synonymplasma membrane(Biological term) generally refers to cell membrane
The cell membrane is mainly composed ofphospholipidConstitutiveFlexibleOfSemipermeable membrane, film thickness 7 ～ 8nm, forAnimal cellFor example, the outside of the membrane is in contact with the external environment.Its main function is to selectively exchange substances and absorbNutrients, discharge metabolic waste, secrete and transportprotein。[1]
Phospholipid bilayerIt is the basic scaffold of cell membrane.The main components of cell membrane are protein and lipid, containing a small amount of sugar.Some lipids and sugars combine to form glycolipids, and some proteins and sugars combine to form glycoproteins.
3. Chemical composition
cell membrane[2]
The cell membrane is mainly composed of lipids (mainly phospholipids), proteins and sugars;Protein and lipid are the main components.Under the electron microscope, it can be divided into three layers, that is, there is an electronic dense band about 2.5 nm thick on both the inside and outside of the membrane, and there is a band 2.5 nm thick in the middleTransparent bandThe total thickness is about 7.0~7.5nm.This structure is not only found in various cell membranesOrganelle membrane(e.gmitochondrion、Endoplasmic reticulum) also has a similar structure.[3]
brief introduction
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Cell membrane is a barrier to prevent extracellular substances from entering cells freely, which ensuresIntracellular environmentThe relative stability of thebiochemical reaction It can run orderly.But cells must exchange information, matter and energy with the surrounding environment to complete specificphysiological functionTherefore, cells must have a set ofMaterial transportThe system is used to obtain the required substances and discharge the metabolic waste.It is estimated that proteins related to substance transport on cell membrane account forNuclear gene15~30% of the encoded protein, and the energy used by cells in material transport reaches two-thirds of the total energy consumed by cells.
Cell membraneplasma membrane(plasmalemma), located inProtoplastPeripheral, close fittingcell wallThe function of the membrane structure is to protect the interior.The main substances that make up the plasma membrane are proteins and lipids, as well as a small amount of polysaccharides and trace amounts ofnucleic acid、metal ionAnd water inelectron microscopeNext, useOsmium tetroxideThe fixed cell membrane has obvious "dark light dark" three parallel bands, with inner and outer layersScotomaComposed of protein molecules, the middle layerBright bandIt is composed of bilayer lipid molecules, whose thickness is about 2.5 nm, 3.5 nm and 2.5 nm respectively. Such films are calledUnit membrane(unit membrane) orBiomembrane（biomembrane）。
physiological function
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The cell membrane has importantphysiological function, which enables cells to maintain stable metabolisminternal environmentIt can also regulate and select substances to enter and exit cells.Cell membrane passes throughPinocytosis（pinocytosis）、Phagocytosis(emphasis) orExocytosis(exocytosis) absorption, digestion andEffluxSubstances outside and inside the cell membrane.stayCell recognition、signal transmission , cellulose synthesis andMicrofibrilThe plasma membrane also plays an important role in the assembly of.Some intercellularInformation exchangeIt is not realized by the receptors on the cell membrane, such as someCell secretionOfSterolsThese substances can be used as signals to exchange information with other cells, but these substances do not bind to receptors on the cell membrane, but pass through the cell membranecytoplasmSome of the receptors in the cell combine to mediate the exchange of information between two cells.
Research progress of cell membrane structure
cell membrane
In the middle of the 19th century, KW. Mageli foundcell surfaceObstructivedyestuffEntry phenomenon, promptmembrane structureThe existence of;1899 EOverton found that substances with large fat solubility are easy to enter the cell, and speculated that they should be lipid barriers.In 1925, EGorter and FFor GrendelacetoneExtract erythrocyte membrane structure and calculatered blood cellThe flat area of the membrane is aboutSurface areaTwice of, proposeLipid bilayerModel. Premises for establishment: a. All lipids of red blood cells are on the membrane;b.acetoneComplete extraction;c.RBCThe average surface area was estimated correctly.(70%~80% is low);After 40 years, Bar repeated the experiment and found that the flat area of erythrocyte membrane should not be 70%~80%, but 1.5 times of the protein surfaceDry filmArea is 99μm²，The wet film area is 145μm²。The two errors are balanced, and the result is basically correct.
