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Membrane Function
- -seperate cells from environment to create defined intracellular space
- -allow selective transport in/out of cell
- -provide location for specialized pathways and processes (energy creation in mitochondria)
- -nervous system communication through changes in electrical potential across membranes
- -surface for receptors that read physiological signs and facilitate change from them
- -mediate cell-cell recognition and interaction
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Membrane Structure
- -impermeable to charged and polar molecules
- -5 to 8 nm thick (different thicknesses at different parts of membrane)
- -asymetrical(different on each face)
- -fluid and dynamic
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Membrane Composition
- -primarily lipid(phospholipids, glycoshingolipids and sterols) and protein
- -more protein in membrane the more active it is
- -outer and inner faces can have different composition, this serves different purposes (nothing happens by accident kinda thing)
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Peripheral Membrane Proteins
- -associate with membrane through charge-charge interactions or H bonding
- -weakest link to membrane, can easily be removed by change in temp or pH
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Integral Membrane Proteins
- -span through the entire bilayer
- -can be extracted from membrane with detergents (dish soap)
- -protein direction matters
- -transmembrane portion hydrophobic, extra and inter cellular portions hydrophyllic
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Types of Trans-Membrane (integral) Proteins
- -Beta barrel and alpha helical bundles
- -alpha helical bundles almost always have 7 alpha helices
- -alpha helices H bond with each other amide-carbonyl to satisfy bonding arrangements
- -inter membrane portion structure can be predicted by AA sequence
- -stretches of 20 hydrophobic residues (AA) are likely membrane spanning
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Greasy Fingers
- -proteins that are anchored to the membrane through lipid attachment that serve as anchors, this allows for them to be easily removed by a lipase
- -GPI anchored proteins always at outer face
- -fatty acyl or prenylated proteins always to innner face
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Membrane Fluid Mosaic
- -lateral movement of protein and lipids is very rapid and constant
- -temperature can affect membrane fluidity
- -too cold=ordered paracrystalline gel state
- -too hot=liquid disordered state
- -ideal temp=liquid ordered state
- -cells can adjust membrane composition to maintain liquid ordered state
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Transbilayer Movement
- -flipping from facing out, to facing in or vice versa
- -requires catalyst, flippase
- -slow process because polar head group must pass through hydrophobic trans membrane environment
- -membrane lipids are initially produced on inner face and some must be flipped to outer face
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Lateral Membrane Movement
- -higher order structures can be "fenced" withing membrane, restricting movement
- -membrane proteins can also be linked to internal structures, which again limits lateral movement
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Membrane Rafts
- -glycosphingolipids form clusters (rafts) that exclude glycerophospholipids
- -longer, saturated acyl groups of sphingolipids form better associations with cholesterol in outer layer which allows thicker and more ordered rafts
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Membrane Fusion
- -easily and frequently occurs because they are non covalent bonds
- Generally Involves
- 1)appropriate recognition
- 2)close association of membrane surfaces, with exclusion of water molecules
- 3)localized disruption of the membranes to allow fusion of outer leaflets (hemifusion)
- 4)fusion of bilayers to form single bilayer
- 5)regulation
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Botox and Botulism
- -botulism found in canned foods and improperly handled meats
- -botulinic toxin cleaves SNARE and SNAP25 proteins which are neurotransmitter proteins, this causes paralysis and eventually death
- -botox is a localized injection to cleave the proteins for both muscle spasms and cosmetic applications
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Simple Diffusion
- -non polar gases (O2 and CO2) and hydrophobic molecules can directly cross the membrane when moving down the concentration gradient
- -no energy needed, does not require carrier protein and not saturable
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Facilitated Diffusion
- -uses membrane transporters to lower the activation energy barrier for crossing the membrane
- -no energy needed
- -two kinds, channels and carriers
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Facilitated Diffusion Channels
- -can only transport molecules down the concentration gradient
- -molecules move through very quickly (door analogy) because channels bind substrate very weakly
- -cannot be saturated
- -allow movement of polar molecules by lining the interior of the channel with other polar molecules
- -molecules must have appropriate size, charge and geometry to use
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Facilitated Diffusion Carriers
- -protein binds specific solute (molecule) and transports it down concentration gradient
- -does not require energy
- -can become saturated becaue it moves slowly (turnstile vs door)
- -occurs with glucose permease of erythrocytes, facilitated diffusion occurs 50,000 times faster than regular diffusion, specific for movement of D-glucose
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Coupled Transport
- -electroneutral exchange of ions prevents the electrical potential across the membrane from changing
- -chloride bicarbonate exchange protein
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Active Transport
- -uses the input of energy(ATP) to move molecules against their concentration gradient
- -ATP is used to force a conformational change in the transport/carrier protein that favors the movement of the molecule against the gradient
- -can become saturated
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F-Type ATPase
-uses a proton gradient in reverse to generate ATP in mitochondria and chloroplasts
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V-Type ATPase
-pumps protons across a membrane to acidify the contents of cell
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ABC Transporters
- -active transport
- -pump things out of the cell
- -multi drug transporter pumps can cause medicine to be ineffective because the body is so good at expelling unknown substances
- -contain two ATP binding domains
- -ATP-Binding Cassette transporters
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Secondary Active Transport
- -uses the energy created by moving molecules down their concentration gradient as power to move other molcules up their own concentration gradient
- -ex. glucose uptake-glucose can move against its gradient by paying the price of moving 2 Na ions down their concentration and electrical gradient (implication being that no Na ions to move means no glucose can be moved either)
- -requires carriers, can be saturated
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Voltage Gated Ion Selective Channels
- different from ion transporters 3 ways
- -much faster
- -no saturation limits
- -gated (can be opened and closed in response to cellular events)
- -carbonyls of alpha helices line up facing the inside of channel which allows them to take the place of water for H bonding
- -channels are narrow and very specific K+ channels allow K+ to move through 10,000 times faster than a Na+ ion
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