Flow mosaic model[7]: Yesmembrane structureA hypothetical model of.lipidThe double layers of material molecules form a membraneBasic structureAnd membraneproteinIt is combined with the inner and outer surfaces of the lipid layer, or embedded in the lipid layer, or penetrated the lipid layer and partially exposed on the inner and outer surfaces of the membrane.phospholipidAnd protein have a certain fluidity, so that the membrane structure is in a state of constant change.
The flow mosaic model believes that:cell membraneThe structure is composed of liquid lipidMolecular layerMovableGlobular proteinAnd formed.With the continuous innovation and improvement of scientific research technology, the flow mosaic model has gradually been improved, which is the basis of the recognized membrane structure model.
This model has two structural characteristics: one is the fluidity of the membrane,Membrane proteinandMembranous lipidCan move laterally;The second is the distribution of membrane proteinsAsymmetrySome proteins are embedded in the membrane orOuter surface, some embedded or spannedPhospholipid bilayer。
Because scientists areTransmembrane transport of substancesA series of questions were raised in the research of the phenomenon, so we started fromBiomembraneThe structure of biomembrane has been deeply explored.
Lipid bilayer
In 1895, E. Overton used more than 500 kinds of chemistrymaterialyesplant cellOfpermeabilityAfter tens of thousands of experiments, it is found that the permeability of cell membrane to different substances is different:lipidCompared with substances insoluble in lipids, substances that can not dissolve in lipids are easier to enter cells through the cell membrane.So he proposed that membrane is composed of lipids.
In 1935, J. Danielli&H. Davson found thatplasma membraneOfsurface tensionThe tension at the oil-water interface is much lower than that at the oil-water interface. It is speculated that there is protein in the membrane, thus the "protein" is proposed-lipid-ProteinSandwichModel.It is believed that the plasma membrane consists of bilayer lipid moleculesSurface adhesionOf protein.In 1959, a modified model was proposed on the basis of the above, and it was believed that there was also penetration on the membraneLipid bilayerA protein channel for hydrophilic substances to pass through.
In 1959, J. D. Robertson obtained a clear picture of cell membrane with ultrathin sectioning technologyelectron microscopeWe can see the dark light dark of the cell membraneThree layer structure, about 7.5nm thick, it is composed of a bilayer lipid molecule about 3.5nm thick and a protein about 2nm thick on the inner and outer surfaces.He proposed“Unit membrane model”Hypothesis: continuousLipid bilayerAs the main body of the membrane, the non-polar end of phospholipid faces the inner side of the membrane, the polar end faces the outer sides of the membrane, and the protein is in a single layerPeptide chainThickness, passing throughElectrostatic actionIt combines with the polar end of phospholipid to form a three-layer structure of protein phospholipid protein, which is calledUnit membrane。He proposed that eukaryotic cells and prokaryotic cells have the same membrane structure.The main disadvantage of the unit membrane model is that it describes the structure of biofilm as static and unchangeable, which obviously contradicts the diversity of membrane functions.
Introduction to the model
In 1970, Larry Frye and Michael Lipids and other scientists used different cell membranes for human and mouseFluorescent antibodyAfter marking, let twocell fusion，Hybrid cellOne half is red fluorescent and the other half is green fluorescent. After being placed for a period of time, two fluorescent antibodies are founduniform distribution。This experiment, as well as other relevant experimental evidence, shows that the cell membrane has fluidity.
On the basis of new observation and experimental evidence, some scholars have proposed some molecules related to biomembraneStructural model。Of which, 1972Sanger(S. J. Singer) andNixon(G. Nicolson)Flow mosaic modelIt is accepted by most people.[1]
Crystal mosaic model
In 1975, Wallach proposedCrystal mosaic model 。On the basis of flow mosaic model, he further emphasized:reversibilityChange.This kind of change area is distributed on the membrane in the form of dots.The phase transition is shown asMembranous lipidA synergistic effect of molecules, that is, more than dozens of lipid molecules phase change at the same time.The phase transition of membrane lipid is affected by temperature, the nature of lipid itself, other components in the membrane, pH and divalencecationConcentration and other factors.
plate mosaic model
In 1977, Jain and White proposedplate mosaic model Its content is essentially the same as the lattice mosaic model.They believe that: in the flow of fatMolecular layerThere are many lipid "plates" with different sizes, larger stiffness and independent movement（Ordered structureThe two regions are in a continuous dynamic equilibrium.
summary
Although there are many kinds of biomembrane structures, the basic contents of the widely accepted structural models tend to be consistent.The main points and characteristics are basically the same, including the molecular composition and structural characteristics of the membrane.
primary coverage
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Model Overview
In 1972, S. J. Singer and G. Nicolson summarized the achievements of membrane structure models and various new research technologies at that time, and proposed the fluid mosaic model of biomembrane.
Fluid mosaic model refers to the flow mosaic model of membrane.This model is a membrane structure model proposed for cell membrane, which describes that the membrane is asymmetric in structure and functionLipid bilayerThe protein is distributed on the surface and inside of the membrane in a mosaic pattern, and can move within the membrane.This model can also be applied to the membrane with subcellular structure.[2]
fundamental theory
The fluid mosaic model of biomembrane believes that:
1、 The phospholipid bilayer constitutes the basic scaffold of biomembrane, which is not static.Of which phospholipidHydrophilicityHead towards both sides, drain waterLipophilicityThe tail of the is facing inward.
2、 Spherical membrane protein molecules are combined with the phospholipid bilayer in various mosaic forms. Some are embedded on the surface of the phospholipid bilayer, some are completely or partially embedded in the phospholipid bilayer, and some penetrate the entire phospholipid bilayer.This reflects the asymmetry inside and outside the membrane structure.In addition, most membrane protein molecules areFunctional protein。
3、 Most protein molecules and phospholipid molecules can move in the form of transverse diffusion, which reflects that the membrane has certain fluidity.
4、 On the surface of the cell membrane, there is a layer of glycoprotein formed by the combination of protein and sugar on the cell membrane, called glycoproteinGlycocalyx 。It plays an important role in cell life activities.For example: digestive tract andrespiratory tractepitheliumcell surfaceIts glycoprotein has the function of protection and lubrication;The sugar coating is closely related to the recognition of cell surface, such as between cells, or between cells and othersmacromoleculeA word or language used to communicate with each other.exceptglycoproteinIn addition, the cell membrane surface is also composed of sugars and lipid moleculesGlycolipids。[1]
main features
The flow mosaic model has two main characteristics.The first feature is that protein is not stretchedLamellaIt is embedded in the phospholipid bilayer with folded spheres. The degree of protein binding to membrane lipids depends on the nature of amino acids in membrane proteins.The second feature is that the membrane has certain fluidity and is no longer closedFlaky structureTo meet the needs of various cell functions.
Comprehensive evaluation
This model emphasizes the fluidity and asymmetry of membrane structure, and gives a more scientific explanation of the structure and function of cell membrane. It is widely accepted and supported by many experiments.
The flow mosaic model is not perfect in some aspects, such as ignoringInorganic ionAnd water, ignoring the effect of protein molecules on the fluidity of membrane lipid moleculesControl function, ignoring the fluidity of each part of the membraneNonuniformitywait.
Structural characteristics
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Mosaic
Phospholipid bilayerAnd protein mosaic;Or arrange them into alternating facets in 2D.
mobility
Liquidity Overview
The fluidity of the membrane is one of the basic characteristics of the cell membrane structure, and it is also the normal function of the cell membranenecessary condition。The fluidity of membrane refers to theSportiness, which includes the movement of membrane lipids andMembrane proteinThe movement of.Over the years, with the help of constantly innovative experimental technology, the research on membrane structural characteristics has been further deepened and made continuous progress.
① Temperature: at a certain temperature, the lipid molecules are separated from the liquidCrystalline state(can flow with certain shape and volumeState of matter）Change into gelatinous (non flowing) crystalline state.This can causePhaseThe changing temperature is calledPhase transition temperature。Whenambient temperatureAbove the phase transition temperature, membrane lipid molecules are in flowingLiquid crystal state； But below the phase transition temperature, it is in a non flowing crystalline state.phase transition of membrane lipidThe lower the temperature, the greater the fluidity of membrane lipid;On the contrary, the higher the phase change temperature is, the smaller the fluidity of membrane lipids will be.
② Membranousfatty acidChain: The chain of fatty acid with high saturation degree is closely and orderly arranged, so the fluidity is low;andUnsaturated fatty acidDue to the existence of unsaturated bonds, the arrangement of molecules is loose and disordered, and the phase transition temperature is reduced, thus enhancing the fluidity of the membrane.The length of fatty acid chain also has an impact on the fluidity of membrane lipids: with the growth of fatty acid chain, the opportunities for chain tail interaction increase, easy to agglutinate (phase change temperature increases), and the fluidity decreases.
③cholesterolThe regulation of cholesterol on membrane lipid fluidity changes with temperature.stayPhase transition temperatureAbove all, it can weaken the mobility of the fatty acid chain of phospholipids, thus reducing the fluidity of membrane lipids.Below the phase transition temperature, cholesterol canPhospholipid fatty acidThe interaction of chains alleviates the drastic decline of membrane lipid fluidity caused by low temperature.
In addition to the above factors, membrane lipids andMembrane proteinDegree of integration, environmentalionic strengthAnd pH value will affect the fluidity of membrane lipid.
The flow of membrane lipid is the main factor that causes the flow of cell membrane. To sum up, there are six main ways of membrane lipid movement.
The proteins in the cell membrane can also move in the way of lateral diffusion.The fluidity of membrane proteins has been fully confirmed by experiments.David Frye and Michael Edidin use fluorescent antibodiesimmunolabellingTo determineCell surface antigenDistribution of.First use greenFluorescent labelingAntibody treatment culture ofmiceHuman cells were treated with another antibody labeled with red fluorescence.Because the antibodyantigenOfSpecific bindingAnd make the treated mouse cells and human cells emit green and red fluorescence respectively.InactivatedSendai virusUnder the mediation, mouse cells and humancell fusion, formedHybrid cellHalf green and half red.After incubation at 37 ℃ for 40 minutes, the two-color fluorescence was evenly distributed on the surface of the hybrid cells.The uniform distribution of fluorescent spots is mainly due to the fact that the antigen bound with fluorescent antibody passes throughLateral diffusionAnd redistribute.
The movement of membrane proteins is also affected by many factors.Interaction between protein and lipid in membraneIntrinsic proteinAndExtrinsic proteinInteraction, formation of membrane protein complexes, membrane proteins andCytoskeletonThe role ofRestriction proteinThe flow of quality.
Membrane proteins mainly include the followingSports form：
① Random movement Some proteins can move randomly across the membrane.The rate of movement is higher than that of manual operationLipid bilayerThe measured value should be low.
② Directional movement Some proteins are special and move directionally in the membrane.For example, some membrane proteins can move from the head of the cell to the tail on the membrane.
Cell plasma membraneOfAsymmetry(membrane asymmetry) refers to the uneven distribution of various components in the plasma membrane lipid bilayer, including the uneven variety and quantity.(withLipid bilayerThe hydrophobic end of the molecule is the boundary, and the biomembrane can be divided into two layers: near cytoplasmic surface and non cytoplasmic surface.The structure and function of the inner and outer layers of the biofilm are very different, and this difference is called the asymmetry of the biofilm.)
The main components of the membrane are protein, lipid and sugar. The asymmetry of the membrane mainly refers to the asymmetry of the distribution of these components and the asymmetry of these molecules in the direction.
Asymmetry of membrane lipids[4]
Asymmetry of membrane lipids
The asymmetry of membrane lipids is manifested in the different proportions of various lipids distributed in the lipid bilayerCellsThe asymmetry of membrane lipids of.
Asymmetry of membrane proteins
Each membrane protein has a specific arrangement direction in the membrane, which is suitable for its function. This is the main factor of membrane protein asymmetry.The asymmetry of membrane proteins includesPeripheral proteinAsymmetrical distribution andIntegrinInside and outsideamino acidThe purpose of residual number is asymmetric.
The asymmetry of membrane lipid, membrane protein and membrane sugar distribution leads to the asymmetry of membrane function anddirectional, guaranteedLife activitiesIs highly ordered.[6]
