1

CellMembraneBIOL222

Ch.6

Overview:PlasmaMembrane

•  Plasmamembrane

•  boundarythatseparatesthelivingcellfromitssurroundings

•  Selec4vepermeability

•  Allowanceofsomesubstancestocrossmoreeasilythanothers

CellMembraneModel

•  Fluidmosaicmodel

•  Membraneisafluidstructurewitha“mosaic”ofvariousproteinsembeddedinit

•  Phospholipids

•  mostabundantlipidinthe

plasmamembrane

•  amphipathicmolecules

•  containhydrophobic

andhydrophilicregions

2

Fig.7-2

Hydrophilic head

WATER

Hydrophobic tail

WATER

TheFluidityofMembranes•  Phospholipids

•  Canmovewithinthebilayer

•  Mostofthelipids,andsomeproteins,driHlaterally

•  Rarelydoesamoleculeflip-floptransverselyacrossthemembrane

Lateral movement (∼107 times per second)

Flip-flop (∼ once per month)

Fig.7-7

Fibers of extracellular matrix (ECM)

Glyco- protein

Microfilaments of cytoskeleton

Cholesterol

Peripheral proteins

Integral protein

CYTOPLASMIC SIDE OF MEMBRANE

GlycolipidEXTRACELLULAR SIDE OF MEMBRANE

Carbohydrate

PLAY

3

•  Peripheralproteins

•  boundtothesurfaceofthemembrane

•  Integralproteins

•  Penetratethehydrophobiccore

•  transmembraneproteins

•  Integralproteinsthatspanthemembrane

•  Hydrophobicregionsofanintegralprotein

•  consistofoneormorestretchesofnonpolaraminoacids

•  oHencoiledintoalphahelices

MembraneProteins

N-terminus

C-terminus

α HelixCYTOPLASMIC SIDE

EXTRACELLULAR SIDE

•  SixmajorfuncNonsofmembrane

proteins:

•  Transport

•  EnzymaNcacNvity

•  SignaltransducNon

•  Cell-cellrecogniNon

•  Intercellularjoining

•  ASachmenttothe

cytoskeletonandextracellular

matrix(ECM)

MembraneProteins

(a) Transport

ATP

(b) Enzymatic activity

Enzymes

(c) Signal transductionSignal transduction

Signaling molecule

Receptor

(d) Cell-cell recognition

Glyco- protein

(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

MembraneCarbohydrates

•  Membranecarbohydrates

–  Usedbycellstorecognizeeachother

•  BybindingrecogniNonproteinstosurfacecarbohydrates

•  Glycolipids

•  Carbohydratescovalentlybondedtolipids

•  Glycoproteins

•  Carbohydratesbondedtoproteins

•  varyamongspecies,individuals,andevencell

typesinanindividual

4

SynthesisandSidednessofMembranes•  MembraneshavedisNnctinsideand

outsidefaces

•  AsymmetricaldistribuNon

•  Proteins

•  Lipids

•  Associatedcarbohydrates

•  DeterminedwhenthemembraneisbuiltbytheERandGolgiapparatus

•  ComponentsontheinsideofERorGolgivesicle

•  enduponexteriorofcellmembrane

ER1

Transmembrane glycoproteins

Secretory protein

Glycolipid

2Golgi apparatus

Vesicle

3

4

Secreted protein

Transmembrane glycoprotein

Plasma membrane:Cytoplasmic faceExtracellular face

Membrane glycolipid

ThePermeabilityoftheLipidBilayer

•  Hydrophobic(nonpolar)molecules

•  Dissolveinthelipidbilayerandpassthroughthemembranerapidly

•  Suchashydrocarbons

•  Butonlyverysmallones

•  Polarmolecules

•  Donotcrossthemembraneeasily

•  Repelledbyinnerhydrophobicregion

•  Suchassugars

TransportProteins

•  Transportproteins

•  Allowpassageofhydrophilic

substancesacrossthemembrane

•  channelproteins

•  hydrophilicchannelthatcertain

moleculesorionscanuseasa

tunnel

•  Aquaporins

•  Facilitatethepassageof

water

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•  Carrierproteins

•  BindtomoleculesandchangeshapetoshuSlethemacrossthemembrane

•  Specificforthesubstanceitmoves

TransportProteins

Transport

•  Diffusion

•  Tendencyformoleculesto

spreadoutevenlyintothe

availablespace

•  DuetoBrownian

movement

•  Dynamicequilibrium

•  Asmanymoleculescross

onewayascrossinthe

otherdirecNon

Fig.7-11Molecules of dye Membrane (cross section)

WATER

Net diffusion Net diffusion Equilibrium

(a) Diffusion of one solute

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

(b) Diffusion of two solutes

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•  Passivetransport

•  Noenergyinputrequired

•  SimpleDiffusion

•  Substancesmove(diffuse)down

theirconcentra4ongradient

•  differenceinconcentraNonof

asubstancefromoneareato

another

TypesofTransport:Passive–SimpleDiffusion

TypesofTransport:Passive-Osmosis

•  Osmosis

•  diffusionofwateracrossa

selecNvelypermeablemembrane

•  fromtheregionoflower

soluteconcentraNon

•  totheregionofhigher

soluteconcentraNon

•  Waterwillfollowthesolutes

Lower concentration of solute (sugar)

Fig.7-12

H2O

Higher concentration of sugar

Selectively permeable membrane

Same concentration of sugar

Osmosis

7

WaterBalanceofCellsWithoutWalls•  Tonicity

•  abilityofasoluNontocauseacelltogainorlosewater

•  IsotonicsoluNon

•  SoluteconcentraNonoutsideisthesameasthatinsidethecell

•  Nonetwatermovementacrosstheplasmamembrane

•  HypertonicsoluNon

•  SoluNonwithhighersoluteconcentraNon

•  HypotonicsoluNon

•  SoluNonwithlowersoluteconcentraNon

Hypotonic solution

(a) Animal cell

(b) Plant cell

H2O

Lysed

H2O

Turgid (normal)

H2O

H2O

H2O

H2O

Normal

Isotonic solution

Flaccid

H2O

H2O

Shriveled

Plasmolyzed

Hypertonic solution

•  HypertonicorhypotonicenvironmentscreateosmoNcproblemsfororganisms

•  Osmoregula4on

•  Controlofwaterbalance

•  NecessaryadaptaNonforlifeinsuchenvironments

•  TheproNstParamecium

•  Hypertonictoitspondwaterenvironment

•  HasacontracNlevacuolethatactsasapump

WaterBalanceofCellsWithoutWalls

Filling vacuole 50 µm

(a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm.

Contracting vacuole

(b) When full, the vacuole and canals contract, expelling fluid from the cell.

WaterBalanceofCellswithWalls

•  PlantcellinahypotonicsoluNon

•  SwellsunNlthewallopposes

uptake

•  Cellisnowturgid(firm)

•  Ifisotonic

•  Nonetmovementof

waterintothecell

•  thecellbecomes

flaccid(limp)

•  Plantmaywilt

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•  Plasmolysis

•  Hypertonicenvironment

•  Plantcellslosewater

•  membranepulls

awayfromthe

wall

•  usually

lethal

WaterBalanceofCellswithWalls

TypesofTransport:Passive-FacilitatedDiffusion

•  Facilitateddiffusion

•  transportproteinsfacilitate(enable)passivemovementofmoleculesacrosstheplasmamembrane

•  Channelproteins

•  corridorsthatallowaspecificmoleculeoriontocrossthemembrane

•  Aquaporinsforfacilitateddiffusionofwater

•  IonchannelsthatopenorcloseinresponsetoasNmulus(gatedchannels)

EXTRACELLULAR FLUID

Channel protein

(a) A channel protein

Solute CYTOPLASM

Solute Carrier protein

(b) A carrier protein

TypesofTransport:Ac4veTransport

•  Ac4vetransport

•  movessubstancesagainsttheirconcentraNongradient

•  Requiresenergy

•  intheformofATP

•  Performedbyspecificproteins

embeddedinthemembranes

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•  AcNvetransport

•  allowscellstomaintainconcentraNongradientsthatdifferfromtheirsurroundings

•  Sodium-potassiumpump

TypesofTransport:Ac4veTransport

2

EXTRACELLULAR

FLUID [Na+] high [K+] low

[Na+] low [K+] high

Na+ Na+

Na+

Na+ Na+

Na+

CYTOPLASM ATP

ADP P

Na+ Na+

Na+

P 3

K+

K+ 6

K+

K+

5 4

K+

K+

P P

1

Fig.7-16-7

IonPumpsMaintainMembranePoten4al

•  Membranepoten4al

•  Voltagedifferenceacrossamembrane

•  VoltageiscreatedbydifferencesinthedistribuNonofposiNveandnegaNveions

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•  Electrogenicpump

•  transportproteinthatgenerates

voltageacrossamembrane

•  sodium-potassiumpump

•  majorelectrogenicpump

ofanimalcells

•  protonpump

•  Mainelectrogenicpump

•  ofplants,fungi,andbacteria

MembranePoten4al

EXTRACELLULAR FLUID

H+

H+

H+

H+

Proton pump

+

+

+

H+

H+

+

+

H+

–

–

–

–

ATP

CYTOPLASM

–

TypesofTransport:Ac4veTransport-Exocytosis

•  Exocytosis

•  Transportvesiclesmigratetothemembrane,fusewithit,andreleasetheircontents

•  Usedbysecretorycellstoexporttheirproducts

TypesofTransport:Ac4veTransport-Endocytosis

•  Endocytosis

•  celltakesinmacromolecules

•  byformingvesiclesfromtheplasma

membrane

•  reversalofexocytosis,involvingdifferent

proteins

•  Threetypesofendocytosis:

1.  Phagocytosis(“cellulareaNng”)

2.  Pinocytosis(“cellulardrinking”)

3.  Receptor-mediatedendocytosis

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Fig.7-20a

PHAGOCYTOSIS

CYTOPLASM EXTRACELLULAR FLUID

Pseudopodium

“Food” or other particle

Food vacuole Food vacuole

Bacterium

An amoeba engulfing a bacterium via phagocytosis (TEM)

Pseudopodium of amoeba

1 µm

Ac4veTransport:Endocytosis-Phagocytosis

Fig.7-20b

PINOCYTOSIS

Plasma membrane

Vesicle

0.5 µm

Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM)

Ac4veTransport:Endocytosis-Pinocytosis

Fig.7-20c

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor Coat protein

Coated pit

Ligand

Coat protein

Plasma membrane

0.25 µm

Coated vesicle

A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs)

Ac4veTransport:ReceptorMediatedEndocytosis

12

Youshouldnowbeableto:1.  Definethefollowingterms:amphipathicmolecules,aquaporins,

diffusion

2.  ExplainhowmembranefluidityisinfluencedbytemperatureandmembranecomposiNon

3.  DisNnguishbetweenthefollowingpairsorsetsofterms:peripheralandintegralmembraneproteins;channelandcarrierproteins;osmosis,facilitateddiffusion,andacNvetransport;hypertonic,hypotonic,andisotonicsoluNons

4.  Explainhowtransportproteinsfacilitatediffusion

5.  Explainhowanelectrogenicpumpcreatesvoltageacrossamembrane,andnametwoelectrogenicpumps

6.  Explainhowlargemolecules/substancesaretransportedacrossacellmembrane