cell membrane transport [Compatibility Mode]

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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


Cell membrane

diffusion

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Cell membrane



EQUILIBRIUM



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Facilitated Diffusion through a membrane

Cell membrane



Protein channel


Cell membrane

diffusion



Protein channel


Cell membrane

diffusion



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.


Facilitated diffusion


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.


Osmosis

Cell membranepartially

ermeable.Sugar molecule

DILUTE SOLUTIONCONCENTRATED SOLUTION

.

VERY Low conc. ofwater molecules.High water potential.


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



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Osmosis


ermeable..

Low conc. of watermolecules. Highwater potential.

OSMOSIS


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

Osmosis


ermeable..

OSMOSIS



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.


ReceptorProteins

Theseproteinsareusedinintercellularcommunication.Inthisanimationyoucanseethea

hormonebindingtothereceptor.Thiscausesthereceptorproteinreleaseasignaltoperform

some

action.


Cotransport alsousestheprocessofdiffusion.Inthiscaseamoleculethatismovingnaturally

intothecellthroughdiffusionisusedtodraganothermoleculeintothecell.Inthisexample

glucosehitchesaridewithsodium.


Thesearecarrierproteins.Theydonotextendthroughthemembrane.Theybondand

dragmoleculesthroughthebilipid layerandreleasethemontheoppositeside.




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Vesiclemediatedtransport

Vesicles andvacuoles thatfusewiththecellmembranemaybeutilizedtoreleaseortransport

chemicalsoutofthecellortoallowthemtoenteracell.Exocytosis isthetermappliedwhen

transportisoutofthecell.


CellMembrane Function Endocytosis

Thecellmembranecanalsoengulfstructuresthataremuchtoolargetofitthroughtheporesinthemembraneproteinsthis

processisknownasendocytosis.Inthisprocessthemembraneitselfwrapsaroundtheparticleandpinchesoffavesicle

insidethecell.Inthisanimationanameba engulfsafoodparticle.


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