CH 7 - membranes Flashcards
timeline for development of the fluid mosaic model
lipid nature of membrane: lipophilic substances visibly penetrate plant root hairs; there’s something fatty about cell membranes
lipid monolayer: extract lipids from a known number of RBC, estimate their area, mix lipids with a hydrocarbon, and pour it onto water, let the hydrocarbon evaporate, phospholipids form a monolayer, estimate the area of the liquid monolayer = twice that of the RBC; cell membranes are a bilayer!
lipid bilayer: cell membranes more permeable to ions and sugars
lipid bilayer plus protein sheets: sandwich with some proteins forming pores
unit membrane: TEM - all membranes = dark light dark, protein lipid protein; the dark lines are too thin; protein-lipid ratios vary a lot
fluid mosaic model
membrane protein structure
lipid raft
fluid mosaic model
proteins pass through the bilayer or are stuck to the surface; lipids and proteins move about side-to-side freely; lipids= fluid, uniform distribution; proteins=mosaic, uniform distribution
membranes are two quite fluid layers of lipids with proteins localized within and on the lipid layers and oriented in a specific manner with respect to the inner and outer membrane surfaces
evolution of fluid mosaic model
many proteins have their movement restricted
lipids may form patches of semi-solid “lipid-rafts”
leaflet
each monolayer is called a leaflet
P face
inner monolayer
E face
outer monolayer
membrane lipids
phospholipids = phosphoglycerides and sphingolipids
glycolipids - have sugar but no phosphate
steroids
thin layer chromatography
can separate membrane lipids
based on hydrophobicity
stationary phase: hydrophilic
mobile phase: hydrophobic
the more hydrophobic/less hydrophilic, the MORE the lipid travels
movement of phospholipid molecules within membranes (Fig 7-10, p. 167)
transverse diffusion aka flip-flop = rare
rotation
lateral diffusion
FRAP
Fluorescence Recovery After Photobleaching (Fig 7-11, p. 169)
Tm = transition temperature
temperature at which a bilayer becomes fluid (“melts”) when warmed from a solid gel-like state
high Tm
greater tendency to be in gel state
favored by long fatty acid tails and saturated fatty acid tails
adding cholesterol to artificial bilayer
no clear peak Tm
membrane is in between gel and liquid
may be a function of steroids in membranes
homeothermic adaptation
regulating membrane fluidity in response to changes in temperature
bacterial response to decreasing temperature
some bacteria respond to decreasing temperature be decreasing fatty acid chain length, other bacteria add double bonds
lipid raft hypothesis
cholesterol, sphingolipids, and certain proteins form gel-like (not so fluid) rafts that float in liquid sea of the other lipids and proteins
evidence of lipid raft hypothesis from detergent extractions (slide 10-25 and 13.12)
detergents are amphipathic molecules that break down phospholipid bilayers
mild (non-ionic) detergents dissolve some membrane lipids and proteins while others are insoluble, detergent-resistant
isolate resistant components on sucrose gradient
resistant material is enriched in sphingolipids, cholesterol, and certain proteins
implies that mild detergents dissolve the liquid phase of the membrane leaving semi-solid rafts intact because they are detergent resistant
evidence of lipid raft hypothesis from artificial membranes
pure phosphoglyceride artificial membrane –> Tm
phosphoglycerides plus cholesterol –> no clear Tm
phosphoglycerides plus cholesterol plus sphingolipids –> lipid rafts
evidence of mosaic of proteins from freeze fracture (Fig 7-17, Fig 7-16, Fig 7-18)
Particles are evident. How do we know the particles are proteins? Number of particles correlated with abundance of membrane protein.
Artificial phospholipid bilayers don’t have particles, unless you add protein too. Conclusion: proteins are distributed uniformly across the cell membrane.
peripheral membrane proteins
proteins that can be separated from the membrane with high salt or a change in pH —> will affect electrostatic interactions
integral membrane proteins
proteins that require detergents to be dissolved
what types of residues would be found in the transmembrane segments?
hydrophobic
hydrophobicity (hydrophathy) analysis
easy way to predict integral membrane protein structure
gene sequence –> amino acid sequence –> running average of the hydropathy index
functions of membrane proteins (slide 8.9)
- enzyme
- membrane transport
- receptors
- intercellular joining
- adhesion to extracellular matrix
- electron transport
plasma membrane proteins
may be attached to the cytoskeleton, providing cell shape Ex. RBC
glycosylation
process by which sugars are attached to proteins to form glycoproteins
plasma membranes display oligosaccharides. glycolipids make some contribution but most oligosaccharides are attached to glycoproteins. such glycolipids and glycoproteins are glycosylated.
evidence for mobility of membrane proteins (Fig 7-28, p. 187; Fig 7-29, p.188)
–mixing of membrane proteins that occurs when cells from two different species are fused and the membrane proteins are labeled with specific fluorescent antibodies
–exposure of vesicles to an electric field causes the membrane particles to migrate to one end of the vesicle - if proteins were not mobile, this would not happen
some proteins are not free to move around. they may be:
attached to cytoskeleton
part of a lipid raft - the raft can move, but they can’t
restricted by cell-cell junctions - polarized cells have membrane proteins restricted by tight junctions to one part of the membrane
