Plasma membrane was once calledcell membrane(cell membrane) refers to an extremely thin membrane surrounding the cell surface, mainly composed ofMembranous lipidandMembrane proteinConsists of.The basic function of cell plasma membrane is to maintain the cellMicroenvironmentIs relatively stable, and participates in the external environmentSubstance exchange, Energy andinformation transfer。In addition, it plays an important role in the survival, growth, division and differentiation of cells.
Chinese name
Cell plasma membrane
Foreign name
plasma membrane
Structure
Membrane lipids and membrane proteins
Role
Material exchange, energy and information transmission
EukaryoteIn addition to the cell surface membrane, there are many kinds of cells separated by the membrane in the cytoplasmOrganelleOf these organellesmembrane structureSimilar to plasma membranes, but with different functions, these membranes are called internal membranes.Endometrium includesnucleusMembraneEndoplasmic reticulumMembrane, Golgi membrane, etc.Since bacteria have no inner membrane, they are thinCytoplasmic membraneActing on the cytoplasmic membrane.
It is the basic component of membrane lipid, accounting for more than 50% of the whole membrane lipid.Phospholipids are amphipathic molecules orAmphiphilic moleculeOr facultative molecules.
It only exists on the eukaryotic cell membrane, and the content generally does not exceedMembranous lipid1/3 of,plant cellThe content in the membrane is less.It is a bisexual molecule.The function is to improveLipid bilayerThe mechanical stability of can adjust the fluidity of double layers of grease and reduceWater solubilityCorporealpermeability。
Liposome(lipoome) is based on the phospholipid moleculesaqueous phaseStableLipid bilayerWith the trend of membrane, a small amount of phospholipids are put into aqueous solution, which can self assemble into a lipid bilayer spherical structure. This structure is called liposome, so liposomes are artificially prepared continuous lipid bilayer spherical lipid capsules.Liposomes can be used asBiomembraneOfresearch model , and can be used asBiomacromolecule（DNA molecule）And drugsCarrierTherefore, liposomes are used to study the relationship between membrane lipids andMembrane proteinAnd its biological properties.Building missilesArtificial liposomeWhen inserting antibodies, it is not only necessary to wrap the molecules or drugs to be carried into the internal aqueous phase of the liposome, but also to make some modifications on the membrane of the liposome, such as inserting antibodies to facilitate the targeting of the liposome after entering the body.
Function of membrane lipids
(1) Membrane lipid is the skeleton of cell membrane structure;
(2) Maintain conformation and provide an environment for membrane protein to function;
(3) It is necessary for some enzyme driving functions.
Skeleton and specialized structure of cell surface
Membrane skeletonRefers to the relationship between the cell membrane andMembrane proteinConnected byfibrinConstituentgrid structure(meshwork), which is involved in maintaining the shape of the plasma membrane and assisting the plasma membrane to complete a variety ofphysiological function。It is characterized by high viscosity and strong tensile capacity.
mammalNo mature red blood cellsnucleusandEndometrial systemThe cell membrane has both good elasticity and high strength, and the proteins of the cell membrane and membrane skeleton are relatively easy to purify and analyze.The plasma membrane of red blood cells is broken after hypotonic treatment,ContentsRelease, leaving a shell that remains intact, calledBlood shadow。Therefore, it is an ideal material for studying membrane framework.
Change processedionic strength: The bands of blood shadow protein and actin disappear, the shape of blood shadow changes, and the fluidity of membrane increases.
Treat with Triton-100:Band 3 proteinAnd some blood group glycoproteins disappeared, and the shape of blood shadow basically remained unchanged.
Use non-ionicDetergentTreatment: All lipids, blood group glycoproteins and most band 3 proteins disappeared, and the shape of blood shadow basically remained unchanged.
Membrane protein
Membrane protein is the main embodiment of membrane function.Nuclear genomeAbout 30% of the encoded proteins are membrane proteins.According to andMembranous lipidThe membrane proteins are divided into internalprotein），Peripheral protein(peripheral protein) andLipoanchored protein（lipid-anchored protein）。
Binding mode of protein and membrane
Binding mode of protein and membrane
①，②Integrin；③，④Lipoanchored protein; ⑤, ⑥Peripheral protein
The intrinsic protein is also calledIntegrin, embedded into the interior of the grease bilayer at different degrees, some are full spanMembrane protein（tansmembrane proteins）。Membrane proteins are amphoteric molecules.It is tightly bound to the membrane, and can only be washed down from the membrane with detergent. SDS and Triton X100 are commonly used.
