Ch5 Cell Membranes

8/12/2019 Ch5 Cell Membranes

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Chapter 5

Membrane Structure,

Synthesis and Transport

Membrane Structure

Fluidity of Membranes

Synthesis of Membrane Components

Membrane TransportTransport Proteins

Exocytosis and Endocytosis

Key Concepts:


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M embrane: The flui d mosaic modelCharacter istics of the membrane:

Fluidity: membrane is fluid (why?)Selective permeability: membrane is selectively permeable (why?)Components of the membrane

M embrane transport: Passive tr ansport: Passive diffusion & Facilitated diffusion

Active transport: Primary active transport & Secondary activetransportTr ansport of larger molecules: Exocytosis & Endocytosis:

Endocytosis: Receptor mediated endocytosis, Pinocytosis & Phagocytosis

F unction and types of transport proteins:ChannelsTransporters

Types of transporters: Uniporter, Symporter, Antiporter

Specif ic examples of tr ansport: Sodium Potassium Pump


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The framework of the membrane is the phospholipidbilayer

Phospholipids are amphipathic moleculesHydrophobic (water-fearing) region faces in

Hydrophilic (water-loving) region faces out

Membranes also contain proteins and carbohydratesThe two leaflets (halves of bilayer) are asymmetrical, with different amounts of each component

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Membrane Structure


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Cytosol

Glycoprotein

GlycolipidCarbohydrate

Polar

Nonpolar

Polar

Integralmembraneprotein

Phospholipidbilayer

Cholesterol(found only inanimal cells)

Peripheral membraneproteins

Cytosolicleaflet

Extracellularleaflet

Extracellular environment


HO


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Proteins bound to membranes

Integral or intrinsic membrane proteinsTransmembrane proteins

Region(s) are physically embedded in the hydrophobicportion of the phospholipid bilayer

Lipid-anchored proteins An amino acid of the protein is covalently attached to a lipid

Peripheral or extrinsic membrane proteinsNoncovalently bound either to integral membraneproteins that project out from the membrane,or to polar head groups of phospholipids

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Approximately 25% of All Genes

Encode Transmembrane ProteinsMembranes are important medically as well as biologically

Computer programs can be used to predict the number oftransmembrane proteins

Estimated percentage of membrane proteins is substantial:20 30% of all genes may encode transmembrane proteins

This trend is found throughout all domains of life includingarchaea, bacteria, and eukaryotes

Function of many genes is unknown study may providebetter understanding and better treatments for disease


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Freeze Fracture Electron Microscopy (FFEM)

A specialized form of TEM usedto analyze the interior of thephospholipid bilayer

Sample is frozen in liquid nitrogenand fractured with a knife

Due to the weakness of the centralmembrane, the leaflets separateinto the P face (Protoplasmic facenext to the cytosol) and the E face(Extracellular face)

Can provide significant detailabout membrane protein form


Transmembrane protein

Direction of fracture

P face exposed

P face E face

E face exposed

E face

P face

Extracellularleaflet

Cytosolicleaflet

The McGraw-Hill Companies, Inc./Al Tesler, photographer


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(a) Spontaneous lipid movements (b) Lipid movement via flippase

Lateral movement

Rotational movement

Flip-flop

Flippase

ATP ADP + P i


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Lipid rafts

Certain lipids associate strongly with eachother to form lipid rafts

A group of lipids floats together as a unitwithin the larger sea of lipids in the membrane

Composition of lipid raft is different than restof membrane

High concentration of cholesterol

Unique set of membrane proteins

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Factors affecting fluidity

Length of fatty acyl tailsShorter acyl tails are less likely to interact, whichmakes the membrane more fluid

Presence of double bonds Double bond creates a kink in the fatty acyl tail,making it more difficult for neighboring tails to

interact and making the bilayer more fluidPresence of cholesterol

Cholesterol tends to stabilize membranes

Effects vary depending on temperature 16


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Experiments on lateral movement

Larry Frye and Michael Edidin experiment, 1970

Demonstrated the lateral movementof membrane proteins

Mouse and human cells were fused

Temperature treatment 0 C or 37 C

Mouse membrane protein H-2 fluorescently labeledCells at 0 C label stays on mouse side

