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    Membrane Structure and Function

    -plasma membrane acts as a barrier between cells

    and the surrounding.

    -plasma membrane is selective permeable

    -consist of lipids, proteins and carbohydrates

    -major lipids are phospholipid which is amphipathic

    = contain both hydrophobic and hydrophilic region

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    The Fluid Mosaic Model :current model of membrane

    1972, S.J.Singer and G.Nicolson proposed this

    model: the membrane is amosaic of proteinmolecules bobbing in a

    fluid bilayer ofphospholipid.

    - membrane is held together by hydrophobic interaction

    -some membrane proteinsmove along cytoskeletal fibers bymotors proteins

    -some membrane proteins are immobileby attachment to thecytoskelton

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    Membranes are fluid

    -phospholipids can drift laterally in the plane of themembrane (an average lipid molecule can diffuse the length of a large

    bacterial cells (~ 2 m) in about 1 second) = lateral movement

    (frequently)-also, phospholipids can migrate from the monolayeron one side to that on the other = flip-flop (rarely)

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    -Temperature and lipid composition determine

    fluidity of the membrane-At low temperature, membrane is less fluid andbecause the phospholipids are more closely

    packed.

    -Membranes rich in

    unsaturated fatty acids aremore fluid that thosedominated by saturated

    fatty acids because thekinks in the unsaturatedfatty acid tails preventtight packing.

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    -Steroid cholesterol which is wedged between

    phospholipids also effects membrane fluidity.

    -At warm temperature, it makes membrane less

    fluid by restraining the movement ofphospholipid.

    -At low temperature, themembrane remains fluidbecause cholesterol

    hinders the close packingof the phospholipids.

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    Membranes are mosaic of structure andfunction

    A membrane is a collage of different proteinsembedded in the fluid matrix of the lipid bilayer

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    2 major populations of membrane proteins:-integral proteins -peripheral proteins

    Integral proteins penetrate the

    hydrophobic core of the lipid bilayer.

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    Many of the integral proteins are

    transmembrane proteins which completelyspan the membrane.

    -The hydrophobic region of anintegral protein consist of oneor more stretches of nonpolaramino acids, usually coiledinto a-helices.-The hydrophilic parts are

    exposed to the aqueoussolutions on either side of themembrane.

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    Peripheral proteins are not embedded in the

    lipid bilayer at all.They are appendage loosely bound to thesurface of the membrane, often to he exposed

    parts of integral proteins.

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    -The lipid and proteins of plasma membrane also

    covalently bound to carbohydrates (~15 sugar units)= glycolipid and glycoprotein which are restricted tothe exterior surface and are important for cell-cell

    recognition.

    -e.g.variation ofoligosaccharideson the RBC

    determineshuman bloodgroup A,B,AB andO.

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    -Membrane has distinct inside and outside

    faces.

    -This asymmetrical

    orientation begins duringsynthesis of newmembranein the endoplasmicreticulum

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    Some Functions of Membrane Protiens

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    Traffic across Membranes

    Biological membrane is a

    supramolecular structure = many molecules

    ordered into a higher level of organization

    with emergent properties beyond those of

    the individual molecules.

    The supramolecular structure of the

    membrane results in selective permeability.

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    Permeability of the membrane

    -lipid bilayer is permeable to small nonpolarmolecules: CO2 and O2-but it is impermeable to ions and polarmolecules: glucose, sucrose, Na+, H+, Cl-

    -transport proteins enable cell membranespermeable to specific ions and polar moleculesincluding H2O

    Thus, the selective permeability of a membrane

    depends on both the discriminating barrier of the lipid

    bilayer and the specific transport proteins.

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    Transport Across Membrane

    - passive transport = diffusion across amembrane down the concentration gradient

    - osmosis = passive transport of water

    - facilitated diffusion = specific proteinsfacilitate the passive transport of water and

    selected solutes- active transport = pumping of solutes againsttheir concentration gradients

    - cotransport = membrane protein couples thetransport of one solute to another- exocytosis and endocytosis transport large

    molecules

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    Passive transport is diffusion across a

    membrane

    -diffusion of a substance across a biological

    membrane is called passive transport

    -molecules diffuse across membrane downits concentration gradient

    -spontaneous process: does not need energy

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    Osmosis is the passive transport of water

    Hypertonic solution = solution with higherconcentration of solute

    Hypotonic solution = solution with lower soluteconcentrationIsotonic solution = solution of equal solute

    concentration

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    U-shaped vessel with a selectively

    permeable membrane separating 2 sugarsolution of different concentration.The membrane pores are too small for

    sugar molecule to pass but large enough forwater

    Water diffuses acrossmembrane fromhypotonic solution to the

    hypertonic solution.This diffusion of water =osmosis

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    Direction of osmosis is determined only

    by a different in total solute concentration.Water diffuses from hypotonic to a

    hypertonic even if the hypotonic solution has

    more kinds of solutes.

    (Water from seawater (containing a

    great variety of solutes) will diffuse to a veryconcentrated sugar solution)

    For isotonic solutions, water movesacross membrane in both direction at an

    equal rate.

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    Water balance of cells

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    The protist Paramecium lives in pondwater which is hypotonic to the cell.

    Its contractile vacuole forces water outof the cell as fast as it enters by osmosis.

    Osmoregulation: the control of water balance

    f f

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    Specific proteins facilitate the passive transportof water and and selected solutes

    Facilitate diffusion = diffusion of polar

    molecules (including H2O) and ions with thehelp of transport proteins spanning themembrane

    Model # 1: The purpletransport protein forms a

    channel through whichwater molecules or aspecific solute can pass.

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    Model # 2: Transportprotein alternates between

    2 conformations, moving asolute across themembrane as the shape ofthe protein changes.

    The proteins can transport the solute in

    either direction, with the net movement being

    down the concentration gradient of thesolute.

    A ti t t

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    Active transport:

    -pumping of solutes against their concentrationgradient-require energy in the form of ATP

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    The sodium-potassium pump

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    Comparison between passive and active transport

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    An electrogenic pump = a transport protein

    that generates voltage across a membranesodium-potassium pumpproton pump transport positive charge in

    the form of hydrogen ions

    Voltage generatedacross membranes

    can be trapped forcellular work suchas in cotansport.

    Cotransport: a membrane protein couples the

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    Cotransport: a membrane protein couples thetransport of 2 solutes

    A substance that

    has been pumpedacross a membrane

    can do work as it leaksback by diffusion.Plant cells uses

    hydrogen gradient generated by proton pumps

    to drive active transport of amino acids, sugarand other nutrients

    E t i d d t i t t f l

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    Exocytosis and endocytosis: transport of large

    molecules (proteins, polysaccharides)

    Exotytosis = secretion of

    macromolecules fromcells by the fusion of

    vesicles with the plasmamembrane.

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    Endocytosis = the process which cell takes inmacromolecules and particulate matter by

    forming new vesicles from the plasmamembrane.

    3 types of endocytosis:

    phagocytosis = celular eating

    pinocytosis = cellular drinking

    receptor-mediated endocytosis

    Phagocytosis

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    Phagocytosis

    The cell engulfs a particle by wrappingpseudopodia around it and packaging it within

    a vacuole: food vacuole.Food vacuole fuses with lysosome to bedigested.

    Pinocytosis

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    Pinocytosis

    The cell gulps droplets of extracellularfluid into tiny vesicles.

    Pinocytosis is unspecific in the substancesit transports.

    Receptor mediated endocytosis

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    Receptor-mediated endocytosis

    Specific receptor proteins embedded inthe membrane bind to the ligands.

    e.g. transport of cholesterol into the cells.