Sinh học - Chapter 5: The working cell

Chapter 5The Working Cell0IntroductionSome organisms use energy-converting reactions to produce light in a process called bioluminescence.Many marine invertebrates and fishes use bioluminescence to hide themselves from predators.Scientists estimate that 90% of deep-sea marine life produces bioluminescence.The light is produced from chemical reactions that convert chemical energy into visible light.© 2012 Pearson Education, Inc.Figure 5.0_1Chapter 5: Big IdeasMembrane Structureand FunctionEnergy and the CellHow Enzymes FunctionCellular respirationFigure 5.0_2IntroductionBioluminescence is an example of the multitude of energy conversions that a cell can perform.Many of a cell’s reactionstake place in organelles anduse enzymes embedded in the membranes of these organelles.This chapter addresses how working cells use membranes, energy, and enzymes.© 2012 Pearson Education, Inc.MEMBRANE STRUCTURE AND FUNCTION© 2012 Pearson Education, Inc.5.1 Membranes are fluid mosaics of lipids and proteins with many functionsMembranes are composed ofa bilayer of phospholipids withembedded and attached proteins,in a structure biologists call a fluid mosaic.© 2012 Pearson Education, Inc.5.1 Membranes are fluid mosaics of lipids and proteins with many functionsMany phospholipids are made from unsaturated fatty acids that have kinks in their tails.These kinks prevent phospholipids from packing tightly together, keeping them in liquid form.In animal cell membranes, cholesterol helpsstabilize membranes at warmer temperatures andkeep the membrane fluid at lower temperatures.© 2012 Pearson Education, Inc.Figure 5.1Fibers ofextracellularmatrix (ECM)Enzymatic activityPhospholipidCholesterolCYTOPLASMCYTOPLASMCell-cellrecognitionGlycoproteinIntercellularjunctionsMicrofilamentsof cytoskeletonATPTransportSignaltransductionReceptorSignalingmoleculeAttachment to the cytoskeletonand extracellular matrix (ECM)5.1 Membranes are fluid mosaics of lipids and proteins with many functionsMembrane proteins perform many functions.Some proteins help maintain cell shape and coordinate changes inside and outside the cell through their attachment to the cytoskeleton and extracellular matrix.Some proteins function as receptors for chemical messengers from other cells.Some membrane proteins function as enzymes.© 2012 Pearson Education, Inc.5.1 Membranes are fluid mosaics of lipids and proteins with many functionsSome membrane glycoproteins are involved in cell-cell recognition.Membrane proteins may participate in the intercellular junctions that attach adjacent cells to each other.Membranes may exhibit selective permeability, allowing some substances to cross more easily than others.© 2012 Pearson Education, Inc.5.2 EVOLUTION CONNECTION: Membranes form spontaneously, a critical step in the origin of lifePhospholipids, the key ingredient of biological membranes, spontaneously self-assemble into simple membranes.The formation of membrane-enclosed collections of molecules was a critical step in the evolution of the first cells.© 2012 Pearson Education, Inc.Figure 5.2Water-filledbubble made ofphospholipidsFigure 5.2QWaterWater5.3 Passive transport is diffusion across a membrane with no energy investmentDiffusion is the tendency of particles to spread out evenly in an available space.Particles move from an area of more concentrated particles to an area where they are less concentrated.This means that particles diffuse down their concentration gradient.Eventually, the particles reach equilibrium where the concentration of particles is the same throughout.© 2012 Pearson Education, Inc.5.3 Passive transport is diffusion across a membrane with no energy investmentDiffusion across a cell membrane does not require energy, so it is called passive transport.The concentration gradient itself represents potential energy for diffusion.© 2012 Pearson Education, Inc.Figure 5.3AMolecules of dyeMembranePoresNet diffusionNet diffusionEquilibriumFigure 5.3BNet diffusionNet diffusionNet diffusionNet diffusionEquilibriumEquilibrium5.4 Osmosis is the diffusion of water across a membraneOne of the most important substances that crosses membranes is water.The diffusion of water across a selectively permeable membrane is called osmosis.© 2012 Pearson Education, Inc.5.4 Osmosis is the diffusion of water across a membraneIf a membrane permeable to water but not a solute separates two solutions with different concentrations of solute,water will cross the membrane,moving down its own concentration gradient,until the solute concentration on both sides is equal. © 2012 Pearson Education, Inc.Figure 5.4OsmosisSolute moleculewith cluster ofwater moleculesWatermoleculeSelectivelypermeablemembraneSolutemoleculeH2OLowerconcentrationof soluteHigherconcentrationof soluteEqualconcentrationsof solute5.5 Water balance between cells and their surroundings is crucial to organismsTonicity is a term that describes the ability of a solution to cause a cell to gain or lose water.Tonicity mostly depends on the concentration of a solute on both sides of the membrane.© 2012 Pearson Education, Inc.5.5 Water balance between cells and their surroundings is crucial to organismsHow will animal cells be affected when placed into solutions of various tonicities? When an animal cell is placed intoan isotonic solution, the concentration of solute is the same on both sides of a membrane, and the cell volume will not change,a hypotonic solution, the solute concentration is lower outside the cell, water molecules move into the cell, and the cell will expand and may burst, or a hypertonic solution, the solute concentration is higher outside the cell, water molecules move out of the cell, and the cell will shrink. © 2012 Pearson Education, Inc.5.5 Water balance between cells and their surroundings is crucial to organismsFor an animal cell to survive in a hypotonic or hypertonic environment, it must engage in osmoregulation, the control of water balance.© 2012 Pearson Education, Inc.5.5 Water balance between cells and their surroundings is crucial to organismsThe cell walls of plant cells, prokaryotes, and fungi make water balance issues somewhat different.The cell wall of a plant cell exerts pressure that prevents the cell from taking in too much water and bursting when placed in a hypotonic environment.But in a hypertonic environment, plant and animal cells both shrivel.© 2012 Pearson Education, Inc.Figure 5.5AnimalcellPlantcellTurgid(normal)FlaccidShriveled(plasmolyzed)PlasmamembraneLysedNormalShriveledHypotonic solutionIsotonic solutionHypertonic solutionH2OH2OH2OH2OH2OH2OH2O5.6 Transport proteins can facilitate diffusion across membranesHydrophobic substances easily diffuse across a cell membrane.However, polar or charged substances do not easily cross cell membranes and, instead, move across membranes with the help of specific transport proteins in a process called facilitated diffusion, whichdoes not require energy andrelies on the concentration gradient.© 2012 Pearson Education, Inc.5.6 Transport proteins can facilitate diffusion across membranesSome proteins function by becoming a hydrophilic tunnel for passage of ions or other molecules.Other proteins bind their passenger, change shape, and release their passenger on the other side.In both of these situations, the protein is specific for the substrate, which can be sugars, amino acids, ions, and even water.© 2012 Pearson Education, Inc.5.6 Transport proteins can facilitate diffusion across membranesBecause water is polar, its diffusion through a membrane’s hydrophobic interior is relatively slow.The very rapid diffusion of water into and out of certain cells is made possible by a protein channel called an aquaporin.© 2012 Pearson Education, Inc.Figure 5.6SolutemoleculeTransportprotein5.7 SCIENTIFIC DISCOVERY: Research on another membrane protein led to the discovery of aquaporinsDr. Peter Agre received the 2003 Nobel Prize in chemistry for his discovery of aquaporins.His research on the Rh protein used in blood typing led to this discovery.© 2012 Pearson Education, Inc.Figure 5.75.8 Cells expend energy in the active transport of a soluteIn active transport, a cellmust expend energy tomove a solute against its concentration gradient.The following figures show the four main stages of active transport.