Cells at 37 C label moves over entire fused cell

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Not all integral membrane proteinscan move

Depending on the cell type, 10 70% of membraneproteins may be restricted in their movement

Integral membrane proteins may be bound tocomponents of the cytoskeleton , which restricts theproteins from moving laterally

Membrane proteins may be also attached to moleculesthat are outside the cell , such as the interconnectednetwork of proteins that forms the extracellular matrix

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Cytoskeletal filament

Linker protein

Cytosol

Extracellular matrix

Fiber in the extracellularmatrix

Plasmamembrane


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Glycosylation

Process of covalently attaching a carbohydrateto a protein or lipid

Glycolipid carbohydrate to lipidGlycoprotein carbohydrate to protein

Can serve as recognition signals for othercellular proteins

Often play a role in cell surface recognition

Helps protect proteins from damage

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The plasma membrane is selectively permeable

Allows the passage of some ions and moleculesbut not others

This structure ensures that: Essential molecules enter

Metabolic intermediates remain

Waste products exit

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Membrane Transport


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Ways to move across membranes

Passive transportRequires no input of energy down or with gradient

Passive diffusion Diffusion of a solute througha membrane without transport protein

Facilitated diffusion Diffusion of a solute througha membrane with the aid of a transport protein

Active transportRequires energy up or against gradient

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(b) Facilitated diffusion passive transport (c) Active transport(a) Diffusion passive transport

ATP

ADP + P i


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Phospholipid bilayer barrier

Barrier to hydrophilic molecules and ions dueto hydrophobic interior

Rate of diffusion depends on chemistry of solute andits concentration

Example: Diethylurea diffuses 50 times faster throughthe bilayer than urea, due to nonpolar ethyl groups

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NH NH

O

C

O

CNH2 NH2 CH 3 CH 2 CH 2 CH 3Urea Diethylurea



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Tonicity

IsotonicEqual water and solute concentrations on eitherside of the membrane

HypertonicSolute concentration is higher (and waterconcentration lower) on one side of the membrane

HypotonicSolute concentration is lower (and waterconcentration higher) on one side of the membrane

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Osmosis

Water diffuses through a membrane from anarea with more water to an area with lesswater

If the solutes cannot move, water movementcan make the cell shrink or swell as waterleaves or enters the cell

Osmotic pressure the tendency for waterto move into any cell

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2%sucrosesolution

1 liter ofdistilled water

1 liter of10% sucrose

solution

1 liter of2% sucrose

solution

HypertonicConditions

IsotonicConditions

aka: crenate


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Isotonic

Hypertonic

Hypotonic

Outside the cell Inside the cell

The solution andcell are isotonic

The solution ishypertonic to the cell

The solution ishypotonic to the cell


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Osmosis in animal cells

aka crenation


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Osmosis in plant cells

aka:plasmolysi


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Which solution is hypertonic to the other?the cell contents

the environment


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Transport proteins

Transport proteins enable biological membranesto be selectively permeable (will allow diffusion

or not)

2 classesChannels (porins)

Transporters


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Channel Proteins

Form an openpassageway,normally polar inside.

i.e. Aquaporins


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Animal cells mustmaintain a balancebetween extracellularand intracellular soluteconcentrations to maintaintheir size and shape

Crenation shrinkageof a cell in a hypertonicsolution

Osmotic Lysis swellingand bursting of a cell in a

hypotonic solution 39

Osmosis in animal cells

Cells are initially inan isotonic solution.

Cells undergo shrinkage(crenation) because waterexits the cell.

Cells swell and mayundergo osmotic lysisbecause water is takeninto the cell.

Place inhypertonicsolution.

Place inhypotonicsolution.

Cellsmaintainnormalshape.

H2O

Red blood cell

H2O

(a) Osmosis in animal cells



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A cell wall prevents majorchanges in cell size

Turgor pressure pushesplasma membrane againstcell wall

Maintains shape and size

Plasmolysis plants wiltingbecause water leaves plantcells

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Osmosis in plant cellsCell is initially in anisotonic solution.

Place inhypertonicsolution.

Place inhypotonicsolution.

Cellsmaintainnormalshape.

