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Outline - Membranes

1. Fluid Mosaic Model of Membrane Structure

2. Membrane Proteins1. Kinds of membrane proteins2. Membrane protein structure3. Transport MechanismsPassive: Diffusion & Facilitated DiffusionActive: Molecular & Bulk

Plasma Membrane Properties and Structure

Properties:No Free EndsInternal spaceFluid Mosaic Model of Membrane StructureFluid = Phospholipid bilayerMosaic = Embedded Proteins:

Transport proteins: channels and carriers Receptor Proteins: gates, triggers Recognition Proteins: ID Tags

Plasma Membrane Function

• Gate Keeper• Regulation• Communication• Transport• Selectively permeable• Elastic• Protection

Membranes

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Polarhydrophilicheads

Nonpolarhydrophobictails

Polarhydrophilicheads

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Outside Cell

Cytoplasm (inside cell)

Cholesterol Transmembraneproteins

Peripheralprotein

Glycoprotein

CarbohydrateGlycolipid

PhospholipidBilayer

Fluid Mosaic Model of Cell Membrane

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Cell Membrane StructureSummary

1. Phospholipid bilayer

2. ProteinsTransmembraneInterior

3. CarbohydratesAttached to lipids GlycolipidsAttached to proteins Glycoproteins

4. Cholesterol

Outside

Plasmamembrane

InsideTransporter Cell surface

receptorEnzyme

Cell surface identitymarker

Attachment to thecytoskeleton

Cell adhesion

Six Functions of Membrane Proteins

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Phospholipids

Polar areasof protein

Nonpolar areas of

protein

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Anchoring Proteins in the Phospholipid

Bilayer

Types of Transport

• Passive TransportFollows concentration gradientDoes not require energyDirect or via channels

Examples: Diffusion, Facilitated Diffusion and Osmosis

• Active Transport:Against concentration gradientRequires energy

Bulk Transport• Exocytosis and Endocytosis

Moving Molecules into or out of Cells- Overview of Types of TransportI. Passive Transport

1. Always “down” a concentration gradient2. Always involves proteins called

A. ChannelsB. CarriersC. Pores… “porins”

II. Active Transport1. Always “up” a concentration gradient2. Small molecules transported through

A. Protein Pumps3. Large molecules transported by vesicles

A. EndocytosisB. Exocytosis

Fig. 6.12 (TEArt)

Lumpof sugar Sugar

molecule

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Solute dissolves in a solvent.Solutes move from a high to a low concentration.

Diffusion

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Fig. 6.14 (TEArt)

Solutemolecule Water

molecules

Osmosis is Water Diffusion Across a Semipermeable Membrane

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Shriveled cells Normal cells Cells swell & burst

Hypertonicsolution

Isotonicsolution

Hypotonicsolution

Direction of Water Diffusion

Water diffuses out Water diffuses out and in = equilibrium

Water diffuses in

External environment of a cell can vary

Fig. 6.15c (TEArt)

PlasmolysisCell body shrinks

from cell wall

Normal cellTurgor Pressure

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HypertonicExternal Solution

Water Diffusion in Plant CellsIsotonicExternal Solution

HypotonicExternal Solution

Life in a osmotic environment1. Extrusion

e.g. Contractile Vacuoles in Paramecium

2. Isotonic solutionse.g. Blood Protein

3. Live with ite.g. Turgor pressure

Maintaining Osmotic Balance

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1. Multi-pass proteins create openings in the membrane

Moving Molecules into or out of CellsPassive Transport

Passive TransportPassive Transport1. Channels1. Channels2. Carriers2. Carriers3. Pores3. Pores

Solutemolecule

Transportprotein

Multi-PassProtein

Moving Molecules into or out of Cells- Passive Transport – Channels

Passive transport of1) Water-soluble molecules2) Ions

Selectivity filter

Inside cell

Outside cell

K+ ion

K+ ion channel

Side view Top view

Moving Molecules into or out of CellsPassive Transport – Carriers

Outside cell

Inside cell

Passive transport of1) ions2) Sugars3) amino acids

Facilitated Diffusion in Red Blood Cells1) Cl- and bicarbonate ions2) Glucose carrier Porins are transport channels

1.Allow movement of small moleculesWaterIonsOrganic Wastes

2003 Nobel Prize in ChemistryAquaporin Water Channels

Moving Molecules into or out of CellsPassive Transport – Pores

Pleated folds

Porin Protein

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PPPA

PPPANa+

Extracellular

Intracellular

ATP ATP

PPPAATP

PPA

P

ADP

1. Protein in membrane bindsintracellular sodium.

2. ATP phosphorylates proteinwith bound sodium.

3. Phosphorylation causesconformational change inprotein, allowing sodium to leave.

PPA

P

ADP

4. Extracellular potassiumbinds to exposed sites.

K+

PPA

P

ADP+Pi

5. Binding of potassium causesdephosphorylation of protein.

6. Dephosphorylation ofprotein triggers change backto original conformation,potassium moves into cell,and the cycle repeats.

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Example: Active Transport – Sodium-Potassium Pump

Animation

Fig. 6.19 (TEArt)

Outside cell

Inside cell

Na+

Coupledtransportprotein

Sugar

K+

Na/Kpump

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Active Transport - Cotransport

Animation

Bulk Transport Across Membranes• Exocytosis - discharge of material from vesicles at

the cell surface

• Endocytosis - enveloping food– phagocytosis - particulate material– pinocytosis - liquid– receptor-mediated - transport specific molecules

Animation

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Bulk Tranport: Exocytosis

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Cytoplasm

Plasmamembrane

Bulk Transport: EndocytosisCoated pit

ClathrinReceptor protein

Coated vesicle

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Carrier-Mediated Endocytosis

END

Membranes & Transport