According to cellularPhysiological and biochemical characteristicsIt has been speculated that the plasma membrane is a kind of fat gridMolecular layerAnd a sandwich structure composed of protein phospholipid protein.At the same time, the electron microscopic observation also confirmed that the parenchymal membrane was indeed dark light darkThree layer structure。subsequentlyFreeze Etching TechnologyIt showed that there were protein particles in the bilayer membrane;The immunofluorescence technique proved that the protein in the plasma membrane was flowing.According to SJ. Singer et al. proposed in 1972BiomembraneOfFlow mosaic modelThe structural feature is that the skeleton of the biomembrane is a phospholipid bilayer, in which protein molecules are embedded in different ways, and there are sugar molecules on the surface of the cell membrane, formingGlycolipids、glycoprotein；On the internal and external surfaces of the biofilm, the distribution of lipids and proteins is unbalanced, reflecting the different functions on both sides of the membrane;Lipid bilayerWith fluidity, its lipid molecules can move freely, and protein molecules can also move laterally in the lipid bilayer.
Although no one can directly observe the membranemolecular structureHowever, since the 1930s, dozens of hypotheses have been put forward, which have been supported by many experimental facts and thus accepted by most people as SJ. Singer and G50. Nicholsom proposed in 1972Fluid mosaic model（fluid mosaic model）。The basic content of this hypothetical model is that the common structural feature of the membrane is the liquid lipid bilayer as the base frame, which is embedded with proteins with different molecular structures and therefore different physiological functionsα-A helical or globular protein exists.Its limitation is that it does not express the heterogeneity of liquidity. Jain and White proposed the "plate and mosaic model".
Sandwich model and unit membrane model
J. Danielli&H. Davson 1925 found thatsurface 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 a sandwich model of "protein lipid protein" is proposed.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 the membrane also had a protein channel through the lipid bilayer for hydrophilic substances to pass through.
J. D. Robertson 1959Ultrathin sectioning techniqueClear cell membrane photos were obtained, showing a dark light dark three-layer structure with a thickness of about 7.5 nm.This is the so-called "unit membrane" model.It consists of a bilayer lipid molecule about 3.5nm thick and an inner and outer surface about2nmProtein composition.
deficiencies1. Describe the membrane structure as static;2. The functional activity of the membrane cannot be explained;3. Various films have their own specific thickness, not all of which are 7.5nm;4. The difficulty of protein extraction is different;5. The ratio of protein and lipid of various membranes is different.[4]
Flow mosaic model
The flow mosaic model highlights the fluidity and asymmetry of the membrane, and believes that the cell membrane is composed of flowing lipid bilayer and protein.Phospholipid moleculesHydrophobicityOpposite tail, polar head facingaqueous phaseCompose of biomembrane skeleton, protein or embedded inLipid bilayerThe surface, either embedded in it or across the entire lipid bilayer, shows asymmetric distribution.
Disadvantages: 1. It cannot explain how to maintain the integrity and stability of the membrane during the change process;2. Neglect the effect of protein on the fluidity of lipid moleculesControl function；3. Neglecting the fluidity of each part of the membraneNonuniformity。[4]
Lattice model
Wallach proposed the lattice model in 1975.The lattice model is a supplement to the flow mosaic model, emphasizing the flowentirety。Reversible disorder (liquid) and order with membrane grease（Crystalline state）To explain the fluidity of biofilm.Membrane mosaics control the movement of lipid molecules.Mosaicin and the surrounding lipid molecules form a lattice state. These unmovable lipid molecules are called interfacial lipids, and the flowing lipids are distributed in small pieces and dots.So the lipid flow is local, not the whole lipid bilayer is flowing.[4]
Lipid raft(lift lift) is a micro particle rich in cholesterol and sphingomyelin on the plasma membraneDomain（microdomain）。About 70nm in size, it is a dynamic structure, located outside the plasma membraneSmall page。becauseSphingomyelinWith longerSaturated fatty acidChain,Intermolecular forceStronger, so theseRegional structureDense, between disordered liquid and liquid crystal, is called liquid ordered.At low temperatures, these areas can resist non-ionicDetergentTherefore, it is also called detergent resistant membranes (DRMs).The lipid raft is like a platform for protein docking, which is associated with the signal transductionProtein sortingAre closely related.
