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· 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
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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
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· 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
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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
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· 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
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· 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
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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
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§ 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
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· 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
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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
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· 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
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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
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· 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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