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11| Membranes
© 2013 W. H. Freeman and Company
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CHAPTER 11 Membranes
– The function of biological membranes – The structure and composition membranes– Dynamics of membranes– Structure and function of membrane proteins– Transport across biological membranes
Key topics:
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Lipids aggregate into structures in water
• Structures formed depend on:– type of lipid– concentration
• Micelles• Liposomes• Bilayers
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Micelle
• Forms in the solution of amphipathic molecules that have larger head than tail– Fatty acids– Sodium dodecyl sulfate
• Each micelle has from a few dozen to a few thousand lipid molecules
• Aggregation occurs when the concentration of molecules is higher than a certain threshold
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Vesicle (Liposome)
• Small bilayers will spontaneously seal into spherical vesicles
• Vesicle membranes can contain artificially inserted proteins
• The central aqueous cavity can enclose dissolved molecules
• They are useful artificial carriers of molecules (e.g., drugs)
• Vesicles fuse readily with cell membranes or with each other
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Membrane Bilayer• consists of two leaflets of lipid monolayers
– Hydrophilic head groups interact with water
– Hydrophobic fatty acid tails are packed inside
– One leaflet faces the cytoplasm
– Another leaflet faces the extracellular space or the inside of membrane‐enclosed organelle
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What are membranes?
•Complex lipid‐based structures that form pliable sheets
•Composed of a variety of lipids and proteins
• Some membrane lipids and proteins are glycosylated
•All cells have a cell membrane, which separates the cell from its surrounding
•Eukaryotic cells have various internal membranes that divide the internal space into compartments
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Electron Micrograph of Biological Membranes
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Functions of Membranes
• Define the boundaries of the cell• Allow import and export
– Selective import of nutrients (e.g., lactose)– Selective export of waste and toxins (e.g., antibiotics)
• Retain metabolites and ions within the cell• Sense external signals and transmit information into the cell• Provide compartmentalization within the cell
– separate energy‐producing reactions from energy‐consuming ones– keep proteolytic enzymes away from important cellular proteins
• Produce and transmit nerve signals• Store energy as a proton gradient • Support synthesis of ATP
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Common Features of Membranes• Sheet‐like flexible structure, 30–100 Å (3–10 nm) thick• Main structure is composed of two leaflets of lipids (bilayer)
– With the exception of archaebacteria: monolayer of bifunctional lipids
• Form spontaneously in aqueous solution and are stabilized by noncovalent forces, especially hydrophobic effect
• Protein molecules span the lipid bilayer• Asymmetric
– Some lipids are found preferably “inside”– Some lipids are found preferably “outside”– Carbohydrate moieties are always outside the cell– Electrically polarized (inside negative ~ –60mV)
• Fluid structures: two‐dimensional solution of oriented lipids
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Fluid Mosaic Model of Membranes
• Proposed in 1972 by Singer and Nicholson (UCSD)
• Lipids form a viscous, two‐dimensional solvent into which proteins are inserted and integrated more or less deeply
• Integral proteins are firmly associated with the membrane, often spanning the bilayer
• Peripheral proteins are weakly associated and can be removed easily
– Some are noncovalently attached
– Some are linked to membrane lipids
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The Fluid Mosaic Model: Details
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The Composition of Membranes
Lipid composition of membranes is different in:different organisms, tissues, and organelles
• Ratio of lipid to protein varies • Type of phospholipid varies• Abundance and type of sterols varies
• prokaryotes lack sterols• Cholesterol predominant in the plasma membrane, virtually
absent in mitochondria• Galactolipids abundant in plant chloroplasts but almost
absent in animals
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Membrane composition is highly variable in different organisms
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Membrane Structure in Archaea
• Unique glycerol chirality in phospholipids– L‐glycerol in archaea– D‐glycerol in bacteria
• Unique fatty acids– Branched isoprene chains in archaea– Unbranched fatty acid chains in bacteria
• Unique linkages– Ether linkages in archaea– Ester linkages in bacteria
• Membrane topology– Monolayer in some archaea– Bilayer in all bacteria
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Lipid Monolayer in Archaea• Sulfolobus solfataricus and relatives:
– Volcanic hot springs– Temperatures 75–80°C– Acidic environment: pH 2–3
• Better membrane stability by:– Isoprenoid tetraethers with unique alcohols