fromLipid raftFrom the perspective of,Membrane proteinIt can be divided into three categories: ① proteins existing in lipid rafts;Including GPI anchored protein, someTransmembrane protein, Hedgehog protein,diacetylDouble acylated protein, such as:Non receptor tyrosine kinaseSrc、G proteinG ofαSubunit、vascular endothelial cellOfNitric oxide synthase（NOS）；②Proteins in the disordered liquid phase outside the lipid raft; ③Proteins between the two, for example, some proteins are not receivedligandThe affinity for lipid rafts is low, and will be transferred to lipid rafts when oligomerization occurs when ligands are combined.
Lipid raftLike glue, cholesterol inSaturated fatty acidChainedSphingomyelinHigh affinity, butUnsaturated fatty acidLow affinity of chainMethyl-β-Cyclodextrin（methyl-β-Cyclodextrin) to remove cholesterol, anti detergent protein becomes easy to extract.The sphingomyelin in the membrane is mainly located in the outer leaflet, and most of them are involved in the formation of lipid rafts.
It is estimated that the area of the lipid raft may occupy the membraneSurface areaMore than half of.The size of the lipid raft can be adjusted, and small independent lipid rafts may keepSignal proteinIt plays an important role in the closed state. When necessary, these small lipid rafts will be assembled into a large platformSignal molecule(such as receptors) will meet their accessories and start the signal transmission pathway.asAllergen(allergen) is able tomast cellsorBasophilThe IgE antibody on the cell surface and its receptor bridge together to form a largeLipid raft, Lyn in receptor lipid raft (aNon receptor tyrosine kinase）Phosphorylation starts downstream signal transduction and finally triggersAnaphylactic reaction。
The thickness of cell membrane is usually 7~8nm, and one of the most important characteristics of cell membrane is halfPermeabilityOr selective permeability, that is, selectively allowing substances to enter cells through diffusion, infiltration and active transport, so as to ensure the normal metabolism of cells.In addition, most plasma membranes also have hormone receptors, antigenicbinding siteAnd other sites related to cell recognition,immune reactionandCellular communicationPlays an important role in the process.
Basic structure
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Membranous lipid
Cell membrane under electron microscope[5]
eachAnimal cellThere are about 10 ^ 9 lipid molecules on the plasma membrane, that isSquare micrometerThere are about 5x10 ^ 6 lipid molecules on the plasma membrane of.
