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Chapter 05Lecture and
Animation Outline
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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
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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
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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
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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 +
Extracellularenvironment
CytosolCytosolLow [Na
+]High [K +]
High [Na+
]Low [K +]
2 K +
ADP + P iATP
Extracellularenvironment
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.
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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
Extracellularenvironment
The vesicle fuses withthe plasma membraneand releases the cargoto the outside.
4
The proteincoat is shed.
3
The vesicleis releasedfrom theGolgi,carrying cargomolecules.
2
A vesicle loadedwith cargo isformed as a proteincoat wraps aroundit.
1
Exocytosis
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Cytosol
Extracellularenvironment
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.
4
5
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
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II Membranes and Transport in Hippocampus
Cell Membranes: Overview
Membrane Structure
Transport Mechanisms
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Cell Membranes: Summary
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