Intrinsic proteinTransmembrane ofDomainThere are two forms to form hydrophilic channels:Alpha helixForming hydrophilic channels;The second is fromBeta foldingForm a hydrophilic channel.
Membrane proteinCytoplasmic matrixUnilateralCysteineResidueCovalentIt binds fatty acid molecules, inserts between lipid bilayers, and a few proteins covalently bind with glycolipids.
Peripheral protein
Peripheral protein is also calledExtrinsic protein(extraneous protein), water-soluble, distributed on the surface of cell membrane, hydrophilic with protein molecules or lipid molecules on the membrane surface by ionic bonds or other weak bondsPartial combinationTherefore, as long as the ionic strength of the solution is changed or even the temperature is increased, it can be separated from the membrane.
Lipoanchored protein（lipid-anchored protein）
Lipoprotein is anchored by phospholipids or fatty acids and covalently bound.There are two types. One is sugarPhosphatidylinositol(GPI) linked protein, GPI is located in the outer lobule of cell membranePhospholipase CTreatment can release binding proteins.Many cell surface receptors, enzymes,Cell adhesion moleculeAnd causeScrapie of sheepPrPC is hereProteoid。Another class of lipid anchored proteins and inserted plasma membraneInner lobuleLength ofHydrocarbonChain binding.
2. __ E. Gorter&F. Grendel 1925 speculated that the cell membrane was composed of bilayer lipid molecules.
3. __ J. Danielli&H. Davson 1935 proposed "protein-lipid-ProteinSandwichModel. It is believed that the plasma membrane consists of bilayer lipid molecules, inside and outsideSurface adhesionIn 1959, a modified model was proposed, which suggested that the membrane also had a protein channel through the lipid bilayer for hydrophilic substances to pass through.
4. __ J. D. Robertson 1959 Obtained clear cell membrane photos with ultrathin sectioning technology, showing dark light darkThree layer structure, about 7.5nm thick. This is the so-called“Unit membrane"Model. It consists of a bilayer lipid molecule about 3.5nm thick and a protein about 2nm thick on the inner and outer surfaces.
5. ____ S. J. Singer&G. Nicolson 1972 according toImmunofluorescenceTechnology,Freeze etchingThe research results of the technology are proposed on the basis of the "unit membrane" model“Flow mosaic model"。
Flow mosaic characteristics
Composition: The main components are lipids and proteins, and a small amount of sugars.
AsymmetryThe protein is either embedded on the surface of the lipid bilayer, or embedded inside it, or across the entire lipid bilayer, showing asymmetric distribution.
Fluidity: Membrane protein and membrane lipid can move in all directions.
fiveFlip motion6 Rotational isomerization movement
2. Factors affecting membrane flow
It mainly includes lateral diffusion andRotational diffusionTwo types of motion. Rotating diffusion fingerMembrane proteinRotates around an axis perpendicular to the membrane plane. The lateral movement of membrane proteins is restricted by the cytoskeleton and is destroyedMicrofilamentDrugs such asCytochalasin BIt can promote the lateral movement of membrane protein. Available fluorescenceMarking technologyThe fluidity of membrane protein was detected by light decoloration recovery technique.
The fluidity of plasma membrane is to ensure its normal functionnecessary conditionFor example, transmembrane material transport, cellinformation transfer，Cell recognition，cellular immunityCell differentiation and the role of hormones are closely related to membrane fluidity. When membrane fluidity is below a certain threshold, many enzyme activities and transmembrane transport will stop. On the contrary, if the fluidity is too high, membrane dissolution will occur.
Mediate the connection between cells and between cells and matrix;?
Plasma membrane participates in the formation ofcell surfaceSpecial structure
The specialized structure on the cell surface is formed to adapt to a certain environmentsurface structure , such as:Membrane skeleton, flagellum and cilium,microvilliAnd the deformed foot of cells, respectively, and the maintenance of cell morphology,Cell movement, cellularSubstance exchangeAnd other functions
Membrane skeleton
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Erythrocyte plasma membrane protein and membrane skeleton
⒈HaemoglobinAlso called spectrinErythrocyte membrane skeletonIt is the main component of red blood cell membrane protein, but it is not the component of red blood cell membrane protein, accounting for about 30% of the membrane extracted protein. Haemoglobin is the sub membrane protein of red blood cells, which is a long and flexible proteinFibrillar protein, about 100 nm long, composed of two similarSubunit: beta subunit（relative molecular mass220kDa) and α subunit (relative molecular weight 200kDaDimer, two heterodimers head to head connected into a 200 nm longTetramerThe tail ends of 5 or 6 tetramers are connected to the shortActinFiber and pass throughNoncovalent bondAnd external 4.1protein binding The 4.1 band protein combines with the cytoplasmic plane of the transmembrane protein band 3 protein through non covalent bonds to form a "junction complex". These blood shadow proteins form a deformable grid structure under the cytoplasmic plane of the entire cell membrane to maintain the double concave disk shape of red blood cells
⒉ANCHORIN(ankyrin): also called 2.1 proteinAnchoring proteinIt is a relatively large intracellular connexin. Each red blood cell contains about 100000 anchor proteins,relative molecular mass215000. On the one hand, anchor proteinHaemoglobinConnected with the transmembrane cytoplasm with 3 proteinsDomainIn this way, the anchor protein connects the blood shadow protein to the cell membrane with the help of the band 3 protein, which also fixes the skeleton to the plasma membrane. It belongs to the linker protein family and widely exists in variousHistiocyteMedium. Anchorin connectionIntegrinreachCytoskeletonProtein network, which plays a key role in a variety of cell functional activities, participates in the growth, development, and intracellular protein transport of the body,Cell polarityEstablishment and maintenance of,cell adhesion ，signal transduction as well asmRNATranscription, etc
3. Band 3 protein: andBlood group glycoproteinThey are all membrane proteins of red blood cells, so they are named because they are located in the third band when they are separated by PAGE electrophoresisErythrocyte membraneThe content is very high, about 25% of the erythrocyte membrane proteinanionTransport function, so band 3 protein is also called“Anion channel". Band 3 protein is composed of two identicalSubunitDimer, each subunit contains 929amino acid, it is a kind ofglycoproteinIt passes through the plasma membrane 12~14 times, so it is a multiple transmembrane protein
4. Band 4.1 protein
It is a globular protein composed of two subunitsMembrane skeletonThe role ofHaemoglobinBinding to promote the fusion of hemoglobin withActinBinding. The band 4.1 protein itself is not linked to actin, because it has no connection site with actin
5. Glycoprotein
Blood group glycoproteinAlso called salivaglycoprotein(sialo glycoprotein), because it is rich insialic acidBlood group glycoprotein is the first to be measuredaminoThere are several types of acid sequence proteins, including A, B, C, D. Blood group glycoproteins B, C, DErythrocyte membraneMedium concentration is lowBlood group glycoprotein AIt is a single transmembrane glycoprotein, composed of 131amino acid compositionIts hydrophilic amino end is exposed on the outside of the membrane, combining 16oligosaccharideSide chainThe basic function of blood group glycoprotein may be that it contains a lot of negative charges in its sialic acid, which prevents red blood cells from accumulating and depositing in blood vessels when passing through narrow blood vessels during circulation
Sealing and connecting the adjacent plasma membraneTight connectionTogether, the molecules in the solution are prevented from penetrating into the body along the cell gap
also calledzonula occludens (zonula occlusens), the main form of closed connection, exists invertebrateEpithelial cells. The junction region is formed by proteinsRidge lineThe ridge line is mainly composed of two transmembrane proteins, claudin and occludin, andPeripheral proteinZO composition. Tight connection has the functions of sealing (preventing the dispersion of soluble substances), isolation (isolating the free end of upper epidermal cells from the membrane protein on the cell membrane of the basal surface) and support
The main function of tight connection is to close the seams between adjacent cells, prevent molecules in solution from infiltrating into the body along the cell gap, thus ensuring the bodyinternal environmentIs relatively stable;Gastrointestinal epithelium, bladder epithelium, brainblood capillaryendotheliumas well asSertoli cells of testisThere are tight connections between them. The latter two constituteBrain blood barrierAnd testis blood barrier, which can protect these important organs and tissues from foreign bodiesSmall moleculeFor example, the tight junction of small intestinal epithelial cells leaks 10 thousand times more Na+than that of bladder epithelium
Anchor connection
Anchoring the skeleton system of adjacent cells or connecting cells to the matrix to form a
Desmosome（desmosome）It is a button like structure formed between adjacent cellsMacula densaconnectIntermediate fibre, in the middleCadherinIt is mainly distributed in tissues under tension, such as skin, mouth, test tube and heart muscle.
Hemidesmosome is located inepithelial cellsthe base levelAndBasement membraneIt differs from desmosomes in that: ① desmosomal plaques are formed only on the inner side of the plasma membrane, and the other side is the basement membrane; ②TransmembraneConnexinbyintegrin Instead of cadherin; ③The attachment proteins in cells areKeratin.
