludes the passage of larger molecules . The cell membrane is selectively permeable and able to regulate what enters and exits
2607-418: The endoplasmic reticulum , which inserts the proteins into a lipid bilayer. Once inserted, the proteins are then transported to their final destination in vesicles, where the vesicle fuses with the target membrane. The cell membrane surrounds the cytoplasm of living cells, physically separating the intracellular components from the extracellular environment. The cell membrane also plays a role in anchoring
2686-419: The fluid mosaic model of S. J. Singer and G. L. Nicolson (1972), which replaced the earlier model of Davson and Danielli , biological membranes can be considered as a two-dimensional liquid in which lipid and protein molecules diffuse more or less easily. Although the lipid bilayers that form the basis of the membranes do indeed form two-dimensional liquids by themselves, the plasma membrane also contains
2765-404: The liquid crystalline state . It means the lipid molecules are free to diffuse and exhibit rapid lateral diffusion along the layer in which they are present. However, the exchange of phospholipid molecules between intracellular and extracellular leaflets of the bilayer is a very slow process. Lipid rafts and caveolae are examples of cholesterol -enriched microdomains in the cell membrane. Also,
2844-410: The paucimolecular model of Davson and Danielli (1935). This model was based on studies of surface tension between oils and echinoderm eggs. Since the surface tension values appeared to be much lower than would be expected for an oil–water interface, it was assumed that some substance was responsible for lowering the interfacial tensions in the surface of cells. It was suggested that a lipid bilayer
2923-556: The plasma membrane or cytoplasmic membrane , and historically referred to as the plasmalemma ) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extracellular space). The cell membrane consists of a lipid bilayer , made up of two layers of phospholipids with cholesterols (a lipid component) interspersed between them, maintaining appropriate membrane fluidity at various temperatures. The membrane also contains membrane proteins , including integral proteins that span
3002-415: The 1970s. Although the fluid mosaic model has been modernized to detail contemporary discoveries, the basics have remained constant: the membrane is a lipid bilayer composed of hydrophilic exterior heads and a hydrophobic interior where proteins can interact with hydrophilic heads through polar interactions, but proteins that span the bilayer fully or partially have hydrophobic amino acids that interact with
3081-637: The absorption rate of nutrients. Localized decoupling of the cytoskeleton and cell membrane results in formation of a bleb . The content of the cell, inside the cell membrane, is composed of numerous membrane-bound organelles , which contribute to the overall function of the cell. The origin, structure, and function of each organelle leads to a large variation in the cell composition due to the individual uniqueness associated with each organelle. The cell membrane has different lipid and protein compositions in distinct types of cells and may have therefore specific names for certain cell types. The permeability of
3160-863: The basal to the lateral surface of the cell or vice versa in accordance with the fluid mosaic model . Tight junctions join epithelial cells near their apical surface to prevent the migration of proteins from the basolateral membrane to the apical membrane. The basal and lateral surfaces thus remain roughly equivalent to one another, yet distinct from the apical surface. Cell membrane can form different types of "supramembrane" structures such as caveolae , postsynaptic density , podosomes , invadopodia , focal adhesion , and different types of cell junctions . These structures are usually responsible for cell adhesion , communication, endocytosis and exocytosis . They can be visualized by electron microscopy or fluorescence microscopy . They are composed of specific proteins, such as integrins and cadherins . The cytoskeleton
3239-564: The bilayer. The cytoskeleton is able to form appendage-like organelles, such as cilia , which are microtubule -based extensions covered by the cell membrane, and filopodia , which are actin -based extensions. These extensions are ensheathed in membrane and project from the surface of the cell in order to sense the external environment and/or make contact with the substrate or other cells. The apical surfaces of epithelial cells are dense with actin-based finger-like projections known as microvilli , which increase cell surface area and thereby increase
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3318-656: The cell because they are responsible for various biological activities. Approximately a third of the genes in yeast code specifically for them, and this number is even higher in multicellular organisms. Membrane proteins consist of three main types: integral proteins, peripheral proteins, and lipid-anchored proteins. As shown in the adjacent table, integral proteins are amphipathic transmembrane proteins. Examples of integral proteins include ion channels, proton pumps, and g-protein coupled receptors. Ion channels allow inorganic ions such as sodium, potassium, calcium, or chlorine to diffuse down their electrochemical gradient across
