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TRANSPORT ACROSS THE CELL MEMBRANE 4/2/2011

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MEMBRANEPHYSIOLOGY

Dr.K.S.BasnetAssociateProfessor

2067

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Transport through cell

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Transport through cell membranes

The phospholipid bilayer is a good barrier around cells, especially towater soluble molecules. However, for the cell to survive somema er a s nee o e a e o en er an eave e ce .

There are 4 basic mechanisms:

1. DIFFUSION and FACILITATED DIFFUSION

2. OSMOSIS

3. ACTIVE TRANSPORT

4. BULK TRANSPORT

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Diffusionofliquids

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Diffusion is the net movement of molecules (or ions)from a region of their high concentration to a region oftheir lower concentration.

emo ecu esmove ownaconcentrat ongra ent.

Moleculeshavekineticenergy,whichmakesthemmoveabout

randomly.

theyareevenlyspreadout.

Thisiswhenthereisnonetmovementofmoleculesfromeither

side.

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DIFFUSIONDiffusion is a PASSIVE process which means no energy is used to make

the molecules move, they have a natural kinetic energy.

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Diffusion of Bromine

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Diffusion of Bromine

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Diffusion through a membrane

Cell membrane

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Inside cell Outside cell

Diffusion through a membrane

Cell membrane

diffusion

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Inside cell Outside cell

Diffusion through a membrane

Cell membrane

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Inside cell Outside cell

EQUILIBRIUM

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Facilitated Diffusion through a membrane

Cell membrane

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Inside cell Outside cell

Protein channel

Facilitated Diffusion through a membrane

Cell membrane

diffusion

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Inside cell Outside cell

Protein channel

Facilitated Diffusion through a membrane

Cell membrane

diffusion

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Inside cell Outside cell

Protein channel

EQUILIBRIUM

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Facilitated Diffusion:Molecules will randomly move through the opening like pore, by diffusion. Thisrequires no energy, it is a PASSIVE process. Molecules move from an area of high

concentration to an area of low conc.

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Facilitated diffusion

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Osmosis

The diffusion of water from an area ofig concentration o water mo ecu es

(high water potential) to an area of lowconcentration of water (low waterpotential) across a partially permeable

mem rane.

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Osmosis

Cell membranepartially

ermeable.Sugar molecule

DILUTE SOLUTIONCONCENTRATED SOLUTION

.

VERY Low conc. ofwater molecules.High water potential.

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Inside cell Outside cellVERY High conc. ofwater molecules.High water potential.

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Osmosis

Cell membranepartially

ermeable..

Low conc. of watermolecules. Highwater potential.

OSMOSIS

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Inside cell Outside cellHigh conc. of watermolecules. Highwater potential.

Osmosis

Cell membranepartially

ermeable..

OSMOSIS

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Inside cell Outside cell

EQUILIBRIUM. Equal water concentration on each side. Equalwater potential has been reached. There is no net movement of

water

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Endocytosis isthecasewhenamoleculecausesthecellmembranetobulgeinward,forminga

vesicle.Phagocytosis isthetypeofendocytosis whereanentirecellisengulfed.Pinocytosis is

whentheexternalfluidisengulfed.Receptormediatedendocytosis occurswhenthematerialto

betransportedbindstocertainspecificmoleculesinthemembrane.Examplesincludethe

transportofinsulinandcholesterolintoanimalcells.

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Cotransport alsousestheprocessofdiffusion.Inthiscaseamoleculethatismovingnaturally

intothecellthroughdiffusionisusedtodraganothermoleculeintothecell.Inthisexample

glucosehitchesaridewithsodium.

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ReceptorProteins

Theseproteinsareusedinintercellularcommunication.Inthisanimationyoucanseethea

hormonebindingtothereceptor.Thiscausesthereceptorproteinreleaseasignaltoperform

some

action.

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Cotransport alsousestheprocessofdiffusion.Inthiscaseamoleculethatismovingnaturally

intothecellthroughdiffusionisusedtodraganothermoleculeintothecell.Inthisexample

glucosehitchesaridewithsodium.

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Thesearecarrierproteins.Theydonotextendthroughthemembrane.Theybondand

dragmoleculesthroughthebilipid layerandreleasethemontheoppositeside.

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Vesiclemediatedtransport

Vesicles andvacuoles thatfusewiththecellmembranemaybeutilizedtoreleaseortransport

chemicalsoutofthecellortoallowthemtoenteracell.Exocytosis isthetermappliedwhen

transportisoutofthecell.

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CellMembrane Function Endocytosis

Thecellmembranecanalsoengulfstructuresthataremuchtoolargetofitthroughtheporesinthemembraneproteinsthis

processisknownasendocytosis.Inthisprocessthemembraneitselfwrapsaroundtheparticleandpinchesoffavesicle

insidethecell.Inthisanimationanameba engulfsafoodparticle.

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