Desmosome exists in tissues that bear strong tension, such as skin and mouth,esophagusStratified scale at the same placeepithelial cellsAnd in the heart muscle (Figure 11-7). A button like structure is formed between adjacent cells, the gap between cell membranes is about 30nm, and there iscytoplasmAttachment proteins, such as plakoglobin,desmoplakin(desmoplakin), forming aMacula densa. There are spotsIntermediate fibreThe nature of intermediate fibers varies with cell type, such as:Keratin silk(keratin fillments), inCardiac myocyteIn, it isdesmin filament（desmin filaments）。Desmoglein and desmocollin were found in the middle of desmosomes.Therefore, the intermediate fibers in the adjacent cells form the penetrability through the cytoplasmic spot and cadherinCytoskeletonNetwork.
Hemidesmosome is similar to desmosome in structure and located atepithelial cellsBase surface andBasement membrane(Figure 11-9), it differs from desmosomes in that: ① desmosomal plaques are formed only on the inner side of the plasma membrane, and the other side is the basement membrane; ②The transmembrane connexin isintegrin (integrin) instead of cadherin, integrin isExtracellular matrixReceptor protein; ③The attachment proteins in cells areKeratin（keratin）。
Adhesive tape
Adhesive tape(adhesion belt)Encircling cell, generally located at the top and side of epithelial cellsTight connectionBelow.The gap between adjacent cells at the adhesive band is about 15-20 nm.
Adhesive tapeUnder the plasma membrane ofActinBundle,CadherinIt binds to actin bundles through attachment proteins.Therefore, actin in adjacent cellsTowIt weaves a wide network of cadherins and attachment proteins to unite adjacent cells.
Adhesive spot
Adhesive spot(adhesion plaque) is located between cells andExtracellular matrixBetween, viaintegrin (integrin) Connect the actin bundles in cells with the matrix. The plasma membrane at the junction is discoid, which is called adhesive spot.
IntercellularcouplingA pathway that provides signaling substances to provide a specific cell with its“position information"And influence its differentiation according to its location.
tumor cellThe connection between gaps is significantly reduced or disappeared, and the gap connection is similar“tumourInhibitor ".
Cell adhesion molecules are transmembrane glycoproteins, and the molecular structure consists of three parts: ①Extracellular region，Peptide chainOfN-endSection withsugar chain, responsible for the identification with ligands; ②Transmembrane area, mostly a transmembrane area; ③CytoplasmPeptide chainC-endPart, generally small, or directly connected with the skeleton components under the plasma membrane, or with intracellular chemistrySignal moleculeConnect to activatesignal transduction pathway.
Action mode of cell adhesion molecules
The role of cadherin mainly includes the following aspects:
⒈MediationCell junctionIn adult vertebrates, E-cadherin is the main CAM that keeps epithelial cells adhering to each otherAdhesive tapeThe main components ofDesmosomeThe cadherins in are desmoglein and desmocollin
2. Participationcell differentiation, cadherin has a significant effect on the early differentiation of embryonic cells and adult tissues (especially epithelium andNervous tissue）The interaction between embryonic cells (adhesion, separation, migration, and re adhesion) can be determined by regulating the type and quantity of cadherin expression during development, thus affecting the microenvironment of cellsCell differentiation, participating in the process of organ formation
3. Inhibit cell migration in many kinds of cancer tissuescell surfaceThe decrease or disappearance of E-cadherin in the tumor is the premise of invasion and metastasis because the cancer cells are easy to fall off from the tumor. Therefore, E-cadherin is regarded as a metastasis inhibitor
Cadherin
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1. It mediates cell connection. In adult vertebrates, E-cadherin is the main CAM to keep epithelial cells adhering to each other and the main component of adhesive tape
2. It is involved in cell differentiation, and cadherin plays an important role in the early differentiation of embryonic cells and the construction of adult tissues. During development, the interaction between embryonic cells can be determined by regulating the type and quantity of cadherin expression, so as toMicroenvironment, ImpactCell differentiation, participating in the process of organ formation
3. Inhibition of cell migration. E-cadherin on the surface of cells in many kinds of cancer tissues decreases or disappears, so that carcinogenic cells are easy to fall off from the tumor, which is the premise of invasion and metastasis. Therefore, E-cadherin is regarded as a metastasis inhibitor
1、 Cadherin
Cadherin belongs to homophilic CAM, and its role depends on Ca 2+. More than 30 kinds of cadherins have been identified and distributed in different tissues