Volume inside the plasmamembrane shrinks, and themembrane pulls away fromthe cell wall (plasmolysis)due to the exit of water.

A small amount of watermay enter the cell, butthe cell wall preventsmajor expansion.

H2OH2O

(b) Osmosis in plant cells

Vacuole Plant cell



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Freshwater protists likeP a r a m e c i u m have to survivein a strongly hypotonicenvironment

To prevent osmotic lysis,contractile vacuoles takeup water and discharge itoutside the cell

Using vacuoles to removeexcess water maintains a

constant cell volume 41

Osmosis in freshwater protists


60 m

60 m

Filledcontractilevacuole

Vacuoleafter

expellingwater

(all): Carolina Biological Supply/Visuals Unlimited


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Agre Discovered That Osmosis Occurs MoreQuickly in Cells with Transport Proteins That

Allow the Facilitated Diffusion of WaterWater can passively diffuse across plasma membranes,but some cell types allow water to move across themembrane much faster than predicted

Peter Agre and colleagues first identified a protein thatwas abundant in red blood cells, bladder, and kidney cells

Channel-forming Integral Membrane Protein, 28kDa(CHIP28)

Unlike controls, frog oocytes that expressed CHIP28swelled up and lysed when put in a hypotonic medium

CHIP28 was renamed Aquaporin, since it forms achannel that allows water to pass through the membrane


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Oocyte rupturingOocyte

Control CHIP28

1

2

3

4 THE DATA

RNA polymeraseCHIP28 mRNA

Frog oocyte CHIP28 protein

CHIP28 protein

Ribosome

Control

Nucleus Cytosol

Control CHIP28

3 5 minutes

Add an enzyme (RNA polymerase) andnucleotides to a test tube that containsmany copies of the CHIP28 gene. Thisresults in the synthesis of many copiesof CHIP28 mRNA.

Inject the CHIP28 mRNA into frog eggs(oocytes). Wait several hours to allowtime for the mRNA to be translated intoCHIP28 protein at the ER membrane andthen moved via vesicles to the plasmamembrane.

Place oocytes into a hypotonic mediumand observe under a light microscope.As a control, also place oocytes thathave not been injected with CHIP28mRNA into a hypotonic medium andobserve by microscopy .

CHIP28 protein isinserted into theplasma membrane.

CHIP28

DNA

Experimental level Conceptual level

Enzymesand nucleotides

CHIP28mRNA

Courtesy Dr. Peter Agre. From GM Preston, TP Carroll, WP Guggino, P Agre (1992), Appearance of water channels in Xenopus oocytes expressing red cell CHIP28 protein, Science, 256(5055):385 7


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Transport proteins are transmembrane proteinsthat provide a passageway for the movement

of ions and hydrophilic molecules acrossmembranes

Two classes based on type of movementChannels

Transporters

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Transport Proteins


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Channels

Form an open passageway for the

direct diffusion of ionsor molecules acrossthe membrane

Most are gated

example: Aquaporins

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Gate opened

Gateclosed

When a channel is open, a solutedirectly diffuses through thechannel to reach the other side ofthe membrane.


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Transporters

Also known as carriers

Conformational change

transports solute acrossmembrane

Principal pathway foruptake of organic

molecules, such assugars, amino acids,and nucleotides

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Conformational change

Hydrophilic pocket

Solute

For transport to occur, a solute binds in a hydrophilic pocketexposed on one side of the membrane. The transporter thenundergoes a conformational change that switches theexposure of the pocket to the other side of the membrane,

where the solute is then released.



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UniporterSingle molecule or ion

Symporter orcotransporter

Two or more ions ormolecules transportedin same direction

AntiporterTwo or more ions ormolecules transportedin opposite directions

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Transporter typesCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

A single solute moves inone direction.

(a) Uniporter

Two solutes move in thesame direction.

(b) Symporter

Two solutes move in

opposite directions.

