Y khoa, y dược - Cells: The living units

3 Cells: The Living Units: Part BTwo types of active processes:Active transportVesicular transportBoth use ATP to move solutes across a living plasma membraneMembrane Transport: Active ProcessesRequires carrier proteins (solute pumps)Moves solutes against a concentration gradientTypes of active transport:Primary active transportSecondary active transportActive TransportEnergy from hydrolysis of ATP causes shape change in transport protein so that bound solutes (ions) are “pumped” across the membranePrimary Active TransportSodium-potassium pump (Na+-K+ ATPase)Located in all plasma membranesInvolved in primary and secondary active transport of nutrients and ionsMaintains electrochemical gradients essential for functions of muscle and nerve tissuesPrimary Active TransportFigure 3.10Extracellular fluid K+ is released from the pump proteinand Na+ sites are ready to bind Na+ again.The cycle repeats. Binding of Na+ promotesphosphorylation of the protein by ATP.Cytoplasmic Na+ binds to pump protein.Na+Na+-K+ pumpK+ releasedATP-binding siteNa+ boundCytoplasmATPADPPK+ K+ binding triggers release of thephosphate. Pump protein returns to itsoriginal conformation. Phosphorylation causes the protein tochange shape, expelling Na+ to the outside.Extracellular K+ binds to pump protein.Na+ releasedK+ boundPK+PPi123456Figure 3.10 step 1Extracellular fluidCytoplasmic Na+ binds to pump protein.Na+Na+-K+ pumpATP-binding siteCytoplasmK+1Figure 3.10 step 2 Binding of Na+ promotesphosphorylation of the protein by ATP.Na+ boundATPADPP2Figure 3.10 step 3 Phosphorylation causes the protein tochange shape, expelling Na+ to the outside.Na+ releasedP3Figure 3.10 step 4Extracellular K+ binds to pump protein.PK+4Figure 3.10 step 5 K+ binding triggers release of thephosphate. Pump protein returns to itsoriginal conformation.K+ boundPi5Figure 3.10 step 6 K+ is released from the pump proteinand Na+ sites are ready to bind Na+ again.The cycle repeats.K+ released6Figure 3.10Extracellular fluid K+ is released from the pump proteinand Na+ sites are ready to bind Na+ again.The cycle repeats. Binding of Na+ promotesphosphorylation of the protein by ATP.Cytoplasmic Na+ binds to pump protein.Na+Na+-K+ pumpK+ releasedATP-binding siteNa+ boundCytoplasmATPADPPK+ K+ binding triggers release of thephosphate. Pump protein returns to itsoriginal conformation. Phosphorylation causes the protein tochange shape, expelling Na+ to the outside.Extracellular K+ binds to pump protein.Na+ releasedK+ boundPK+PPi123456Depends on an ion gradient created by primary active transportEnergy stored in ionic gradients is used indirectly to drive transport of other solutesSecondary Active TransportCotransport—always transports more than one substance at a timeSymport system: Two substances transported in same directionAntiport system: Two substances transported in opposite directionsSecondary Active TransportFigure 3.11 The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient for Na+ entry into the cell. As Na+ diffuses back across the membrane through a membrane cotransporter protein, it drives glucose against its concentration gradientinto the cell. (ECF = extracellular fluid)Na+-glucosesymporttransporterloadingglucose fromECFNa+-glucosesymport transporterreleasing glucoseinto the cytoplasmGlucoseNa+-K+pumpCytoplasmExtracellular fluid12Figure 3.11 step 1 The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient for Na+ entry into the cell.Na+-K+pumpCytoplasmExtracellular fluid1Figure 3.11 step 2 The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient for Na+ entry into the cell. As Na+ diffuses back across the membrane through a membrane cotransporter protein, it drives glucose against its concentration gradientinto the cell. (ECF = extracellular fluid)Na+-glucosesymporttransporterloadingglucose fromECFNa+-glucosesymport transporterreleasing glucoseinto the cytoplasmGlucoseNa+-K+pumpCytoplasmExtracellular fluid12Transport of large particles, macromolecules, and fluids across plasma membranesRequires cellular energy (e.g., ATP)Vesicular TransportFunctions:Exocytosis — transport out of cell Endocytosis — transport into cellTranscytosis — transport into, across, and then out of cellSubstance (vesicular) trafficking—transport from one area or organelle in cell to anotherVesicular TransportInvolve formation of protein-coated vesiclesOften receptor mediated, therefore very selectiveEndocytosis and TranscytosisFigure 3.12 Coated pit ingestssubstance.Protein-coatedvesicledetaches. Coat proteins detachand are recycled toplasma membrane. Uncoated vesicle fuseswith a sorting vesiclecalled an endosome. Transportvesicle containingmembrane