Integrin is mostly heterophileCell adhesion moleculeIts effect depends on Ca2+. It mediates the interaction between cells and between cellsExtracellular matrixAlmost all animal and plant cells express integrin
Relationship between the two
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Cell envelope
There is a layer of sugar rich structure on the surface of animal cells, calledCell envelope(cell coat) orSugar calyx(glycocalyx). Use heavy metalsdyestuffFor example, after ruthenium red staining, the structure with a thickness of about 10~20 nm can be shown under the electron microscope, and the boundary is not very clearglycoproteinandGlycolipidsProtrusiveOligosaccharide It is actually part of the plasma membrane structure (Figure 4-18)
effect
1. Protective effect: The extracellular coating has a certain protective effect. Removing the extracellular coating will not directly damage the plasma membrane
2，Cell recognition: Cell recognition and compositionCell envelopeOfoligosaccharideThe oligosaccharide chain extends from the plasma membrane glycoprotein and glycolipid, and the monosaccharide residues of each cell oligosaccharide chain have a certain order of arrangementcell surfaceIt is the "fingerprint" of cells, forming the molecular basis for cell recognition. At the same time, there are specific receptors for oligosaccharides on the cell surface, which haveIdentification functionTherefore, cell recognition is essentiallymolecular recognitionTownes and Holtfreter (1955)Gastrula3BlastodermAfter the cells of,Mixed cultureThe results showed that the cells of the three embryonic layers gathered by themselves and participated in the formation of their source embryonic layersEmbryonic cellandmiceAfter the embryo cells are dispersed, they are mixed and cultured. All kinds of cells still gather according to the source tissue, but the aggregated cells only have tissueSpecificityWithout speciesResolution.
3. Determine blood type: blood type is actually different red blood cellssurface antigen There are more than 20 blood types in people. The most basic blood type is AB0redCell plasma membraneThe glycosphingolipid on it is AB0Blood group systemOfBlood group antigen, blood typeImmunological activitySpecificityThe molecular basis ofsugar chainOfGlycosylform.A. The sugar chain structure of B and O blood group antigens is basically the same, but the sugar group at the end of the sugar chain is differentType A bloodThe end of the sugar chain of N - acetylgalactose;Type B bloodIs galactose;Both AB type sugar groups are present,Type O bloodThese two sugar groups are missing
Extracellular matrix
concept
Extracellular matrix,ECM）It refers to being distributed in extracellular spaceCell secretionA network of proteins and polysaccharides
function
Adhesion;support;Change cell microenvironment;Signal function
Collagen is the main water-insoluble fibrin in extracellular matrix.
Type: more than 20 types have been found, and I~IV types are well known.
molecular structure：Collagen fiberBasics ofStructural unityesProcollagenIn the interior of the collagen microscope, the tropocollagen molecules are alternately arranged in parallel in 1/4, and the adjacent tropocollagen proteins are connected head to tail. The N end and C end of the parallel arrangement are covalently bound.
Proteoglycan in cartilage is one of the largest macromolecules, which endows cartilage with gel like properties and anti deformation ability;
Proteoglycans can be regarded as extracellular hormone enrichment andRepository, can work with multiplegrowth factorCombine to complete signal transmission.
Elastin consists of two typessmall peptideOne is that hydrophobic peptides endow molecules with elasticity;The other short peptide is alanine rich andLysineResidueAlpha helix, responsible for the formation of cross-linking between adjacent molecules
Elastin is highly hydrophobicGlycosylationProtein has two obvious characteristics:
Increase cell strength and provide support function;
Functions of information repository: produce multipleoligosaccharinAs a signaling substance, or resistant to diseases, pests, orCell growthAnd developmental signaling substances.
hemicellulose（hemicellulose）:xylose, galactose andglucoseHighly branched polysaccharide with equal composition
MediationMicrofiberTo connect or mediate microfibers and other matrix components（Pectin）Connection
Pectin（pectin）: It contains a large amount of sugar with negative charge, combines with cations such as Ca2+, and is highly hydrated to form gel
Pectin and hemicellulose are connected horizontally and participate incell wallFormation of complex grid structure?
Extensin(extensin): glycoprotein, inPrimary wallMedium content can be up to 15%,
Total sugar accounts for about 65%
Lignin(lignin): water insoluble formed by phenol residuesPOLYMER.
participate inSecondary wallForm and usecovalent bondCrosslinking with cell wall polysaccharide greatly increases the strength and anti degradation of cell wall