membrane lipidMainly byphospholipid、cholesterolAnd a small amountGlycolipidsComposition.In most cell membraneslipidAmong them, phospholipids account for more than 70% of the total, cholesterol does not exceed 30%, and glycolipids does not exceed 10%.Phospholipids can be divided into two categories:glycerolPhosphoglycerides andSphingomyelin(sphingomyelin, SM)。Glycerophospholipids mainly include phosphatidylcholine(Lecithin)（phosphatidylcholine, PC), followed by phosphatidylserine(phosphalidylserine,PS）And phosphatidylEthanolamine(Cephalin)（phosphatidylethanolamine，PE）, phosphatidyl is the leastinositol(phosphatidylinosital, PI )。Phospholipids, cholesterol and glycolipids are amphiphilic molecules.Phospholipidphosphoric acidandBase, hydroxyl groups in cholesterol molecules andsugar chainetc.HydrophilicityThe groups form the hydrophilic end of their respective molecules, while the other end of the molecule is hydrophobicfatty acidHydrocarbon chain.These molecules exist in the plasma membrane in the form of lipid bilayer, and the hydrophilic end facesExtracellular fluidorCytoplasm, hydrophobic fatty acid hydrocarbon chains are opposite each other, formingDrainage zone。The lipid composition in the membrane lipid bilayer is asymmetric, includingamino acidPhospholipids (phosphatidylserine, phosphatidylethanolamine, phosphatidylinositol) are mainly distributed in the inner layer near the cytoplasm of the membrane, while most and all glycolipids of phosphatidylcholine are distributed in the outer layer of the membrane.[6]
Membrane protein
cell membrane
CellsMembrane proteinMembrane proteins (including enzymes) mainly combine with membrane lipids in two forms:Intrinsic proteinandExtrinsic proteinTwo.The internal protein is directly connected with the hydrophobic part of phospholipid as the hydrophobic partCovalentCombination, with polarity at both ends, running through the inside and outside of the membrane;External proteincovalent bondIt binds to the outer end of the intrinsic protein or to the hydrophilic head of the phospholipid molecule.Such as carrier, specific receptor, enzymesurface antigen 。Surface protein accounting for 20%~30%（Peripheral proteinChargedamino acidOr group——Polar groupBinding with lipids on both sides of the membrane;70%~80%Binding protein(intrinsic protein) passing through one or more hydrophobicα-screw(20~30 drainsAmino acid absorptionAnd 3.6 pieces per circleAmino acid residue, equivalent to film thickness.Adjacentα-The helix is connected by linear chain peptides on both sides of the membrane, that is, hydrophobic hydroxyl groups in the membrane are bound to lipid molecules.In theory, the protein embedded in the lipid layer can float horizontally, so it israndom distributionOf;Regional distribution that can actually exist;(This may be different from theCytoskeletonIt is related to the limited effect of certain protein molecules) to realize its special functions: material, energy andinformation switchingEtc.(Frye and Edidin in 1970 labeled human cells with red glowing alkaline core redSame asGlows greenfluoresceinLabeling membrane protein antibody Labeling in vitromiceThe cells were cultured together, and then they were fused. After 37 ℃ and 40 min, they becameuniform distribution。Photobleaching fluorescenceRecovery method, micro area monitoring)
There are two main types ofTransporter, i.e.:Carrier protein（carrierprotein）AndChannel protein（channel protein）。Carrier protein is also called carrierPermeating enzymePermease and transporter can combine with specific solute, and transfer the solute to the other side of the membrane through their own conformational changes. Some carrier proteins need energy to drive, such as:ATPDrivenIon pump；Some do not need energy toAssisting proliferationTransport of substances, such as valine.Channel proteins are weakly bound to the transported substances. They can form hydrophilic channels. When the channels are opened, they can allow specific solutes to pass through. All channel proteins transport solutes in a way that facilitates diffusion.
The cell membrane wraps up the cell, so that the cell can maintain relative stability and maintain normalLife activities。In addition, the absorption andMetabolitesAll of them pass through the cell membrane.Therefore, the selectivity of the cell membrane allows some molecules to enter or exit the cell, which is calledSelective permeability。This is the most basic function of cell membrane.If cells lose this function, they will die.
Cell shape
In addition to regulating and controlling cells through selective permeationSubstance exchangeIn addition, we can also“EndocytosisAndExocytosis"To help cells absorb liquid droplets and capture food particles from the external environment, and supply cells withNutrientsNeeds.The cell membrane can also receive the stimulation of external signals to make cells respond, thus regulating the life activities of cells.Cell membranes are not just cellularPhysical barrierIt is also an important structure with complex functions in cell life activities.