3397-442: The cell, as well as getting more insight into cell membrane permeability. Lipid vesicles and liposomes are formed by first suspending a lipid in an aqueous solution then agitating the mixture through sonication , resulting in a vesicle. Measuring the rate of efflux from the inside of the vesicle to the ambient solution allows researchers to better understand membrane permeability. Vesicles can be formed with molecules and ions inside
3476-463: The cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be achieved by either passive transport , occurring without the input of cellular energy, or by active transport , requiring the cell to expend energy in transporting it. The membrane also maintains the cell potential . The cell membrane thus works as a selective filter that allows only certain things to come inside or go outside
3555-433: The cell. The cell employs a number of transport mechanisms that involve biological membranes: 1. Passive osmosis and diffusion : Some substances (small molecules, ions) such as carbon dioxide (CO 2 ) and oxygen (O 2 ), can move across the plasma membrane by diffusion, which is a passive transport process. Because the membrane acts as a barrier for certain molecules and ions, they can occur in different concentrations on
3634-465: The description of the cell membrane bilayer structure based on crystallographic studies and soap bubble observations. In an attempt to accept or reject the hypothesis, researchers measured membrane thickness. These researchers extracted the lipid from human red blood cells and measured the amount of surface area the lipid would cover when spread over the surface of the water. Since mature mammalian red blood cells lack both nuclei and cytoplasmic organelles,
3713-417: The ectoplast ( de Vries , 1885), Plasmahaut (plasma skin, Pfeffer , 1877, 1891), Hautschicht (skin layer, Pfeffer, 1886; used with a different meaning by Hofmeister , 1867), plasmatic membrane (Pfeffer, 1900), plasma membrane, cytoplasmic membrane, cell envelope and cell membrane. Some authors who did not believe that there was a functional permeable boundary at the surface of the cell preferred to use
3792-412: The entropy of the system. This complex interaction can include noncovalent interactions such as van der Waals , electrostatic and hydrogen bonds. Lipid bilayers are generally impermeable to ions and polar molecules. The arrangement of hydrophilic heads and hydrophobic tails of the lipid bilayer prevent polar solutes (ex. amino acids, nucleic acids, carbohydrates, proteins, and ions) from diffusing across
3871-603: The equivalent of a plant cell wall . It was also inferred that cell membranes were not vital components to all cells. Many refuted the existence of a cell membrane still towards the end of the 19th century. In 1890, a revision to the cell theory stated that cell membranes existed, but were merely secondary structures. It was not until later studies with osmosis and permeability that cell membranes gained more recognition. In 1895, Ernest Overton proposed that cell membranes were made of lipids. The lipid bilayer hypothesis, proposed in 1925 by Gorter and Grendel, created speculation in
3950-478: The fluidity is fatty acid composition. For example, when the bacteria Staphylococcus aureus was grown in 37 C for 24h, the membrane exhibited a more fluid state instead of a gel-like state. This supports the concept that in higher temperatures, the membrane is more fluid than in colder temperatures. When the membrane is becoming more fluid and needs to become more stabilized, it will make longer fatty acid chains or saturated fatty acid chains in order to help stabilize
4029-454: The fluidity of the membrane. Cholesterol is more abundant in cold-weather animals than warm-weather animals. In plants, which lack cholesterol, related compounds called sterols perform the same function as cholesterol. Lipid vesicles or liposomes are approximately spherical pockets that are enclosed by a lipid bilayer. These structures are used in laboratories to study the effects of chemicals in cells by delivering these chemicals directly to
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#17330861995054108-411: The intensity of light reflected from a sample to the intensity of a membrane standard of known thickness. The instrument could resolve thicknesses that depended on pH measurements and the presence of membrane proteins that ranged from 8.6 to 23.2 nm, with the lower measurements supporting the lipid bilayer hypothesis. Later in the 1930s, the membrane structure model developed in general agreement to be
4187-527: The lipid bilayer of the membranes; they function on both sides of the membrane to transport molecules across it. Nutrients, such as sugars or amino acids, must enter the cell, and certain products of metabolism must leave the cell. Such molecules can diffuse passively through protein channels such as aquaporins in facilitated diffusion or are pumped across the membrane by transmembrane transporters . Protein channel proteins, also called permeases , are usually quite specific, and they only recognize and transport
4266-431: The lipid bilayer through hydrophilic pores across the membrane. The electrical behavior of cells (i.e. nerve cells) is controlled by ion channels. Proton pumps are protein pumps that are embedded in the lipid bilayer that allow protons to travel through the membrane by transferring from one amino acid side chain to another. Processes such as electron transport and generating ATP use proton pumps. A G-protein coupled receptor
