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Membrane Structure and Membrane Structure and FunctionFunctionChapter 7
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What is the Function of The What is the Function of The Plasma Membrane?Plasma Membrane?boundarymust be selectively permeableWhat, then is the structure that
allows the membrane to perform this function successfully?
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PhospholipidsPhospholipidsRecall that phospholipids are
amphipathic (both hydrophilic and hydrophobic)
Artificial membranes showed phospholipids will form a layer in water
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The BilayerThe BilayerWhy are membranes organized
into a bilayer?There are two “watery” areas
that interact with the membrane, outside of the cell and inside the cell
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Science as a ProcessScience as a Process1935 Davson-Danielli
model “The Fat Sandwich”. The evidence:
Both protein and phospholipids were isolated from membranes
Thickness measured equals the bilayer
Phospholipids alone are not as attracted to water as the real membrane surfaces. Therefore coat with proteins!
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Problems with the modelProblems with the model In what way are membrane proteins a
problem for this model?Hint: Membrane proteins are
amphipathicHydrophobic parts of the proteins are
in hydrophilic zones, resulting in an unstable structure
What other problem was there with this model?
The model suggests that all membranes are identical with regard to thickness. They are not!
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Fluid Mosaic Model - 1972Fluid Mosaic Model - 1972In what way does
this model solve the problems?
Hydrophobic parts of proteins are embedded within the membrane
Thickness between different membranes is a function of the proteins
What evidence is there to support this model?
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Fluid Mosaic Model - 1972Fluid Mosaic Model - 1972In what way does
this model solve the problems?
Hydrophobic parts of proteins are embedded within the membrane.
Thickness between different membranes is a function of the proteins
Freeze-fracture technique with electron
microscope
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What evidence is there for What evidence is there for membrane fluidity?membrane fluidity?
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Membrane FluidityMembrane FluidityWhy is it that membrane
phospholipids drift laterally, and rarely flip?
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How is this fluidity How is this fluidity maintained?maintained?Kinks in unsaturated fatty acid
tails of phospholipidsCholesterol
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How are proteins arranged to How are proteins arranged to contribute to membrane function?contribute to membrane function?Membrane proteins contribute to the
mosaic quality of the structureDifferent proteins convey different
properties to each membraneIntegral proteins are inserted within
the membranePeripheral proteins are attached to
membrane surfaceProteins attach to cytoskeleton or to
extracellular fibers to help give animal cells a stronger framework
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Membrane CarbohydratesMembrane CarbohydratesFound only on the outside of the
membraneWhat is their function?Cell to cell recognitionSorting cells into tissues Immune defenseUsually oligosaccharides (15 or
less sugar units)glycolipids or glycoproteins
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How do ions and other polar How do ions and other polar molecules pass into and out of molecules pass into and out of cells?cells?Transport proteinsProvide hydrophilic tunnel for
ionsThey are specific for the
substances they transport
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What determines the direction What determines the direction of traffic across a membrane?of traffic across a membrane?DiffusionWhat causes diffusion? Why is it
spontaneous?Concentration gradient
represents potential energy Since the direction of movement
decreases the free energy of the system, it is spontaneous
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Does the diffusion of more than Does the diffusion of more than one kind of particle work together one kind of particle work together or separately?or separately?
Do the particles stop moving once equilibrium
is reached?
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Does the diffusion of more than Does the diffusion of more than one kind of particle work together one kind of particle work together or separately?or separately?
NO!
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If a molecule can move freely If a molecule can move freely through the phospholipid bilayer, through the phospholipid bilayer, then what always controls the then what always controls the direction of its movement?direction of its movement?Concentration gradientRemember that the
concentration gradient represents potential energy
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OsmosisOsmosisWhat is osmosis?The diffusion of water across a
semi-permeable membrane
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Since water passes freely across the membrane, how can the cell control the direction of osmosis?
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The cell can concentrate solutes that are not permeable to the phospholipid bilayer on one side of the membrane
Which way will water move? Water will follow the solutes What do the terms hypotonic,
hypertonic and isotonic mean?Hypotonic = lower solute concentrationHypertonic = greater solute
concentrationIsotonic = equal solute concentration
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??
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Which way will the water
move?
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WHY?
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Do Water Molecules Stop Do Water Molecules Stop Moving in Isotonic Moving in Isotonic Conditions?Conditions?NoThey continue to diffuse,
however there is no net movement
In general, which way does water move?
From hypotonic to hypertonic
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Water Balance in CellsWater Balance in Cells
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Facilitated DiffusionFacilitated DiffusionWhat is facilitated diffusion?Diffusion of solutes with the help
of transport proteinsIs this a passive or an active
process?passiveWhy do these solutes need a
protein to facilitate their diffusion?They are too polar to pass
through the lipid bilayer
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Active TransportActive TransportWhat is active transport?Pumps molecules across the
membrane against their concentration gradients
Requires energy in the form of ATPUsed to help maintain ionic gradients
across membranesWhat do these ionic gradients
represent?Potential energy
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Membrane PotentialMembrane PotentialMembrane potential is the
voltage across a membraneUsually around -70 mVHow is it maintained?An unequal distribution of anions
inside the cell to cations outside the cell
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What two forces drive the What two forces drive the diffusion of ions?diffusion of ions?Concentration gradient of the ionEffect of membrane potential (charge)
on the ionThis is called the electrochemical
gradientIons diffuse down their electrochemical
gradientA transport protein that generates
voltage across a membrane is called an electrogenic pump
One example is the sodium-potassium pump
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Proton pumps are the main electrogenic pumps of bacteria, fungi and plants.
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Review Review of of passive passive and and active active transpotransportrt
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Review Review of of passive passive and and active active transpotransportrt
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Review Review of of passive passive and and active active transpotransportrt
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What is co-transport ?What is co-transport ?
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Transport of large Transport of large moleculesmoleculesEndocytosis –
◦Phagocytosis – endocytosis of large particulate substances
◦Pinocytosis – endocytosis of fluid and dissolved solutes
Exocytosis -
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PhagocytosisPhagocytosis
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PinocytosisPinocytosis