Outline - Membranes

1. Fluid Mosaic Model of Membrane Structure

2. Membrane Proteins

1. Kinds of membrane proteins

2. Membrane protein structure

Single pass

Multi-pass: Channels, Pores & Carriers

3. Transport MechanismsPassive: Diffusion & Facilitated Diffusion

Active: Molecular & Bulk

Membranes

Fig. 6.2 (TEArt)

Fatty acidPhosphorylated

alcohol

Polar(hydrophilic) region

Nonpolar (hydrophobic) region

Fatty acidGLYCEROL

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Fig. 6.3 (TEArt)

Polarhydrophilicheads

Nonpolarhydrophobictails

Polarhydrophilicheads

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Outside Cell

Cytoplasm (inside cell)

Cholesterol Transmembraneproteins

Peripheralprotein

Glycoprotein

CarbohydrateGlycolipid

Phospholipid Bilayer

Fluid Mosaic Model of Cell Membrane

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Cell Membrane Structure

1. Phospholipid bilayer

2. Proteins

3. Carbohydrates

Attached to lipids Glycolipids

Attached to proteins Glycoproteins

4. Cholesterol

Outside

Plasmamembrane

InsideTransporter Cell surface

receptorEnzyme

Cell surface identitymarker

Attachment to thecytoskeleton

Cell adhesion

Membrane Protein Functions

Phospholipids

Polar areasof protein

Nonpolar areas of

protein

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Anchoring Proteins in the Phospholipid

Bilayer

1. Single-Pass… Anchors

2. Multi-Pass

Channels & Pores

Carriers

Structure of Membrane Proteins

Examples of Single Pass ProteinsReceptors, anchors & enzymes Anchors - Cell Surface Markers

Self-recognition

MHC Antigen

Guanylyl cyclase

Second messenger

Protein anchor

Protein receptor

Protein enzyme

Smooth muscle relaxation

NO Nitric oxide substrate

Unique part of protein

cGMP degradation by

phosphdiesteraseV

Fig. 6.12 (TEArt)

Lumpof sugar

Sugarmolecule

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Solute dissolves in a solvent.

Solutes move from a high to a low concentration.

Diffusion

Fig. 6.14 (TEArt)

Ureamolecule Water

molecules

Osmosis is Water Diffusion Across a Semipermeable Membrane

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Shriveled cells Normal cells Cells swell and

eventually burst

Hyperosmoticsolution

Isoosmoticsolution

Hypoosmoticsolution

Direction of Water Diffusion

Fig. 6.15c (TEArt)

Plasmolysis Cell body shrinks

from cell wall

Normal turgid cellTurgor Pressure

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HyperosmoticExternal Solution

Water Diffusion in Plant Cells

IsoosmoticExternal Solution

HypoosmoticExternal Solution

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Maintaining Osmotic BalanceLife in a osmotic environment1. Extrusion

e.g. Contractile Vacuoles in Paramecium

2. Isoosmotic solutionse.g. Blood Protein

3. Live with ite.g. Turgor pressure

Moving Molecules into or out of Cells- Overview of Passive & Active Transport

I. Passive Transport1. Always “down” a concentration gradient

2. Always involves proteins calledA. ChannelsB. CarriersC. Pores… “porins”

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

A. Protein Pumps3. Large molecules transported by vesicles

A. EndocytosisB. Exocytosis

1. Multi-pass proteins create openings in the membrane

Moving Molecules into or out of Cells- Passive Transport

Passive TransportPassive Transport1. Channels1. Channels2. Carriers2. Carriers

Solutemolecule

Transportprotein

Multi-PassProtein

Moving Molecules into or out of Cells- Passive Transport – Example of a Channel

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 Cells- Passive Transport – Carriers & Facilitated Diffusion

Passive transport of1) ions2) Sugars3) amino acids

Outside cell

Inside cell

Examples of Facilitated Diffusion in Red Blood Cells1) Cl- and bicarbonate ions2) Glucose carrier

Pores & Porin Proteins Allow Water and Small Molecules into Cells

Porins are transport channels

1.Allow movement of small moleculesWaterIonsOrganic Wastes

Porin Protein

Pleated folds

2003 Nobel Prize in ChemistryAquaporin Water Channels

Major Sites of Expression Comments

Aquaporin-0 Eye: lens fiber cells Fluid balance within the lens

Aquaporin-1

Red blood cells Osmotic protection

Kidney: proximal tubule Concentration of urine

Eye: ciliary epithelium Production of aqueous humor

Brain: choriod plexus Production of cerebrospinal fluid

Lung: alveolar epithelial cells Alveolar hydration state

Aquaporin-2 Kidney: collecting ducts Mediates antidiuretic hormone activity

Aquaporin-3 *Kidney: collecting ducts Reabsorbtion of water into blood

Trachea: epithelial cells Secretion of water into trachea

Aquaporin-4

Kidney: collecting ducts Reabsorbtion of water

Brain: ependymal cells CSF fluid balance

Brain: hypothalamus Osmosensing function?

Lung: bronchial epithelium Bronchial fluid secretion

Aquaporin-5Salivary glands Production of saliva

Lacrimal glands Production of tears

Aquaporins are Water Channels

1. Proteins allow transport2. Mechanisms of movement through proteins

1. Passive Transport Channels, carriers & pores Simple Diffusion Facilitated Diffusion

2. Active Transport Molecular Transport Bulk Transport

ExocytosisEndocytosis

How do molecules move across membranes?

PPPA

PPPA

Na+

Extracellular

Intracellular

ATP ATP

PPPA

ATP

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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Active Transport – Sodium-Potassium Pump

Fig. 6.19 (TEArt)

Outside cell

Inside cell

Na+

Coupledtransportprotein

Sugar

K+

Na/Kpump

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Active Transport - Cotransport

Example of Glucose Transport From Intestine to Blood

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 of specific molecules

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Bulk Tranport: Exocytosis

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Cytoplasm

Plasmamembrane

Bulk Transport: Endocytosis

Fig. 6.16c (TEArt)

Coated pit Target molecule

Receptor protein

Coated Vesicle

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Carrier-Mediated Endocytosis

Example of Neurotransmitter Movement from Cell to Cell

Reuptake transporter

END

Membranes & Transport