Ch. 8: Transport Across The Cell Membrane

NOTES: CH 7 part 2 Transport Across the CellMembrane (7.3-7.5)

The Permeability of the Lipid Bilayer Hydrophobic (nonpolar) molecules,such as hydrocarbons, can dissolvein the lipid bilayer and pass throughthe membrane rapidly Polar molecules, such as sugars, donot cross the membrane easily

Transport proteins: membrane proteins that transport specificmolecules or ions across biologicalmembranes:-may provide hydrophilic tunnel thrumembrane-may bind to a substance and physicallymove it across the membrane-are specific for the substance they move

GLUCOSEBindingRecoveryTransportDissociation

Movement across the cell membrane can be:1) PASSIVE cell does not haveto expend energy2) ACTIVE energy-requiringprocess during which atransport protein pumps amolecule across amembrane, against itsconc. gradient; isenergetically “uphill”

7.3 - Passive Transport:DIFFUSION net movement of a substancedown a concentration gradient-results from KE of molecules-results from random molecularmovement-continues until equilibrium isreached (molecules continueto move but there is no netdirectional movement)

Molecules of dyeMembrane (cross section)WATERNet diffusionDiffusion of one soluteNet diffusionEquilibrium

Net diffusionNet diffusionDiffusion of two solutesNet diffusionNet diffusionEquilibriumEquilibrium

7.3 - Passive Transport:OSMOSIS diffusion of water across a selectivelypermeable membrane; water moves downits concentration gradient-continues until equil. is reached-at equil. water moleculesmove in both directionsat same rate

OUTSIDETHE CELLINSIDETHE CELL

Effects of Osmosis on Water Balance The direction of osmosis is determinedonly by a difference in total soluteconcentration Water diffuses across a membrane fromthe region of lower solute concentrationto the region of higher soluteconcentration

Lowerconcentrationof solute (sugar)Higherconcentrationof sugarH2OSelectivelypermeable membrane: sugar molecules cannot passthrough pores, butwater molecules canOsmosisSame concentrationof sugar

Water Balance of Cells Without Walls Isotonic solution: solute concentration is thesame as that inside the cell; no net watermovement across the plasma membrane Hypertonic solution: solute concentration isgreater than that inside the cell; cell loseswater Hypotonic solution: solute concentration isless than that inside the cell; cell gains waterWATER MOVES FROM HYPO TO HYPERTONIC!!!

Animals and other organisms without rigid cellwalls have osmotic problems in either a hypertonicor hypotonic environment To maintain their internal environment, suchorganisms must have adaptations forosmoregulation, the control of water balance The protist Paramecium, which is hypertonic to itspond water environment, has a contractile vacuolethat acts as a pump

Filling vacuoleContracting vacuole50 µm50 µm

Water Balance of Cells with Walls Cell walls help maintain water balance A plant cell in a hypotonic solution swells until thewall opposes uptake; the cell is now turgid (firm) If a plant cell and its surroundings are isotonic,there is no net movement of water into the cell; thecell becomes flaccid (limp), and the plant may wilt In a hypertonic environment, plant cells losewater; eventually, the membrane pulls away fromthe wall, a usually lethal effect called plasmolysis

RECAP: In cells with cell walls: in a HYPERTONIC environment,PLASMOLYSIS occurs; cells shriveland usually die in a HYPOTONIC environment, watermoves into cell, causing it to swell; cellbecomes more TURGID.

Hypotonic solutionIsotonic solutionHypertonic solutionAnimalcellH2OH2OTurgid ellH2OH2OH2OPlasmolyzed

7.3 - Passive Transport:FACILITATED DIFFUSION diffusion of solutes across a membrane, withthe help of transport proteins;

Facilitated Diffusion:Passive Transport Aided by Proteins Channel proteins provide corridors thatallow a specific molecule or ion to crossthe membrane Carrier proteins undergo a subtlechange in shape that translocates thesolute-binding site across the membrane

EXTRACELLULARFLUIDChannel proteinSoluteCYTOPLASM

Carrier proteinSolute

7.4 - Active transport uses energy tomove solutes against their gradients Facilitated diffusion is still passive becausethe solute moves down its concentrationgradient Some transport proteins, however, canmove solutes against their concentrationgradients

The Need for Energy in Active Transport Active transport moves substances againsttheir concentration gradient Active transport requires energy, usually in theform of ATP Active transport isperformed by specificproteins embeddedin the membranes

Passive transportActive transportATPDiffusionFacilitated diffusion

Examples of Active Transport protein“pumps”:1) Sodium-Potassium Pump:-actively pumps Na ions out / K ions in-in every pump cycle, 3 Na leave and 2K enter cell-Na and K are moved against theirgradients (both concentration and electricpotential!)

EXTRACELLULAR [Na ] highFLUID[K ] lowNa Na Na Na Na Na Na Na CYTOPLASM[Na ] low[K ] highNa Cytoplasmic Na bonds tothe sodium-potassium pumpPATPPADPNa binding stimulatesphosphorylation by ATP.Phosphorylation causesthe protein to change itsconformation, expelling Na to the outside.Loss of the phosphaterestores the protein’soriginal conformation.K is released and Na sites are receptive again;the cycle repeats.PPExtracellular K bindsto the protein, triggeringrelease of the phosphategroup.

OUTSIDEINSIDE

Maintenance of Membrane Potentialby Ion Pumps Membrane potential is the voltagedifference across a membrane Two combined forces, collectively called theelectrochemical gradient, drive thediffusion of ions across a membrane:-A chemical force (the ion’s concentrationgradient)-An electrical force (the effect of themembrane potential on the ion’s movement)

Membrane Potential: voltage acrossmembrane; in most cells the interior isnegatively charged w/respect to outside-favors diffusion of cations intocell and anions out of cell Electrochemical Gradient: diffusiongradient resulting from the combinedeffects of membrane potential and conc.gradient

**The Na -K pump maintains themembrane potential HOW?**

ELECTROGENIC PUMPS: An electrogenic pump is a transportprotein that generates the voltage acrossa membrane The main electrogenic pump of plants,fungi, and bacteria is a PROTON PUMP.

Examples of Active Transport protein“pumps”:2) Proton Pump: pumps protons (H ions) outof the cell, creating a proton gradient (protonsare more concentrated outside themembrane than inside) this is anenergetically “uphill” process!-protons then diffuse back into cell-the force of the proton pushing back throughthe membrane is used to power theproduction of ATP

––ATPEXTRACELLULARFLUID H H Proton pumpH – H H – CYTOPLASM–H

3) Cotransport / Coupled Channels:process where a single ATP-powered pumpactively transports one solute and indirectlydrives the transport of other solutes againsttheir conc. gradients.-Example: plants use a proton pumpcoupled with sucrose-H transport toload sucrose into specialized cells

– H ATPH – H Proton pumpH – H – H Sucrose-H cotransporterDiffusionof H H –– Sucrose

7.5 - Bulk transport across the plasmamembrane occurs by exocytosis andendocytosis Small molecules and water enter or leavethe cell through the lipid bilayer or bytransport proteins Large molecules, such as polysaccharidesand proteins, cross the membrane viavesicles

BULK TRANSPORT:EXOCYTOSIS & ENDOCYTOSIS transport of large molecules (e.g.proteins and polysaccharides) across cellmembrane

Exocytosis*exporting macromoleculesby fusion of vesicles w/theplasma membrane*vesicle buds from ER orGolgi and migrates to plasmamembrane*used by secretory cells toexport products (e.g. insulin inpancreas)Endocytosis

ExocytosisEndocytosis*exporting macromoleculesby fusion of vesicles w/theplasma membrane*importing macromoleculesby forming vesicles derivedfrom plasma membrane*vesicle buds from ER orGolgi and migrates to plasmamembrane*vesicle forms in localizedregion of plasma membrane*used by secretory cells toexport products (e.g. insulin inpancreas)*used by cells to incorporateextracellular substances (e.g.macrophage engulfs abacterial cell)

EXOCYTOSIS In exocytosis, transport vesiclesmigrate to the membrane, fuse with it,and release their contents

ENDOCYTOSIS In endocytosis, the cell takes inmacromolecules by forming vesicles fromthe plasma membrane Endocytosis is a reversal of exocytosis,involving different proteins

Three types of Endocytosis:1) Phagocytosis: part of the cell membrane engulfslarge particles or even entire cells (“cell eating”)

Three types of Endocytosis:2) Pinocytosis: part of the cellmembrane engulfs smalldissolved substances or fluiddroplets in vesicles (“celldrinking”)

Three types of Endocytosis:3) Receptor-Mediated Endocytosis: importing of specificmacromolecules by receptor proteins bind to a specificsubstance which triggers the inward budding of vesiclesformed from COATED PITS (how mammalian cells take upcholesterol)

RECEPTOR-MEDIATED ENDOCYTOSISCoat proteinReceptorCoatedvesicleCoatedpitLigandA coated pitand a coatedvesicle proteinPlasmamembrane0.25 µm

Passive transport ATP Active transport. Examples of Active Transport protein “pumps”: 1) Sodium-Potassium Pump: -actively pumps Na ions out / K ions in-in every pump cycle, 3 Na leave and 2 . 7.5 - Bulk transport