componentsmoves to the plasmamembrane for recycling.Fused vesicle may (a) fusewith lysosome for digestionof its contents, or (b) deliverits contents to the plasmamembrane on theopposite side of the cell(transcytosis).Protein coat(typicallyclathrin)Extracellular fluidPlasmamembraneEndosomeLysosomeTransportvesicle(b)(a)Uncoatedendocytic vesicleCytoplasm123456Figure 3.12 step 1 Coated pit ingestssubstance.Protein coat(typicallyclathrin)Extracellular fluidPlasmamembraneCytoplasm1Figure 3.12 step 2 Coated pit ingestssubstance.Protein-coatedvesicledetaches.Protein coat(typicallyclathrin)Extracellular fluidPlasmamembraneCytoplasm12Figure 3.12 step 3 Coated pit ingestssubstance.Protein-coatedvesicledetaches. Coat proteins detachand are recycled toplasma membrane. Protein coat(typicallyclathrin)Extracellular fluidPlasmamembraneCytoplasm123Figure 3.12 step 4 Coated pit ingestssubstance.Protein-coatedvesicledetaches. Coat proteins detachand are recycled toplasma membrane. Uncoated vesicle fuseswith a sorting vesiclecalled an endosome.Protein coat(typicallyclathrin)Extracellular fluidPlasmamembraneEndosomeUncoatedendocytic vesicleCytoplasm1234Figure 3.12 step 5 Coated pit ingestssubstance.Protein-coatedvesicledetaches. Coat proteins detachand are recycled toplasma membrane. Uncoated vesicle fuseswith a sorting vesiclecalled an endosome.Protein coat(typicallyclathrin)Extracellular fluidPlasmamembraneEndosomeTransportvesicleUncoatedendocytic vesicleCytoplasm12345 Transportvesicle containingmembrane componentsmoves to the plasmamembrane for recycling.Figure 3.12 step 6 Coated pit ingestssubstance.Protein-coatedvesicledetaches. Coat proteins detachand are recycled toplasma membrane. Uncoated vesicle fuseswith a sorting vesiclecalled an endosome.Fused vesicle may (a) fusewith lysosome for digestionof its contents, or (b) deliverits contents to the plasmamembrane on theopposite side of the cell(transcytosis).Protein coat(typicallyclathrin)Extracellular fluidPlasmamembraneEndosomeLysosomeTransportvesicle(b)(a)Uncoatedendocytic vesicleCytoplasm123456 Transportvesicle containingmembrane componentsmoves to the plasmamembrane for recycling.Phagocytosis—pseudopods engulf solids and bring them into cell’s interiorMacrophages and some white blood cells EndocytosisFigure 3.13aPhagosome(a)PhagocytosisThe cell engulfs a large particle by forming pro-jecting pseudopods (“false feet”) around it and en-closing it within a membrane sac called a phagosome. The phagosome is combined with a lysosome. Undigested contents remain in the vesicle (now called a residual body) or are ejected by exocytosis. Vesicle may or may not be protein-coated but has receptors capable of binding to microorganisms or solid particles.Fluid-phase endocytosis (pinocytosis)—plasma membrane infolds, bringing extracellular fluid and solutes into interior of the cell Nutrient absorption in the small intestine EndocytosisFigure 3.13bVesicle(b)PinocytosisThe cell “gulps” drops of extracellular fluid containing solutes into tiny vesicles. No receptors are used, so the process is nonspecific. Most vesicles are protein-coated.Receptor-mediated endocytosis — clathrin - coated pits provide main route for endocytosis and transcytosisUptake of enzymes low-density lipoproteins, iron, and insulin EndocytosisFigure 3.13cVesicleReceptor recycledto plasma membrane(c)Receptor-mediatedendocytosisExtracellular substances bind to specific receptor proteins in regions of coated pits, enabling the cell to ingest and concentrate specific substances (ligands) in protein-coated vesicles. Ligands may simply be released inside the cell, or combined with a lysosome to digest contents. Receptors are recycled to the plasma membrane in vesicles.Examples: Hormone secretion Neurotransmitter release Mucus secretion Ejection of wastes ExocytosisFigure 3.14a1 The membrane-bound vesicle migrates to the plasma membrane.2 There, proteinsat the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins).The process of exocytosisExtracellularfluidPlasma membraneSNARE (t-SNARE)SecretoryvesicleVesicleSNARE(v-SNARE) Molecule tobe secretedCytoplasmFusedv- andt-SNAREs3 The vesicleand plasma membrane fuse and a pore opens up.4 Vesiclecontents are released to the cell exterior.Fusion pore formedAlso see Table 3.2Summary of Active ProcessesProcessEnergy SourceExamplePrimary active transportATPPumping of ions across membranesSecondary active transportIon gradientMovement of polar or charged solutes across membranesExocytosisATPSecretion of hormones and neurotransmittersPhagocytosisATPWhite blood cell phagocytosisPinocytosisATPAbsorption by intestinal cellsReceptor-mediated endocytosisATPHormone and cholesterol uptake