(c) Antiporter


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Question

A cell is placed in an hypertonic solution. Which way will thewater move?

a. Into the cell

b. Out of the cellc. No net movement


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Question

Gated channels which open when a chemical binds to it is a. Ligand gated channel

b. Leakage channel

c. Mechanically gated channel

d. Voltage gated channel

e. All can open in response to chemical binding


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Question

What type of transport protein can move 2 or more differentmolecules in opposite directions?

a. Uniporter

b. Antiporterc. Symporter

d. Multiporter

e. Diporter


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Active transportMovement of a solute across a membraneagainst its gradient from a region of lowconcentration to higher concentration

Energetically unfavorable and requires theinput of energy

Primary active transport uses a pump

Directly uses energy to transport solute

Secondary active transport uses a differentgradient

Uses a pre-existing gradient to drive transport


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Extracellularenvironment

(a) Primary active transport (b) Secondary active transport

ATP ADP + P iSucrose

H+Cytosol

A H + /sucrose symporter uses the H +

gradient to transport sucrose against aconcentration gradient into the cell.

A pump actively exportsH+ against a gradient.


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ATP-driven ion pumps generateion electrochemical gradients

Na + /K +-ATPase Actively transports Na + and K + against their gradientsusing the energy from ATP hydrolysis

3 Na + are exported for every 2 K + imported into cellAntiporter ions move in opposite directions

Electrogenic pump exports one net positive (+) charge

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3 Na +

Na + /K+-ATPase

Nerve cell

(a) Active transport bythe Na + / K +-ATPase (b) Mechanism of pumping

E1

E2

E2

E1

ADPP

P i

3 Na +

2 K +


CytosolCytosolLow [Na

+]High [K +]

High [Na+

]Low [K +]

2 K +

ADP + P iATP


2 K +

3 Na +

ATP

3 Na + bind from cytosol.ATP is hydrolyzed. ADPis released and phosphate(P) is covalently attachedto the pump, switching itto the E2 conformation.

3 Na + arereleased outsideof the cell.

2 K + bind fromoutside of thecell.

Phosphate (P i) is released,and the pump switchesto the E1 conformation.2 K + are released intocytosol. The processrepeats.

1 2 43


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Used to transport large molecules such as proteinsand polysaccharides

Exocytosis

Material inside the cell packaged into vesicles andexcreted into the extracellular medium

EndocytosisPlasma membrane invaginates (folds inward) to form a

vesicle that brings substances into the cellThree types of endocytosis:

Receptor-mediated endocytosisPinocytosisPhagocytosis

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Exocytosis and Endocytosis


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Plasma membrane

Golgi

apparatus

Proteincoat

Vesicle

Cargo

Cytosol


The vesicle fuses withthe plasma membraneand releases the cargoto the outside.

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The proteincoat is shed.

3

The vesicleis releasedfrom theGolgi,carrying cargomolecules.

2

A vesicle loadedwith cargo isformed as a proteincoat wraps aroundit.

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Exocytosis


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Cytosol


Receptor

CargoInvagination

Coat protein

Lysosome

Cargo is releasedinto the cytosol.

Cargo binds to receptor and receptors aggregate.The receptors cause coat proteins to bind to the

surrounding membrane. The plasma membraneinvaginates as coat proteins cause a vesicle toform.

1The vesicle isreleased in the cell.

2

The proteincoat is shed.

3 The vesicle fuses withan internal organellesuch as a lysosome.

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Receptor-mediated endocytosis


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I Chapter 4: Cell Membrane Structure and Function in Audesirkhttp://wps.prenhall.com/esm_audesirk_bloe_7/17/4453/1140182.cw/index.html Media Activities

4.1 Membrane Structure and Transport Pre-quizActivityPost-qui z4.2 OsmosisPre-quiz ActivityPost-quiz

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Cell Membranes: Overview

Membrane Structure

Transport Mechanisms

Membrane Proteins

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http://www.hippocampus.org/HippoCampus/Biology;jsessionid=A1F9977D639B10E4E8DA26617C1E0CB0

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http://wps.prenhall.com/esm_audesirk_bloe_7/17/4453/1140182.cw/index.htmlhttp://www.hippocampus.org/HippoCampus/Biology;jsessionid=A1F9977D639B10E4E8DA26617C1E0CB0http://www.hippocampus.org/HippoCampus/Biology;jsessionid=A1F9977D639B10E4E8DA26617C1E0CB0http://wps.prenhall.com/esm_audesirk_bloe_7/17/4453/1140182.cw/index.html

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Ch5 Cell Membranes