© 2012 Pearson Education, Inc.Figure 5.8_s1TransportproteinSoluteSolute binding1Figure 5.8_s2TransportproteinSoluteADPATPPSolute bindingPhosphateattaching21Figure 5.8_s3TransportproteinSoluteADPATPPPProteinchanges shape.Solute bindingPhosphateattachingTransport213Figure 5.8_s4TransportproteinSoluteADPATPPPPProteinchanges shape.Phosphatedetaches.Solute bindingPhosphateattachingTransportProteinreversion43215.9 Exocytosis and endocytosis transport large molecules across membranesA cell uses two mechanisms to move large molecules across membranes.Exocytosis is used to export bulky molecules, such as proteins or polysaccharides.Endocytosis is used to import substances useful to the livelihood of the cell.In both cases, material to be transported is packaged within a vesicle that fuses with the membrane.© 2012 Pearson Education, Inc.5.9 Exocytosis and endocytosis transport large molecules across membranesThere are three kinds of endocytosis.Phagocytosis is the engulfment of a particle by wrapping cell membrane around it, forming a vacuole.Pinocytosis is the same thing except that fluids are taken into small vesicles.Receptor-mediated endocytosis uses receptors in a receptor-coated pit to interact with a specific protein, initiating the formation of a vesicle.Figure 5.9_1PhagocytosisEXTRACELLULARFLUIDCYTOPLASMPseudopodium“Food” orother particleFoodvacuoleFoodbeingingestedFigure 5.9_2PinocytosisPlasma membranePlasma membraneVesicleFigure 5.9_3Receptor-mediated endocytosisPlasma membraneReceptorSpecificmoleculeCoatedpitCoatedvesicleCoat proteinCoatedpitMaterial boundto receptor proteinsENERGY AND THE CELL© 2012 Pearson Education, Inc.5.10 Cells transform energy as they perform workCells are small units, a chemical factory, housing thousands of chemical reactions.Cells use these chemical reactions forcell maintenance,manufacture of cellular parts, andcell replication.© 2012 Pearson Education, Inc.5.10 Cells transform energy as they perform workEnergy is the capacity to cause change or to perform work.There are two kinds of energy.Kinetic energy is the energy of motion.Potential energy is energy that matter possesses as a result of its location or structure.© 2012 Pearson Education, Inc.Figure 5.10FuelEnergy conversionWaste productsGasolineOxygenOxygenGlucoseHeatenergyCombustionKinetic energyof movementEnergy conversion in a carEnergy conversion in a cellEnergy for cellular workCellular respirationATPATPHeatenergyCarbon dioxideCarbon dioxideWaterWater5.10 Cells transform energy as they perform workHeat, or thermal energy, is a type of kinetic energy associated with the random movement of atoms or molecules.Light is also a type of kinetic energy, and can be harnessed to power photosynthesis.© 2012 Pearson Education, Inc.5.10 Cells transform energy as they perform workChemical energy is the potential energy available for release in a chemical reaction. It is the most important type of energy for living organisms to power the work of the cell.© 2012 Pearson Education, Inc.5.10 Cells transform energy as they perform workThermodynamics is the study of energy transformations that occur in a collection of matter.Scientists use the wordsystem for the matter under study andsurroundings for the rest of the universe.© 2012 Pearson Education, Inc.5.10 Cells transform energy as they perform workTwo laws govern energy transformations in organisms. According to thefirst law of thermodynamics, energy in the universe is constant, andsecond law of thermodynamics, energy conversions increase the disorder of the universe.Entropy is the measure of disorder, or randomness.© 2012 Pearson Education, Inc.5.10 Cells transform energy as they perform workCells use oxygen in reactions that release energy from fuel molecules.In cellular respiration, the chemical energy stored in organic molecules is converted to a form that the cell can use to perform work.© 2012 Pearson Education, Inc.5.11 Chemical reactions either release or store energyChemical reactions eitherrelease energy (exergonic reactions) orrequire an input of energy and store energy (endergonic reactions).