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Membrane composition is highly variable in different organelles
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Membrane bilayers are asymmetric
• Two leaflets have different lipid compositions• Outer leaflet is often more positively charged• Phosphatidylserine outside has a special meaning:
– Platelets: Activates blood clotting– Other cells: Marks the cell for destruction
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Membrane bilayers are asymmetric
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Membrane composition and asymmetry
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Functions of Proteins in Membranes
• Receptors: detecting signals from outside– Light (opsin)– Hormones (insulin receptor)– Neurotransmitters (acetylcholine receptor)– Pheromones (taste and smell receptors)
• Channels, gates, pumps– Nutrients (maltoporin)– Ions (K‐channel) – Neurotransmitters (serotonin reuptake protein)
• Enzymes– Lipid biosynthesis (some acyltransferases)– ATP synthesis (F0F1 ATPase/ATP synthase)
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Three Types of Membrane Proteins
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Peripheral Membrane Proteins
• Associate with the polar head groups of membranes• Relatively loosely associated with membrane
– through ionic interactions with the lipids or aqueous domains of integral membrane proteins
• Removed by disrupting ionic interactions either with high salt or change in pH
• Purified peripheral membrane proteins are no longer associated with any lipids
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Integral Membrane Proteins• Span the entire membrane• Have asymmetry like the membrane
– Different domains in different compartments
• Tightly associated with membrane– Hydrophobic stretches in the protein interact with the hydrophobic regions of the membrane
• Removed by detergents that disrupt the membrane• Purified integral membrane proteins still have phospholipids associated with them
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Integral Membrane Proteins
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Six Types of Integral Membrane Proteins
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Hydropathy plots can predict transmembrane domains
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Amino acids in membrane proteins cluster in distinct regions
• Transmembrane segments are predominantly hydrophobic• Tyr and Trp cluster at nonpolar/polar interface• Charged amino acids are only found in aqueous domains
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Membrane proteins also contain ‐sheets
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Lipid Anchors• Some membrane proteins are lipoproteins • They contain a covalently linked lipid molecule
– Long‐chain fatty acids– Isoprenoids– Sterols– Glycosylated phosphatidylinositol (PGI)
• The lipid part can become part of the membrane• The protein is now anchored to the membrane
– reversible process– allows targeting of proteins– Some, such as GPI anchors are found only on the outer face of plasma
membrane
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Lipid‐linked Membrane Proteins
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Farnesylation of Proteins• Proteins can be targeted to the inner leaflet of the plasma membrane by farnesylation
• Primary sequence of the protein contains a signature for farnesylation: CaaX– C is a conserved Cys – “a” is usually an aliphatic amino acid– “X” is Met, Ser, Glu or Ala
• This reaction is catalyzed by farnesyl transferase• Nonfarnesylated proteins do not go to the membrane and are inactive– Promising cancer therapy (onco‐Ras)
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Cell membranes are asymetric• Every component of the membrane exhibits asymmetry• Lipids
– Outer and inner leaflets have different lipid compositions• Proteins
– Individual peripheral membrane proteins are only associated with one side of the membrane
– Integral membrane proteins have different domains on different sides of the membrane.
– Specific lipid modification of proteins targets the protein to a specific leaflet
• Carbohydrates– Only on the outside of cells
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Physical Properties of Membranes• Dynamic and flexible structures• Can exist in various phases and undergo phase transitions
• Not permeable to large polar solutes and ions• Permeable to small polar solutes and nonpolar compounds
• Permeability can be artificially increased by chemical treatment– When we want to get DNA into the cell
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Membrane Phases• Depending on their composition and the temperature, lipid bilayer can be in gel or fluid phase– Gel phase: individual molecules do not move around– Fluid phase: individual molecules can move around
• Heating causes phase transition from the gel to fluid• Under physiological conditions, membranes are more fluid‐like than gel‐like – Must be fluid for proper function
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Membrane Phases
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Organisms can adjust the membrane composition
• Membrane fluidity is determined mainly by the fatty acid composition.
