1

TheNeedforEnergyinAc1veTransport

• Ac1vetransport

– movessubstancesagainsttheirconcentra2ongradient

– Requiresenergy

• intheformofATP

– Performedbyspecificproteins

embeddedinthemembranes

Animation: Active TransportAnimation: Active Transport

• Ac2vetransport

– allowscellstomaintainconcentra2ongradientsthatdifferfromtheirsurroundings

• Sodium‐potassiumpump

Ac1veTransport

2

EXTRACELLULARFLUID

[Na+] high[K+] low

[Na+] low[K+] high

Na+

Na+

Na+

Na+

Na+

Na+

CYTOPLASMATP

ADPP

Na+

Na+

Na+

P

3

K+

K+

6

K+

K+

5 4

K+

K+

PP

1

Fig. 7-16-7 Fig. 7-17Passive transport

Diffusion Facilitated diffusion

Active transport

ATP

2

IonPumpsMaintainMembranePoten1al

• Membranepoten1al

– voltagedifferenceacrossamembrane

– Voltageiscreatedbydifferencesinthedistribu2onofposi2veandnega2veions

• Electrochemicalgradient

– Twocombinedforcesdrivediffusionofionsacrossamembrane

• chemicalforce(theion’sconcentra2ongradient)

• electricalforce(theeffectofthemembranepoten2alontheion’smovement)

MembranePoten1al

• Electrogenicpump

• transportproteinthatgeneratesvoltageacrossamembrane

• sodium‐potassiumpump

• majorelectrogenicpumpofanimalcells

• Themainelectrogenicpumpofplants,fungi,andbacteriaisaprotonpump

MembranePoten1alFig. 7-18

EXTRACELLULARFLUID

H+

H+

H+

H+

Proton pump

+

+

+

H+

H+

+

+

H+

–

–

–

–

ATP

CYTOPLASM

–

3

Cotransport:CoupledTransportbyaMembraneProtein

• Cotransport

– occurswhenac2vetransportofasoluteindirectlydrivestransportofanothersolute

• Plantscommonlyusethegradientofhydrogenionsgeneratedbyprotonpumpstodriveac2vetransportofnutrientsintothecell

Fig. 7-19

Proton pump

–

–

–

–

–

–

+

+

+

+

+

+

ATP

H+

H+

H+

H+

H+

H+

H+

H+

Diffusionof H+

Sucrose-H+

cotransporter

Sucrose

Sucrose

BulkTransport:ExocytosisandEndocytosis

• Smallmoleculesandwaterenterorleavethecellthroughthelipidbilayerorbytransportproteins

• Largemolecules,suchaspolysaccharidesandproteins,crossthemembraneinbulkviavesicles

• Bulktransportrequiresenergy

Exocytosis

• Exocytosis

– Transportvesiclesmigratetothemembrane,fusewithit,andreleasetheircontents

– Usedbymanysecretorycellstoexporttheirproducts

Animation: Animation: ExocytosisExocytosis

4

Endocytosis

• Endocytosis

– celltakesinmacromoleculesbyformingvesiclesfromtheplasmamembrane

– reversalofexocytosis,involvingdifferentproteins

• Threetypesofendocytosis:

– Phagocytosis(“cellularea2ng”)

– Pinocytosis(“cellulardrinking”)

– Receptor‐mediatedendocytosisAnimation: Animation: Exocytosis Exocytosis and and Endocytosis Endocytosis IntroductionIntroduction

• Phagocytosis

– Cellengulfsapar2cleinavacuole

– Vacuolefuseswithalysosometodigestthepar2cle

Animation: Animation: PhagocytosisPhagocytosis

TypesofEndocytosis

Fig. 7-20a

PHAGOCYTOSIS

CYTOPLASMEXTRACELLULARFLUID

Pseudopodium

“Food” orother particle

Foodvacuole Food vacuole

Bacterium

An amoeba engulfing a bacteriumvia phagocytosis (TEM)

Pseudopodiumof amoeba

1 µm

• Pinocytosis

– moleculesaretakenupwhenextracellularfluidis“gulped”into2nyvesicles

Animation: Animation: PinocytosisPinocytosis

TypesofEndocytosis

5

Fig. 7-20b

PINOCYTOSIS

Plasmamembrane

Vesicle

0.5 µm

Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM)

• Receptor‐mediatedendocytosis

– bindingofligandstoreceptorstriggersvesicleforma2on

– Ligand

• anymoleculethatbindsspecificallytoareceptorsiteofanothermolecule

Animation: Receptor-Mediated Animation: Receptor-Mediated EndocytosisEndocytosis

TypesofEndocytosis

Fig. 7-20cRECEPTOR-MEDIATED ENDOCYTOSIS

ReceptorCoat protein

Coatedpit

Ligand

Coatprotein

Plasmamembrane

0.25 µm

Coatedvesicle

A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs)

Youshouldnowbeableto:

1. Definethefollowingterms:amphipathicmolecules,aquaporins,diffusion

2. Explainhowmembranefluidityisinfluencedbytemperatureandmembranecomposi2on

3. Dis2nguishbetweenthefollowingpairsorsetsofterms:peripheralandintegralmembraneproteins;channelandcarrierproteins;osmosis,facilitateddiffusion,andac2vetransport;hypertonic,hypotonic,andisotonicsolu2ons

6

4. Explainhowtransportproteinsfacilitatediffusion

5. Explainhowanelectrogenicpumpcreatesvoltageacrossamembrane,andnametwoelectrogenicpumps

6. Explainhowlargemoleculesaretransportedacrossacellmembrane