© 2012 Pearson Education, Inc.5.11 Chemical reactions either release or store energyExergonic reactions release energy.These reactions release the energy in covalent bonds of the reactants.Burning wood releases the energy in glucose as heat and light.Cellular respirationinvolves many steps,releases energy slowly, anduses some of the released energy to produce ATP.© 2012 Pearson Education, Inc.Figure 5.11AReactantsEnergyProductsAmount ofenergyreleasedPotential energy of molecules5.11 Chemical reactions either release or store energyAn endergonic reactionrequires an input of energy andyields products rich in potential energy.Endergonic reactionsbegin with reactant molecules that contain relatively little potential energy butend with products that contain more chemical energy.© 2012 Pearson Education, Inc.Figure 5.11BReactantsEnergyProductsAmount ofenergyrequiredPotential energy of molecules5.11 Chemical reactions either release or store energyPhotosynthesis is a type of endergonic process.Energy-poor reactants, carbon dioxide, and water are used.Energy is absorbed from sunlight.Energy-rich sugar molecules are produced.© 2012 Pearson Education, Inc.5.11 Chemical reactions either release or store energyA living organism carries out thousands of endergonic and exergonic chemical reactions.The total of an organism’s chemical reactions is called metabolism.A metabolic pathway is a series of chemical reactions that eitherbuilds a complex molecule or breaks down a complex molecule into simpler compounds.© 2012 Pearson Education, Inc.5.11 Chemical reactions either release or store energyEnergy coupling uses theenergy released from exergonic reactions to driveessential endergonic reactions,usually using the energy stored in ATP molecules.© 2012 Pearson Education, Inc.ATP, adenosine triphosphate, powers nearly all forms of cellular work.ATP consists ofthe nitrogenous base adenine,the five-carbon sugar ribose, andthree phosphate groups.5.12 ATP drives cellular work by coupling exergonic and endergonic reactions© 2012 Pearson Education, Inc.5.12 ATP drives cellular work by coupling exergonic and endergonic reactionsHydrolysis of ATP releases energy by transferring its third phosphate from ATP to some other molecule in a process called phosphorylation.Most cellular work depends on ATP energizing molecules by phosphorylating them.© 2012 Pearson Education, Inc.Figure 5.12A_s1AdeninePPPPhosphategroupATP:AdenosineTriphosphateRiboseFigure 5.12A_s2ADP:AdenosineDiphosphatePPPEnergyH2OHydrolysisRiboseAdeninePPPPhosphategroupATP:AdenosineTriphosphate5.12 ATP drives cellular work by coupling exergonic and endergonic reactionsThere are three main types of cellular work:chemical,mechanical, andtransport.ATP drives all three of these types of work.© 2012 Pearson Education, Inc.Figure 5.12BATPATPATPADPADPADPPPPPPPPPPChemical workMechanical workTransport workReactantsMotorproteinSoluteMembrane proteinProductMolecule formedProtein filament movedSolute transported5.12 ATP drives cellular work by coupling exergonic and endergonic reactionsATP is a renewable source of energy for the cell.In the ATP cycle, energy released in an exergonic reaction, such as the breakdown of glucose,is used in an endergonic reaction to generate ATP.© 2012 Pearson Education, Inc.Figure 5.12CEnergy fromexergonicreactionsEnergy forendergonicreactionsATPADPPHydrolysisPhosphorylationHOW ENZYMES FUNCTION© 2012 Pearson Education, Inc.5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriersAlthough biological molecules possess much potential energy, it is not released spontaneously.An energy barrier must be overcome before a chemical reaction can begin.This energy is called the activation energy (EA).© 2012 Pearson Education, Inc.5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriersWe can think of EAas the amount of energy needed for a reactant molecule to move “uphill” to a higher energy but an unstable state so that the “downhill” part of the reaction can begin.One way to speed up a reaction is to add heat, which agitates atoms so that bonds break more easily and reactions can proceed butcould kill a cell.