The polar region of the membrane intrinsic protein protrudes to the membrane surface, and the non-polar part is buried inside the double layer;
mobility
The cell membrane is composed ofphospholipiddoubleMolecular layerIt is composed of proteins embedded, penetrating and adsorbed on its surface. The hydrophobic tail of the phospholipid bilayer is inside, and the hydrophilic head is outside.Phospholipids constitute the basic scaffold of membrane by molecular layer, which is not static.The phospholipid bilayer is a light oil like liquid with fluidity. Some protein molecules are embedded on the surface of the phospholipid bilayer, some are partially or completely embedded in the phospholipid bilayer, and some span the entire phospholipid bilayer.Most protein molecules can also move.The classic proof is to useSendai virusMediationComplete differentmiceChromocyteAfter a period of time, red and green are evenly dotted around the cell membrane, indicating that the membrane is fluid
The fluidity of phospholipid molecule is affected by some factors, mainlyinfluence factoryes:
① Temperature: at a certain temperature, phospholipid moleculesLiquid crystal state(can flow with certain shape and volumeState of matter）Transformed into gelatinous (non flowing)Crystalline state。This can causePhaseThe changing temperature is calledPhase transition temperature。Whenambient temperatureWhen the phase transition temperature is above, the phospholipid molecules in the cell membrane are in a flowing liquid crystal state;But below the phase transition temperature, it is in a non flowing crystalline state.The lower the phase transition temperature of cell membrane phospholipids, the greater the fluidity of cell membrane phospholipids;On the contrary, the higher the phase transition temperature, the smaller the fluidity of phospholipid molecules in cell membrane.
② Phospholipid molecularfatty 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.So the cell membrane also has fluidity.The length of fatty acid chain also has an impact on the fluidity of cell membrane phospholipid molecules: 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 the fluidity of phospholipid molecules in cell membrane varies with temperature.stayPhase transition temperatureAbove, it can make the fatty acid chain of phospholipidSportinessTo reduce the fluidity of phospholipid molecules in cell membrane.Below the phase transition temperature, cholesterol canPhospholipid fatty acidThe interaction of chains alleviates the drastic decline of membrane phospholipid molecular fluidity caused by low temperature.
⑤Lipid bilayerThe more proteins embedded in, the smaller the membrane fluidity
In addition to the above factors, cell membrane phospholipid molecules andMembrane proteinIn the environmentionic strengthPH value will affect the fluidity of phospholipid molecules in cell membrane.
Membranous lipidThe flow of membrane lipid is the main factor causing the flow of cell membrane. To sum up, there are four main ways of membrane lipid movement.
The movement of membrane proteins is due to therelative molecular massLarger, and subject toCytoskeletonIt is impossible to exercise like membrane lipids.There are mainly the followingSports form：
① Random movement Some proteins can move randomly across the membrane.The rate of movement is lower than that measured with the double layer of artificial grease.
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.
along withenvironment conditionChanges,lipidThe crystal state and liquid crystal state of molecules are tautomeric;
Update state
In cells, membrane components are constantly updated;
Asymmetry
Cell plasma membraneOfAsymmetryIt means that various components in the bilayer of plasma membrane lipid are notuniform distributionIncluding the uneven variety and quantity.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.The asymmetry of membrane lipid, membrane protein and membrane sugar distribution leads to the asymmetry of membrane function anddirectional。It ensures the high order of life activities.
permeability
Matter passing through biologySemipermeable membraneDegree of difficulty.The effect of biological semipermeable membrane on some molecules in the bodypermeabilityIt can be roughly divided into the following three situations: free passage includeswaterMolecule;There areglucose、amino acid、urea、Chloride ionEtc;Protein, sodium and potassium are not easy to penetrate.The existence of permeability, the movement of water inside and outside the cell, the exchange of various substances,PHandosmotic pressureIt has important physiological significance to maintain.In some pathological conditions (such as allergy, traumaburn, hypoxia, etc.), as the normal structure and function of the biological semipermeable membrane are damaged, its permeability is increasedTissue edemaAnd other reactions.
