Chapter 12.5

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  1. ·         Lipids and many membrane proteins are constantly in __, called __. The rapid lateral movement of membrane proteins is visualized by __.
    o   First, a cell-surface component is specifically labeled with a __; and a small region of the surface is viewed through a fluorescence microscope
    • lateral motion, called lateral diffusion.
    • fluorescence recovery after photobleaching
    • fluorescent chromophore
  2. o   The fluorescent molecules in this region are then __.
    o   The fluorescence of this region is then monitored as a function of __by using a light level sufficiently low to prevent __. 
    • destroyed (bleached) by a very intense light pulse from a laser
    • time 
    • further bleaching
  3. ·         If the labeled component is mobile,what happens?
    o   The rate of recovery of fluorescence depends on the __ of the fluorescence-labeled component, which can be expressed in terms of a __. 
    • bleached molecules leave and unbleached molecules enter the illuminated region, resulting in an increase in the fluorescence intensity
    • lateral mobility
    • diffusion coefficient, D
  4. o   The average distance S traversed in time t depends on D according to the expression:
    §  __
    o   The __ in a variety of membranes is about 1 um2s-1.
    §  Thus, a phospholipid molecule diffuses an average distance of__ in one second.
    ·         This rate means __.
    ·         The magnitude of the observed diffusion coefficient indicates __
    • S= (4Dt)^1/2
    • diffusion coefficient of lipids
    •  2 um in 1 second
    • that a lipid molecule can travel from one end of a bacterium to the other in a second
    • that the viscosity of the membrane is about 100 times that of water
  5. ·         In contrast, proteins vary markedly in their __. Some proteins are nearly as __as lipids, whereas others are __. 
    ·         The __, by Singer and Nicolson, allows __ but not __through the membrane; it describes the overall organization of biological membranes
    ·         Membranes, it states, are __.
    • lateral mobility
    • mobile 
    • virtually immobile
    • fluid mosaic model
    • lateral movement
    • rotation 
    • 2D solutions of oriented lipids and globular proteins
  6. ·         The lipid bilayer has a dual role: __ and __. Membrane proteins can __ in the lipid matrix unless restricted by interactions
    ·         Lateral diffusion is rapid, but __ is slow and involves __, which occurs once in several hours
    it is a solvent for integral membrane proteins and a permeability barrier

    • diffuse laterally
    • spontaneous rotation of lipids
    • transverse diffusion or flip flop
  7. o   The free-energy barriers to __are larger for protein molecules than for lipids because proteins have __.
    ·         Membrane asymmetry can be __
    ·         Membrane fluidity is controlled by __ and __
    o   Many membrane processes depend on the __, which depends on the __, which can exist in an __ or __
    • flip-flopping 
    • more extensive polar regions
    • preserved for long periods
    • fatty acid composition and cholesterol content
    • fluidity of the membrane lipids
    • properties of fatty acid chains
    • ordered, rigid state or in a relatively disordered, fluid state
  8. §  The transition from rigid to the fluid state takes place as __
    ·         This __ depends on the __ and __
    o   The presence of __ favors the __ because what?
    o   On the other hand, a __ bends the chain, interfering with the highly ordered packing of fatty acid chains. 

    o   Long hydrocarbon chains __ than do short ones.
    • the temperature is raised above Tm, the melting temperature
    • transition temperature
    • length of the fatty acid chains and on their degree of unsaturation

    • saturated fatty acid residues
    • rigid state
    • their straight hydrocarbon chains interact very favorably with one another

    • cis double bond
    • interact more strongly
  9. ·         Bacteria regulate the fluidity of their membranes by doing what?
    ·         In animals, __is the key regulator of membrane fluidity. What is the structure?
    • varying the number of double bonds and the length of their fatty acid chains
    • cholesterol 
    • Cholesterol contains a bulky steroid nucleus with a hydroxyl group at one end and a flexible hydrocarbon chain at the other end
  10. o   The hydroxyl group forms a hydrogen bond with the __, whereas the hydrocarbon tail of cholesterol is located in the __
    o   the different shape of cholesterol compared with that of phospholipids does what?
    • carbonyl oxygen atom of a phospholipid head group
    • nonpolar core the bilayer
    • disrupts the regular interactions between fatty acid chains
  11. ·         cholesterol can also form specific complexes with lipids that contain the __, including __ and __, and with __, referred to as __
    o   one result of these interactions is the __, making membranes less fluid but also less subject to phase transitions
    §  the presence of __ represents a modification of the original fluid mosaic model for biological membranes
    • sphingosine backbone
    • sphingomyelin and certain glycolipids
    • GPI-anchored proteins
    • lipid rafts
    • moderation of membrane fluidity
    • lipid rafts
  12. o   lipid rafts play a role in __ and may __
    ·       all biological membranes are structurally and functionally __with different components and different enzymatic activities
    • concentrating proteins that participate in signal transduction pathways
    • serve to regulate membrane curvature and budding
    • asymmetric
  13. ·         membrane proteins have a unique orientation because, after synthesis, they are inserted into the membrane in an __, which is preserved since membrane proteins don’t __ and because membranes are __
    ·         lipids too are asymmetrically distributed as a consequence of their __, but this asymmetry is usually not absolute except for glycolipids
    • asymmetric manner
    • rotate form one side of the membrane to the other side
    • always synthesized by the growth of preexisting membranes
    • mode of biosynthesis
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Chapter 12.5
2014-11-07 02:29:38
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