Chapter 7:Chapter 7:
Membrane Structure andMembrane Structure and
FunctionFunction
Overview: Life at the EdgeOverview: Life at the Edge
• The plasma membrane is the boundary that The plasma membrane is the boundary that separates the living cell from its separates the living cell from its surroundingssurroundings
The plasma membrane exhibits The plasma membrane exhibits selective selective permeabilitypermeability, allowing some substances to , allowing some substances to cross it more easily than otherscross it more easily than others
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Cellular membranes are fluid mosaics Cellular membranes are fluid mosaics of lipids and proteinsof lipids and proteins
Phospholipids are the most abundant lipid in Phospholipids are the most abundant lipid in the plasma membranethe plasma membrane
Phospholipids are Phospholipids are amphipathicamphipathic moleculesmolecules, , containing hydrophobic and hydrophilic containing hydrophobic and hydrophilic regionsregions
The The fluid mosaic model fluid mosaic model states that a states that a membrane is a fluid structure with a membrane is a fluid structure with a “mosaic” of various proteins embedded in it“mosaic” of various proteins embedded in it
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Hydrophilichead
WATER
Hydrophobictail
WATER
Phospholipidbilayer
Hydrophobic regionsof protein
Hydrophilicregions of protein
(a) Movement of phospholipids
Lateral movement(107 times per second)
Flip-flop( once per month)
• Phospholipids in the plasma membrane can move within the bilayer
As temperatures cool, membranes switch As temperatures cool, membranes switch from a fluid state to a solid statefrom a fluid state to a solid state
The temperature at which a membrane The temperature at which a membrane solidifies depends on the types of lipidssolidifies depends on the types of lipids
Membranes must be fluid to work properly; Membranes must be fluid to work properly; they are usually about as fluid as salad oilthey are usually about as fluid as salad oil
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(b) Membrane fluidity
Fluid
Unsaturated hydrocarbontails with kinks
Viscous
Saturated hydro-carbon tails
• Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids
At warm temperatures cholesterol restrains At warm temperatures cholesterol restrains movement of phospholipidsmovement of phospholipids
At cool temperatures, it maintains fluidity by At cool temperatures, it maintains fluidity by preventing tight packingpreventing tight packing
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Cholesterol affects in membraneCholesterol affects in membrane
Cholesterol
Cholesterol within the animal cell membrane
Membrane Proteins and Their Membrane Proteins and Their FunctionsFunctions
A membrane is a collage of different A membrane is a collage of different proteins embedded in the fluid matrix of the proteins embedded in the fluid matrix of the lipid bilayerlipid bilayer
Proteins determine most of the membrane’s Proteins determine most of the membrane’s specific functionsspecific functions
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Animation: Animation: Plasma Membrane
Fig. 7-7Fig. 7-7
Fibers ofextracellularmatrix (ECM)
Glyco-protein
Microfilamentsof cytoskeleton
Cholesterol
Peripheralproteins
Integralprotein
CYTOPLASMIC SIDEOF MEMBRANE
GlycolipidEXTRACELLULARSIDE OFMEMBRANE
Carbohydrate
Peripheral proteins Peripheral proteins are bound to the are bound to the surface of the membranesurface of the membrane
Integral proteins Integral proteins penetrate the hydrophobic penetrate the hydrophobic core (often coiled into alpha helicescore (often coiled into alpha helices
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Six major functions of membrane proteins:Six major functions of membrane proteins: TransportTransport Enzymatic activityEnzymatic activity Signal transductionSignal transduction Cell-cell recognitionCell-cell recognition Intercellular joiningIntercellular joining Attachment to the cytoskeleton and extracellular Attachment to the cytoskeleton and extracellular
matrix (ECM)matrix (ECM)
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(a) Transport
ATP
(b) Enzymatic activity
Enzymes
(c) Signal transduction
Signal transduction
Signaling molecule
Receptor
(d) Cell-cell recognition
Glyco-protein
(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)
Carbohydrates in Cell-Cell RecognitionCarbohydrates in Cell-Cell Recognition
• Cells recognize each other by binding to surface Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma molecules, often carbohydrates, on the plasma membranemembrane
Membrane carbohydrates may Membrane carbohydrates may be covalently bonded to lipids be covalently bonded to lipids (forming (forming glycolipidsglycolipids) or more ) or more commonly to proteins (forming commonly to proteins (forming glycoproteinsglycoproteins))
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Cell to cell recognition
Glyco-protein
The Permeability of the Lipid The Permeability of the Lipid BilayerBilayer
Hydrophobic (nonpolar) molecules, such as Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidlypass through the membrane rapidly
Polar molecules, such as sugars, do not cross the Polar molecules, such as sugars, do not cross the membrane easilymembrane easily
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Transport ProteinsTransport Proteins
Transport proteins Transport proteins allow passage of allow passage of hydrophilic substances hydrophilic substances across the membraneacross the membrane
Channel proteinsChannel proteins, have a , have a hydrophilic channel that certain hydrophilic channel that certain molecules or ions can use as a molecules or ions can use as a tunneltunnel
Aquaporins Aquaporins (channel protein)(channel protein) facilitate the passage of waterfacilitate the passage of water
Carrier proteinsCarrier proteins, bind to molecules , bind to molecules and change shape to shuttle them and change shape to shuttle them across the membraneacross the membrane
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Passive transportPassive transport
Diffusion Diffusion is the tendency for molecules to is the tendency for molecules to spread out evenly into the available spacespread out evenly into the available space
Animation: Animation: Diffusion
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Molecules of dye Membrane (cross section)
Net diffusionNet diffusion Equilibrium
WATER