4345-460: The liver, biliary system, pancreas and kidneys. Latent elevations in GGT are typically seen in patients with chronic viral hepatitis infections often taking 12 months or more to present. Individual test results should always be interpreted using the reference range from the laboratory that performed the test, though example reference ranges are 15–85 IU/L for men, and 5–55 IU/L for women. GGT
4424-488: The membrane and serve as membrane transporters , and peripheral proteins that loosely attach to the outer (peripheral) side of the cell membrane, acting as enzymes to facilitate interaction with the cell's environment. Glycolipids embedded in the outer lipid layer serve a similar purpose. The cell membrane controls the movement of substances in and out of a cell, being selectively permeable to ions and organic molecules. In addition, cell membranes are involved in
4503-444: The membrane, but generally allows for the passive diffusion of hydrophobic molecules. This affords the cell the ability to control the movement of these substances via transmembrane protein complexes such as pores, channels and gates. Flippases and scramblases concentrate phosphatidyl serine , which carries a negative charge, on the inner membrane. Along with NANA , this creates an extra barrier to charged moieties moving through
4582-539: The membrane. Bacteria are also surrounded by a cell wall composed of peptidoglycan (amino acids and sugars). Some eukaryotic cells also have cell walls, but none that are made of peptidoglycan. The outer membrane of gram negative bacteria is rich in lipopolysaccharides , which are combined poly- or oligosaccharide and carbohydrate lipid regions that stimulate the cell's natural immunity. The outer membrane can bleb out into periplasmic protrusions under stress conditions or upon virulence requirements while encountering
4661-407: The membrane. Membranes serve diverse functions in eukaryotic and prokaryotic cells. One important role is to regulate the movement of materials into and out of cells. The phospholipid bilayer structure (fluid mosaic model) with specific membrane proteins accounts for the selective permeability of the membrane and passive and active transport mechanisms. In addition, membranes in prokaryotes and in
4740-408: The membrane. The ability of some organisms to regulate the fluidity of their cell membranes by altering lipid composition is called homeoviscous adaptation . The entire membrane is held together via non-covalent interaction of hydrophobic tails, however the structure is quite fluid and not fixed rigidly in place. Under physiological conditions phospholipid molecules in the cell membrane are in
4819-417: The membrane. Additionally, the amount of cholesterol in biological membranes varies between organisms, cell types, and even in individual cells. Cholesterol, a major component of plasma membranes, regulates the fluidity of the overall membrane, meaning that cholesterol controls the amount of movement of the various cell membrane components based on its concentrations. In high temperatures, cholesterol inhibits
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#17330861995054898-436: The membranes were seen but mostly disregarded as an important structure with cellular function. It was not until the 20th century that the significance of the cell membrane as it was acknowledged. Finally, two scientists Gorter and Grendel (1925) made the discovery that the membrane is "lipid-based". From this, they furthered the idea that this structure would have to be in a formation that mimicked layers. Once studied further, it
4977-430: The mitochondria and chloroplasts of eukaryotes facilitate the synthesis of ATP through chemiosmosis. The apical membrane or luminal membrane of a polarized cell is the surface of the plasma membrane that faces inward to the lumen . This is particularly evident in epithelial and endothelial cells , but also describes other polarized cells, such as neurons . The basolateral membrane or basolateral cell membrane of
5056-401: The most common. Fatty acids may be saturated or unsaturated, with the configuration of the double bonds nearly always "cis". The length and the degree of unsaturation of fatty acid chains have a profound effect on membrane fluidity as unsaturated lipids create a kink, preventing the fatty acids from packing together as tightly, thus decreasing the melting temperature (increasing the fluidity) of
5135-435: The movement of phospholipid fatty acid chains, causing a reduced permeability to small molecules and reduced membrane fluidity. The opposite is true for the role of cholesterol in cooler temperatures. Cholesterol production, and thus concentration, is up-regulated (increased) in response to cold temperature. At cold temperatures, cholesterol interferes with fatty acid chain interactions. Acting as antifreeze, cholesterol maintains
5214-433: The non-polar lipid interior. The fluid mosaic model not only provided an accurate representation of membrane mechanics, it enhanced the study of hydrophobic forces, which would later develop into an essential descriptive limitation to describe biological macromolecules . For many centuries, the scientists cited disagreed with the significance of the structure they were seeing as the cell membrane. For almost two centuries,
5293-406: The plasma membrane is the only lipid-containing structure in the cell. Consequently, all of the lipids extracted from the cells can be assumed to have resided in the cells' plasma membranes. The ratio of the surface area of water covered by the extracted lipid to the surface area calculated for the red blood cells from which the lipid was 2:1(approx) and they concluded that the plasma membrane contains