• More fluid membranes require shorter and more unsaturated fatty acids * Remember differences in TM of fatty acids
• At higher temperatures cells need more saturated fatty acids – To maintain integrity
• At lower temperatures cells need more unsaturated fatty acids – To maintain fluidity
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Sterols and hopanols increase membrane rigidity and permeability
• Cell membranes of many eukaryotes contain sterols– Cholesterol in animals– Phytosterols in plants– Ergosterol in fungi
• Cell membranes of aerobic prokaryotes contain hopanolsOH
OH
OHO OH
OHOH OH
NH2
OH
OH
cholesterol
ergosterol
one particular hopanol
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Membrane Dynamics: Lateral Diffusion
Individual lipids undergo fast lateral diffusion within the leaflet.
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Membrane Dynamics: Transverse Diffusion
Spontaneous flips from one leaflet to another are rare because the charged head group must transverse the hydrophobic tail region of the membrane.
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Membrane Diffusion: Flippases• Special enzymes—flippases—catalyze transverse diffusion
• Some flippases use energy of ATP to move lipids against the concentration gradient
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Study of Membrane Dynamics: FRAP
• Fluorescence Recovery After Photobleaching (FRAP) allows us to monitor lateral lipid diffusion by monitoring the rate of fluorescence return
• From the rate of return of lipids, the diffusion coefficient of a lipid in the leaflet can be determined
• Rates of lateral diffusion are high (up to 1 m/sec): – a lipid can circumnavigate E.coli cell in one second
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FRAP
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FRAP Step 1:Label Outside of Cell
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FRAP Step 2:Photobleaching
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FRAP Step 3:Measure Recovery
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Movement of a Single Lipid in 56 ms
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Membrane RaftsLipid distribution in a single leaflet is not random or uniform• Lipid rafts
– contain clusters of glycosphingolipids with longer‐than‐usual tails– are more ordered – contain specific doubly or triply acylated proteins– allow segregation of proteins in the membrane
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Caveolin forces membrane curvature
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Other Modes of Membrane Curvature
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Membrane Fusion
• Membranes can fuse with each other without losing continuity
• Fusion can be spontaneous or protein‐mediated• Examples of protein‐mediated fusion are:
– Entry of influenza virus into the host cell– Release of neurotransmitters at nerve synapses
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Examples of Membrane Fusion
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Neurotransmitter Release
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Neurotransmitter Release: Step 1
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Neurotransmitter Release: Step 2
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Neurotransmitter Release: Step 3
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Neurotransmitter Release: Step 4
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Neurotransmitter Release: Step 5
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Neurotransmitter Release: Step 6
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Transport Across Membranes
• Cell membranes are permeable to small nonpolar molecules that passively diffuse through the membrane
• Passive diffusion of polar molecules involves desolvation and thus has a high activation barrier
• Transport across the membrane can be facilitated by proteins that provide an alternative diffusion path
• Such proteins are called transporters or permeases
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Types of Transport
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Polar solutes need alternative paths to cross cell membranes
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Three Classes of Transport Systems
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Glucose Transporter in the Membrane
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Model for Glucose Transport
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There are multiple glucose transporters• A Na+‐glucose symporter and a glucose uniporter operate on opposite sides of epithelial cells
• Cells can also have asymmetry, with distinct proteins confined to one side
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Bicarbonate transporter is an antiporter
Maintains the electrochemical potential across the membrane
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Two Types of Active Transport
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ABC transporters use ATP hydrolysis to drive transport of substrates
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Proton Transport and Chemical Energy of ATP
• Energy of ATP hydrolysis can be used to drive protons through the membrane– pH control in the cell by F‐type ATPase
• Energy of the proton gradient can be used to synthesize ATP– in chloroplast and mitochondrial membranes by ATP synthase
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Proton driven ATPases can function in both directions
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Aquaporins allow rapid water passage through membranes
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Ion channels maintain gradients for active transport
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Cystic fibrosis is caused by a mutation to an ion channel in epithelial cells
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Chapter 11: Summary
• membranes are composed of various lipids and proteins• phospholipids form a selectively permeable bilayer • properties of the bilayer depend on the lipid composition, which
varies strongly from – organism to organism – tissue to tissue– organelle to organelle
• membrane proteins play a variety of structural and functional roles, especially in the transport of solutes across the membrane
• Active transport of solutes across membranes requires ATP but can be accomplished in many different ways
In this chapter, we learned: