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Membrane Structure and Function
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What You Must Know: Why membranes are selectively
permeable. The role of phospholipids, proteins, and
carbohydrates in membranes. How water will move if a cell is placed in
an isotonic, hypertonic, or hypotonic solution.
How electrochemical gradients are formed.
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Cell MembraneA. Plasma membrane is selectively
permeableAllows some substances to cross more
easily than othersB. Fluid Mosaic Model
Fluid: membrane held together by weak interactions
Mosaic: phospholipids, proteins, carbs
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Early membrane model(1935) Davson/Danielli –
Sandwich modelphospholipid bilayer
between 2 protein layersProblems: varying
chemical composition of membrane, hydrophobic protein parts
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The freeze-fracture method: revealed the structure of membrane’s interior
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Fluid Mosaic Model
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PhospholipidsBilayerAmphipathic =
hydrophilic head, hydrophobic tail
Hydrophobic barrier: keeps hydrophilic molecules out
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Membrane fluidityLow temps: phospholipids
w/unsaturated tails (kinks prevent close packing)
Cholesterol resists changes by:limit fluidity at high
tempshinder close packing at
low tempsAdaptations: bacteria in
hot springs (unusual lipids); winter wheat ( unsaturated phospholipids)
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Membrane ProteinsIntegral Proteins Peripheral ProteinsEmbedded in
membraneDetermined by freeze
fractureTransmembrane with
hydrophilic heads/tails and hydrophobic middles
Extracellular or cytoplasmic sides of membrane
NOT embeddedHeld in place by the
cytoskeleton or ECMProvides stronger
framework
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Integral & Peripheral proteins
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Hydrophobic interior
Hydrophilic ends
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Some functions of membrane
proteins
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CarbohydratesFunction: cell-cell recognition; developing
organismsGlycolipids, glycoproteinsEg. blood transfusions are type-specific
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Synthesis and sidedness of membranes
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Selective PermeabilitySmall molecules Small molecules (polar or nonpolar)
cross easily (hydrocarbons, hydrophobic molecules, CO2, O2)
Hydrophobic core prevents passage of ionsions, , large polar molecules large polar molecules
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Passive TransportNO ENERGY (ATP) needed!Diffusion down concentration gradientconcentration gradient (high
low concentration)Eg. hydrocarbons, CO2, O2, H2O
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Diffusion
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Water PotentialWater PotentialWater potential (Water potential (ψψ)): : H2O moves from high ψ
low ψ potentialWater potential equation: Water potential equation:
ψ ψ = = ψψSS + + ψψPP
Water potential (ψψ) = free energy of waterSolute potential (ψψSS) = solute concentration
(osmotic potential)Pressure potential (ψψPP) = physical pressure on
solution; turgor pressure (plants)Pure water: ψPP = 0 MPaPlant cells: ψPP = 1 MPa
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Calculating Solute Potential (Calculating Solute Potential (ψψSS) ) ψψSS = -iCRT = -iCRT
• i = ionization constant (# particles made in water)
• C = molar concentration• R = pressure constant (0.0831 liter bars/mole-
K)• T = temperature in K (273 + 0C)
The addition of solute addition of solute to water lowerslowers the solute potential (more negative) and therefore decreasesdecreases the water potential.
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Where will Where will WATERWATER move? move?From an area of:
higher ψ lower ψ (more negative ψ)low solute concentration high solute
concentrationhigh pressure low pressure
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1. Which chamber has a lower water potential?2. Which chamber has a lower solute potential?3. In which direction will osmosis occur?4. If one chamber has a Ψ of -2000 kPa, and
the other -1000 kPa, which is the chamber that has the higher Ψ?
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Sample ProblemSample Problem1. Calculate the solute potential of a 0.1M
NaCl solution at 25°C.
2. If the concentration of NaCl inside the plant cell is 0.15M, which way will the water diffuse if the cell is placed in the 0.1M NaCl solution?
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Facilitated DiffusionTransport proteinsTransport proteins (channel or carrier
proteins) help hydrophilic substances crossTwo ways:
Provide hydrophilic channel
Loosely bind/carry molecule across
Eg. ions, polar molecules (H2O, glucose)
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Aquaporin: channel protein that allows passage of H2O
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Active TransportRequires ENERGYENERGY (ATP)Proteins transport substances
against concentration gradientconcentration gradient (low high conc.)
Eg. Na+/K+ pump, proton pump
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Electrogenic Pumps: generate voltage across membraneNa+/K+ Pump Proton PumpPump Na+ out, K+ into
cellNerve transmission
Push protons (H+) across membrane
Eg. mitochondria (ATP production)
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Cotransport: membrane protein enables “downhill” diffusion of one solute to drive “uphill” transport of other
Eg. sucrose-H+ cotransporter (sugar-loading in plants)
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Passive vs. Active TransportLittle or no EnergyHigh low
concentrationsDOWN the
concentration gradient
eg. diffusion, osmosis, facilitated diffusion (w/transport protein)
Requires Energy (ATP)
Low high concentrations
AGAINST the concentration gradient
eg. pumps, exo/endocytosis
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OsmoregulationControl solute & water balanceContractile vacuole: “bilge pump” forces out
fresh water as it enters by osmosisEg. paramecium caudatum – freshwater protist
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Bulk TransportTransport of proteins, polysaccharides,
large molecules
Endocytosis: take in macromolecules, form new vesicles
Exocytosis: vesicles fuse with cell membrane, expel contents
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Phagocytosis:“cellular eating” - solids
Pinocytosis:“cellular drinking” - fluids
Receptor-Mediated Endocytosis:Ligands bind to specific receptors on cell surface