boundary lipid bilayer selectively permeable fluid mosaic of … · 2011-11-17 · support the...
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Boundary
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Selectively Permeable
Fluid mosaic of lipids and proteins
Lipid bilayer
Contains embedded proteins
Phosphate head hydrophilic
Fatty acid tailshydrophobic
Arranged as a bilayerFatty acid
Phosphate
Aaaah, one of thosestructure–functionexamples
“repelled by water”
“attracted to water”
Amphipathic
Phospholipids in the plasma membrane Can move within the bilayer two ways
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Lateral movement(~107 times per second)
Flip-flop(~ once per month)
Phospholipids can move laterally
hydrocarbon tails affect membrane fluidity
Unsaturated hydrocarbon tails:
(-C=C-)kinks prevents tight packing incr fluiditySaturated hydrocarbon tails:
(-C-C-) tight packing decr fluidity
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Fluid Viscous
Unsaturated hydrocarbontails (kinks) Saturated hydrocarbon tails
Cholesterol (a steroid) affects fluidity
T fluidityT fluidity
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Figure 7.5
Cholesterol
Transmembrane proteins embedded in phospholipid bilayer
-create semi-permeable channels
lipid bilayermembrane
protein channelsin lipid bilyer membrane
1972: Singer and Nicolsonmembrane proteins are dispersed
and individually inserted into the phospholipid bilayer
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Phospholipidbilayer
Hydrophilic region of protein
Hydrophobic region of protein
fluid structure“mosaic” of various proteins embedded in it
when viewed from the top
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Membrane proteinscan move side-to-side or laterally making the membrane fluid
Freeze-fracture studies support the fluid mosaic model
of membrane structure
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A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers.
A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
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Fibers of
extracellular
matrix (ECM)
Integral proteinsPenetrate the hydrophobic core of the lipid
bilayerAre often transmembrane proteins,
completely spanning the membrane
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EXTRACELLULAR
SIDE
Peripheral proteinsAre appendages loosely bound to the
surface of the membrane
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Cell membrane separates living cell from aqueous environmentthin barrier = 8nm thick
Controls traffic in & out of the cell-allows some substances to cross more easily than
others
-hydrophobic (nonpolar) vs. hydrophilic (polar)
Serves as a cellular barrier / border
polarhydrophilicheads
nonpolarhydrophobictails
polarhydrophilicheads
H2Osugar
lipids
salt
waste
impermeable to polar molecules
Membrane becomes semi-permeable via protein channels
◦ specific channels allow specific material across cell membrane
inside cell
outside cell
sugaraaH2O
saltNH3
2007-2008
Why areproteins the perfect molecule to build structures in the cell membrane?
Within membranenonpolar amino acids hydrophobic
anchors protein into membrane
On outer surfaces of membrane in fluidpolar amino acids
hydrophilic
extend into extracellular fluid & into cytosol
Polar areasof protein
Nonpolar areas of protein
NH2
H+
COOH
Cytoplasm
Retinalchromophore
Nonpolar(hydrophobic)a-helices in thecell membrane H+
Porin monomer
b-pleated sheets
Bacterialoutermembrane
proton pump channel in photosynthetic bacteria;Retinal chromophores (rhodopsin) in human eyes
aquaporin = water channel in bacteria, (& plants, mammals)
conformational change
protein changes shape
H2O
H2O
H+
H+
Water moves rapidly into & out of cells-evidence that there were water channels
protein channels allowing flow of water across cell membrane
1991 | 2003
Peter AgreJohn Hopkins
Roderick MacKinnonRockefeller
Proteins determine membrane’s specific
functions
◦ cell membrane & organelle membranes each have
unique collections of proteins
Classes of membrane proteins:
-peripheral proteins
loosely bound to surface of membrane
ex: cell surface identity marker (antigens)
-integral proteins
penetrate lipid bilayer, usually across whole membrane
transmembrane protein
ex: transport proteins
channels, permeases (pumps)
Outside
Plasmamembrane
Inside
Transporter Cell surfacereceptor
Enzyme
activity
Cell surface identity marker
Attachment to thecytoskeleton
Cell adhesion
“Antigen”
“Channel”
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Transporthydrophilic channel across the membrane;
may be selective for a particular solute. (left)
transport protein shuttles substance across membrane
by changing shape. (right)
Some hydrolyze ATP as an energy source
to actively pump substances across the membrane.
Enzymatic Activityenzyme built into membrane; active site
exposed to substances in the adjacent solution.
Sometimes, several enzymes in a membrane are organized
as a team that carries out sequential steps of a metabolic pathway.
Signal TransductionMembrane protein has a binding site with a specific shape
that fits the shape of a chemical mssgr, such as a hormone.
External mssgr (signal) conformational change
in the protein (receptor)
relays the message to the inside of the cell.
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Cell-Cell recognitionSome glycoproteins serve as ID tags
that are specifically recognized by other cells.
Intercellular JoiningMembrane proteins of adjacent cells may hook together
in various kinds of junctions,
(Ex. gap junctions/tight junctions)
Attachment to cytoskeleton & extracellular matrix (ECM)
Microfilaments/other cytoskeletal elements bind to membrane proteins
-maintains cell shape
-stabilizes localization of certain membrane proteins
Proteins adhered to ECM coordinate extra- & intra-cellular changes
glycoprotein
Cell-Cell Recognition:a cell’s ability to distinguish one type of neighboring cell from another
development - cells tissues organsimmune function - ID & rejection of “foreign”
Membrane Carbohydrates:Interact with the surface molecules of other cells, facilitating cell-cell recognition
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http://www.youtube.com/watch?v=hRsLnQ6MbmA&feature=mfu_in_order&list=UL
http://www.youtube.com/watch?v=2as2bsFhoqk
http://www.youtube.com/watch?NR=1&v=37KNBEfcbsA&feature=fvwp
http://www.youtube.com/watch?v=2as2bsFhoqk
http://www.youtube.com/watch?v=hRsLnQ6MbmA&feature=mfu_in_order&list=UL
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