5372-497: The proposal of the cell theory . Initially it was believed that all cells contained a hard cell wall since only plant cells could be observed at the time. Microscopists focused on the cell wall for well over 150 years until advances in microscopy were made. In the early 19th century, cells were recognized as being separate entities, unconnected, and bound by individual cell walls after it was found that plant cells could be separated. This theory extended to include animal cells to suggest
5451-401: The role of cell-cell recognition in eukaryotes; they are located on the surface of the cell where they recognize host cells and share information. Viruses that bind to cells using these receptors cause an infection. For the most part, no glycosylation occurs on membranes within the cell; rather generally glycosylation occurs on the extracellular surface of the plasma membrane. The glycocalyx
5530-422: The substance to be transported is captured. This invagination is caused by proteins on the outside on the cell membrane, acting as receptors and clustering into depressions that eventually promote accumulation of more proteins and lipids on the cytosolic side of the membrane. The deformation then pinches off from the membrane on the inside of the cell, creating a vesicle containing the captured substance. Endocytosis
5609-414: The surrounding medium. This is the process of exocytosis. Exocytosis occurs in various cells to remove undigested residues of substances brought in by endocytosis, to secrete substances such as hormones and enzymes, and to transport a substance completely across a cellular barrier. In the process of exocytosis, the undigested waste-containing food vacuole or the secretory vesicle budded from Golgi apparatus ,
5688-510: The surrounding water while the hydrophilic "head" regions interact with the intracellular (cytosolic) and extracellular faces of the resulting bilayer. This forms a continuous, spherical lipid bilayer . Hydrophobic interactions (also known as the hydrophobic effect ) are the major driving forces in the formation of lipid bilayers. An increase in interactions between hydrophobic molecules (causing clustering of hydrophobic regions) allows water molecules to bond more freely with each other, increasing
5767-507: The term plasmalemma (coined by Mast, 1924) for the external region of the cell. Cell membranes contain a variety of biological molecules , notably lipids and proteins. Composition is not set, but constantly changing for fluidity and changes in the environment, even fluctuating during different stages of cell development. Specifically, the amount of cholesterol in human primary neuron cell membrane changes, and this change in composition affects fluidity throughout development stages. Material
5846-545: The test. The mechanism for this elevation is unclear. Alcohol might increase GGT production by inducing hepatic microsomal production, or it might cause the leakage of GGT from hepatocytes . Numerous drugs can raise GGT levels, including phenobarbitone and phenytoin . GGT elevation has also been occasionally reported following nonsteroidal anti-inflammatory drugs (including aspirin ), St. John's wort and kava . More recently, slightly elevated serum GGT has also been found to correlate with cardiovascular diseases and
5925-430: The two sides of the membrane. Diffusion occurs when small molecules and ions move freely from high concentration to low concentration in order to equilibrate the membrane. It is considered a passive transport process because it does not require energy and is propelled by the concentration gradient created by each side of the membrane. Such a concentration gradient across a semipermeable membrane sets up an osmotic flow for
6004-547: The vesicle by forming the vesicle with the desired molecule or ion present in the solution. Proteins can also be embedded into the membrane through solubilizing the desired proteins in the presence of detergents and attaching them to the phospholipids in which the liposome is formed. These provide researchers with a tool to examine various membrane protein functions. Plasma membranes also contain carbohydrates , predominantly glycoproteins , but with some glycolipids ( cerebrosides and gangliosides ). Carbohydrates are important in
6083-433: The water. Osmosis, in biological systems involves a solvent, moving through a semipermeable membrane similarly to passive diffusion as the solvent still moves with the concentration gradient and requires no energy. While water is the most common solvent in cell, it can also be other liquids as well as supercritical liquids and gases. 2. Transmembrane protein channels and transporters : Transmembrane proteins extend through
6162-445: Was found by comparing the sum of the cell surfaces and the surfaces of the lipids, a 2:1 ratio was estimated; thus, providing the first basis of the bilayer structure known today. This discovery initiated many new studies that arose globally within various fields of scientific studies, confirming that the structure and functions of the cell membrane are widely accepted. The structure has been variously referred to by different writers as
6241-423: Was in between two thin protein layers. The paucimolecular model immediately became popular and it dominated cell membrane studies for the following 30 years, until it became rivaled by the fluid mosaic model of Singer and Nicolson (1972). Despite the numerous models of the cell membrane proposed prior to the fluid mosaic model , it remains the primary archetype for the cell membrane long after its inception in
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