Substances diffuse down their Substances diffuse down their concentration concentration gradientgradient, the difference in concentration of a , the difference in concentration of a substance from one area to anothersubstance from one area to another
No work must be done to move substances No work must be done to move substances down the concentration gradientdown the concentration gradient
The diffusion of a substance across a The diffusion of a substance across a biological membrane is biological membrane is passive transport passive transport because it requires no energy from the cell to because it requires no energy from the cell to make it happenmake it happen
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Effects of Osmosis on Water BalanceEffects of Osmosis on Water Balance
Osmosis Osmosis is the diffusion of water across a is the diffusion of water across a selectively permeable membraneselectively permeable membrane
Water diffuses across a membrane from the Water diffuses across a membrane from the region of lower solute concentration to the region region of lower solute concentration to the region of higher solute concentrationof higher solute concentration
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Lowerconcentrationof solute (sugar)
Fig. 7-12Fig. 7-12
H2O
Higher concentrationof sugar
Selectivelypermeablemembrane
Same concentrationof sugar
Osmosis
Water Balance of Cells Without WallsWater Balance of Cells Without Walls
Tonicity Tonicity is the ability of a solution to cause a cell is the ability of a solution to cause a cell to gain or lose waterto gain or lose water
Isotonic Isotonic solution: Solute concentration is the solution: Solute concentration is the same as that inside the cell (no movement)same as that inside the cell (no movement)
Hypertonic Hypertonic solution: Solute concentration is solution: Solute concentration is greater than that inside the cell; cell loses watergreater than that inside the cell; cell loses water
Hypotonic Hypotonic solution: Solute concentration is less solution: Solute concentration is less than that inside the cell; cell gains waterthan that inside the cell; cell gains water
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Fig. 7-13Fig. 7-13
Hypotonic solution
(a) Animal cell
(b) Plant cell
H2O
Lysed
H2O
Turgid (normal)
H2O
H2O
H2O
H2O
Normal
Isotonic solution
Flaccid
H2O
H2O
Shriveled
Plasmolyzed
Hypertonic solution
Water Balance of Cells with WallsWater Balance of Cells with WallsA plant cell in a hypotonic solution swells until the wall A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now opposes uptake; the cell is now turgidturgid (firm) (firm)
If a plant cell and its surroundings are isotonic, there is If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell no net movement of water into the cell; the cell becomes becomes flaccid flaccid (limp), and the plant may wilt(limp), and the plant may wilt
In a hypertonic environment, plant cells lose water; In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a eventually, the membrane pulls away from the wall, a usually lethal effect called usually lethal effect called plasmolysisplasmolysis
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Facilitated Diffusion: Passive Transport Facilitated Diffusion: Passive Transport Aided by Proteins (no energy)Aided by Proteins (no energy)
In In facilitated diffusionfacilitated diffusion, transport proteins , transport proteins speed the passive movement of molecules speed the passive movement of molecules across the plasma membrane (down across the plasma membrane (down gradient)gradient)
Channel proteins includeChannel proteins include Aquaporins, for facilitated diffusion of waterAquaporins, for facilitated diffusion of water Ion channels Ion channels that open or close in response to that open or close in response to
a stimulus (a stimulus (gated channelsgated channels))
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Animation: Animation: Facilitated diffusion
The Need for Energy in Active The Need for Energy in Active TransportTransport
Active transport Active transport moves substances moves substances against their concentration gradientagainst their concentration gradient
Active transport requires energy, usually in Active transport requires energy, usually in the form of ATPthe form of ATP
Active transport is performed by Active transport is performed by specificspecific proteins embedded in the membranesproteins embedded in the membranes
Animation: Animation: Active Transport
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2
EXTRACELLULAR
FLUID [Na+] high [K+] low
[Na+] low
[K+] high
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLASM ATP
ADP P
Na+ Na+
Na+
P 3
K+
K+ 6
K+
K+
5 4
K+
K+
P P
1
Fig. 7-16-7Fig. 7-16-7
Passive transport
Diffusion Facilitated diffusion
Active transport
ATP
Bulk transport occurs by exocytosis Bulk transport occurs by exocytosis and endocytosisand endocytosis
Small molecules and water enter or leave Small molecules and water enter or leave the cell through the lipid bilayer or by the cell through the lipid bilayer or by transport proteinstransport proteins
Large molecules, such as polysaccharides Large molecules, such as polysaccharides and proteins, cross the membrane in bulk and proteins, cross the membrane in bulk via vesiclesvia vesicles
Bulk transport requires energyBulk transport requires energy
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ExocytosisExocytosis
In In exocytosisexocytosis, transport vesicles migrate to , transport vesicles migrate to the membrane, fuse with it, and release their the membrane, fuse with it, and release their contentscontents
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EndocytosisEndocytosis
In In endocytosisendocytosis, the cell takes in , the cell takes in macromolecules by forming vesicles from the macromolecules by forming vesicles from the plasma membraneplasma membrane
There are three types of endocytosis:There are three types of endocytosis: PhagocytosisPhagocytosis (“cellular eating”) (“cellular eating”) Pinocytosis Pinocytosis (“cellular drinking”)(“cellular drinking”) Receptor-mediated endocytosisReceptor-mediated endocytosis: binding of : binding of
ligands to receptors triggers vesicle formationligands to receptors triggers vesicle formation
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Animation: Animation: Phagocytosis
PHAGOCYTOSIS
CYTOPLASM EXTRACELLULARFLUID
Pseudopodium
“Food” orother particle Food
vacuole Vesicle
PINOCYTOSIS
Plasmamembrane
RECEPTOR-MEDIATED ENDOCYTOSIS
Ligand
Coatedpit
Coat protein
Coatedvesicle
Receptor