© 2012 Pearson Education, Inc.Figure 5.13AActivationenergy barrierReactantProductsWithout enzymeWith enzymeReactantProductsEnzymeActivationenergy barrierreduced byenzymeEnergyEnergyFigure 5.13A_1Activationenergy barrierReactantProductsWithout enzymeEnergyFigure 5.13A_2Activationenergy barrierreduced byenzymeReactantProductsWith enzymeEnergyEnzymeFigure 5.13QReactantsProductsEnergyProgress of the reactionabc5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriersEnzymesfunction as biological catalysts by lowering the EA needed for a reaction to begin,increase the rate of a reaction without being consumed by the reaction, andare usually proteins, although some RNA molecules can function as enzymes.© 2012 Pearson Education, Inc.5.14 A specific enzyme catalyzes each cellular reactionAn enzymeis very selective in the reaction it catalyzes andhas a shape that determines the enzyme’s specificity.The specific reactant that an enzyme acts on is called the enzyme’s substrate.A substrate fits into a region of the enzyme called the active site.Enzymes are specific because their active site fits only specific substrate molecules.© 2012 Pearson Education, Inc.5.14 A specific enzyme catalyzes each cellular reactionThe following figure illustrates the catalytic cycle of an enzyme.© 2012 Pearson Education, Inc.Figure 5.14_s11Enzyme(sucrase)Active siteEnzyme availablewith empty activesiteFigure 5.14_s221Enzyme(sucrase)Active siteEnzyme availablewith empty activesiteSubstrate(sucrose)Substrate bindsto enzyme withinduced fitFigure 5.14_s3321Enzyme(sucrase)Active siteEnzyme availablewith empty activesiteSubstrate(sucrose)Substrate bindsto enzyme withinduced fitSubstrate isconverted toproductsH2OFigure 5.14_s44321Products arereleasedFructoseGlucoseEnzyme(sucrase)Active siteEnzyme availablewith empty activesiteSubstrate(sucrose)Substrate bindsto enzyme withinduced fitSubstrate isconverted toproductsH2O5.14 A specific enzyme catalyzes each cellular reactionFor every enzyme, there are optimal conditions under which it is most effective.Temperature affects molecular motion.An enzyme’s optimal temperature produces the highest rate of contact between the reactants and the enzyme’s active site.Most human enzymes work best at 35–40ºC.The optimal pH for most enzymes is near neutrality.© 2012 Pearson Education, Inc.5.14 A specific enzyme catalyzes each cellular reactionMany enzymes require nonprotein helpers called cofactors, whichbind to the active site andfunction in catalysis.Some cofactors are inorganic, such as zinc, iron, or copper.If a cofactor is an organic molecule, such as most vitamins, it is called a coenzyme.© 2012 Pearson Education, Inc.5.15 Enzyme inhibitors can regulate enzyme activity in a cellA chemical that interferes with an enzyme’s activity is called an inhibitor.Competitive inhibitorsblock substrates from entering the active site andreduce an enzyme’s productivity.© 2012 Pearson Education, Inc.5.15 Enzyme inhibitors can regulate enzyme activity in a cellNoncompetitive inhibitorsbind to the enzyme somewhere other than the active site, change the shape of the active site, andprevent the substrate from binding.© 2012 Pearson Education, Inc.Figure 5.15ASubstrateEnzymeAllosteric siteActive siteNormal binding of substrateCompetitiveinhibitorNoncompetitiveinhibitorEnzyme inhibition5.15 Enzyme inhibitors can regulate enzyme activity in a cellEnzyme inhibitors are important in regulating cell metabolism.In some reactions, the product may act as an inhibitor of one of the enzymes in the pathway that produced it. This is called feedback inhibition.© 2012 Pearson Education, Inc.Figure 5.15BFeedback inhibitionStartingmoleculeProductEnzyme 1Enzyme 2Enzyme 3Reaction 1Reaction 2Reaction 3ABCD5.16 CONNECTION: Many drugs, pesticides, and poisons are enzyme inhibitorsMany beneficial drugs act as enzyme inhibitors, includingIbuprofen, inhibiting the production of prostaglandins,some blood pressure medicines,some antidepressants,many antibiotics, and protease inhibitors used to fight HIV.Enzyme inhibitors have also been developed as pesticides and deadly poisons for chemical warfare.© 2012 Pearson Education, Inc.Figure 5.16