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31 Cards in this Set
- Front
- Back
Biomembranes (functions)
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separate the cell from the outside world
separate the organelles from the inside of the cell (compartmentalize important processes and activities) diverse function depending on location |
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"unit" membrane structure
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at the electron microscope level, membranes share common structure
resemble railroad track with 2 densely stained lines, separated by a relatively clear space on the cell surface, many proteins are glycosylated |
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What are the major constituents of the cell membrane
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phospholipids
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Phospholipid structure
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polar head group
phosphate glycerol backbone 2 FA tails |
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What does the carboxyl group of the FA do?
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if free, it would be ionized
usually, it is linked to other groups to form esthers of amides |
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sphingolipids make up what % of membrane phospholipids?
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15-20%
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Cholesterol: classification and function in membranes
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a sterol
a membrane plasticiser: inserts in membrane to alter packing of phospholipids |
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Lipid composition of membranes
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different membranes have very different lipid composition depending on function
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why do membranes spontaneously form?
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hydrophobic molecules aggregate to minimize SA contact with water.
At the air-water interface, hydrophobic FAs tails of a lipid monolayer will stick up to avoid association with water |
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What determines lipid packing (and whether a micelle or bilayer will form)
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the shape of the lipid
lysosome lipids: lipase removed one acyl chain, more conical in shape phospholipids are cylindrical |
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multilamellar phospholipids
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if you just shake up phospholipids in water, an onionlike structure results
to make vesicles you must sonicate or mix with detergent and slowly dialize away detergent. |
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How do we know that membranes are bilayers?
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Indirectly: differential scanning calorimetry allows us to see that lipid melting occurs in much the same way each time. (for a single sample)
X-ray diffraction data was the definitive evidence for bilayer arrangement |
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Phospholipids most common in the outer hemileaflet:
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sphingomyelin, phosphatadylcholine
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Phospholipids most common in the inner hemileaflet
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Phophatadylthanolamine, phosphatadylserine
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Lipid bilayers are:
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dynamic, noncovalent structures
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FRAP
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Fluorescence recovery after photobleaching:
attach fluorophors to phospholipid heads, bleach with laser beam, the observe diffusion |
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lipid molecule diffusion (lateral and transverse)
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lateral: 1 micon/sec
transverse: much slower, need flipases |
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Transport definition
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the movement of molecules across membranes
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Why do phospholipids form bilayers?
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The molecule is cyllindrical in shape, which allows sheet formation
Planar sheets are thermodynamically unstable because the hydrophobic edge of the plane is exposed to water, therefore they tend to fold in to make spheres. |
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Leakage (diffusion) plot
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plot of uptake (Yaxis) v. [S] (x axis) is linear
V=k[S] |
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protein mediated transport plot
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plot of uptake (y axis) v. [S] (x axis) shows michaelis menton kinetics
V=(Vmax[S])/(Km + [S]) |
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sum of both leakage and protein mediated transport plot
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plot of uptake (y axis) v. [S] (x axis) shows a mix between linear and michaelis-menton kinetics
V=k[S] + (Vmax[S])/(Km + [S]) |
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is protein mediated or diffusion more common in cells
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sum of both is most common
inhibit the protein-mediated process to obtain leakage only rate, then determine protein-mediated rate from that |
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Passive transport
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The distribution of solute across the membrane is identical to the distribution that would be achieved without a transport protein (ie simple diffusion)
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Active transport
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A distribution of solute across a membrane that is not that produced by simple diffusion
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Primary active transporters
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Exploit high-energy intermediates (atp hydrolysis) to catalyze rapid net solute movement against a concentration gradient
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Secondary active transporters
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Exploit Na+, K+ or H+ gradients to drive a molecule against an electrochemical gradient
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2 factors govern the rate of diffusion
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1) the membrane solubility of a specific molecule
2) the size of the molecule that diffuses across the cell membrane |
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Permiability depends on 3 factors:
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1) partitioning into the membrane
2) mobility within the membrane 3) thickness of the membrane |
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Equation for permiability
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P=(K x Dm)/λ
where: P = the permiability coefficient K= partitioning coefficient Dm= diffusion coefficient (related to mobility within the membrane) λ= thickness of the membrane |
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Why do larger molecules have lower Dm?
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The lipid bilayer contains “void space”
o The mobility within the membrane depends on 2 factors: • 1) frequency of appearance of void spaces • 2) the average void space volume in the membrane o so even if a molecule can partition into the bilayer, it may be diffusionally restricted by the amount of void volume in the membrane • increasing temperature can increase void space volume, thus increasing permeability of some molecules |