Cell membrane function
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(1) Separating, forming cells andOrganelle, cellularLife activitiesProvide relatively stableInternal environmentThe area of the membrane is greatly increased, improving the biological function occurring on the membrane;
twoAssisting proliferation: Non fat soluble substances inMembrane proteinWith the help ofPotential differenceThe process of transmembrane diffusion is called assisted diffusion.No energy consumption, but carrier is required.Three characteristics of assisted diffusion: 1. Specificity: in memoryion channelOr the carrier generally transports only one substance.2. Saturation: that is, when the transported substance increases to a certain limit, the transport rate will no longer increase, because the number of ion channels or carriers is limited.3、Competitive inhibitionWhen ion channels or carriers in memory transport two or more substances at the same time, one substanceConcentration increaseWill weaken the transport of another substance.4. Classification of membrane proteins: 1Channel protein2 gate channel protein 3 specific protein (transport by changing its own conformation through contact), such as:glucoseget intored blood cell。
threeActive transportation: ion orSmall moleculeUnder the action of the "pump" on the membrane, the substance is subject to reverse concentration difference or reverse potential differenceTransmembrane transportProcess, calledActive transshipment(Active transportation).Active transport consumes a lot of heat and requires a carrier.yesSelectivity。E.g. Iodine ingressKelpGlucose enters cells other than red blood cells.
fourEndocytosisExocytosis: transportmacromoleculeOr agglomerate material.The process of substances entering cells from outside through the movement of cell membrane is called endocytosis.Including swallowing and swallowing.Liquid substances enter the cell for swallowing, such as intestinal epitheliumNutrientsAbsorption;Solid matter entering the cell is phagocytosis, such asGranulocyteThe process of engulfing bacteria.Exocytosis is a process that is sent from intracellular to extracellular through the movement of cell membrane.CellularMetabolitesandGlandular cellThe secretion ofExocytosisIt needs energy to complete.
1. Cell is the smallest unit of matter from inanimate to living (regardless ofViruses）, deep analysis of cellularenergy flowIt is helpful to understand the fundamental difference between living matter and nonliving matter.
unicellularAlgaeAnd bacteria or isolated single cellsDissociative state, usually spherical or nearly spherical.multicellular In plants, cells areTight arrangementThey are often irregular due to mutual extrusionpolyhedron。higher plantThe cells in the body have a fine division of labor, and their shapes are extremely diverse.For example, cells that transport water and nutrients（Vessel elementandSieve tubeMolecules), in the shape of a long cylinder, and connected into a connected "pipe" to facilitate the transportation of substances;The supporting cells (fibers) are generally long spindle shaped and gather into bundles to strengthen the supporting function;Roots that absorb water and fertilizerHair cellIt produces a long tubular protrusion outward, which increases itscontact area (Figure 1-2).The diversity of these cell shapes is the result of the adaptation of cell morphology to its function.
Generally speaking, the size of plant cells is very small, and the diameter of most cells is 10~100μm. It is difficult to distinguish with the naked eye.Some people think that the size of cell volume is mainly affected bynucleusIt is restricted by the scope that can be controlled. If the volume is small, thenSurface areaLarge, conducive to material exchange between cells and the outside world.However, the size of cells in different species and different parts varies greatly.
Research History
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1. E. Overton 1895 found that substances soluble in fat easily penetrate the cell membrane of plants, while substances insoluble in fat are not easy to penetrate the cell membrane, so it is speculated that the cell membrane is composed of continuouslipidMaterial composition.
Water soluble substances are difficult to pass through plasma membrane
2. For E. Gorter&F. Grendel 1925Organic solventExtracted human erythrocyte plasma membranelipidComponent, spread it on the water surface, measureMembranous lipidThe expanded area is twice the cell surface area, so it is speculated that the cell membrane is composed of double lipid molecules.
3. Proposed by J. Danielli&H. Davson 1935SandwichModel
6. In October 2020, the research team of Shanghai Plant Adversity Biology Research Center, Center for Excellence and Innovation in Molecular Plant Science, Chinese Academy of Sciences, revealed a link between cell membrane andchloroplastThe importance ofsignal transmission Approaches.The study revealed someVegetable proteinHow to relate to cell membrane and perceivepathogenWhen they exist, how do they transfer from the cell membrane to the inside of the chloroplast, "warning" that the chloroplast is a threat.[7]
