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LIPID MEMBRANES (lipid rafts (mammalian cell membranes contain regions…
LIPID MEMBRANES
lipid rafts
mammalian cell membranes contain regions enriched in sphingolipids and cholesterol - lipid rafts
ordered regions of the membrane, distinct from surroundings
certain membrane proteins preferentially associate with lipid rafts
able to float freely through the membrane
unsaturated chains have kinks which do not accommodate cholesterol so well
allow compartmentalisation of cellular processes
cholesterol associates with glycosphingolipids due to their long, saturated fatty acids
associated with signal transduction and protein trafficking
explains how larger integral proteins can be accommodated
questions remain about their transient nature, lifetime - some controversy over whether they are artefacts
what is a biological membrane
defines the external boundary of the cell
separates compartments (mitochondria, nucleus, chloroplast, Golgi body, endoplasmic reticulum)
acts as a barrier to diffusion
pumps solutes in/out of cell
maintains proton gradient needed for ATP production
receptors recognise extracellular chemical signals – communicate them to interior
lipid bilayer embedded with proteins
(solute transport, enzymatic activity, signalling)
membranes are assymetric
slow rate of transverse diffusion allows membranes to be asymmetric
in bacteria, lipids are added to the inside of the membrane
so only the inner/cytoplasmic monolayer is added to
since transverse diffusion is slow, the newly-added lipids rarely flip between leaflets spontaneously
Flippases and floppases use ATP to move specific phospholipids from one leaflet to the other
Scramblases move lipids in either direction toward equilibrium
asymmetry can also be generated and maintained by enzymes
Eukaryotic cells establish asymmetry in the ER or Golgi and membrane fragments flow from these organelles to other membranes
movement of lipid within membrane
lateral diffusion
movement within a leaflet
rapid e.g. a lipid can diffuse the length of a bacterial cell in 1 second at 37oC
2D lateral diffusion
liquid phase transitions
fluidity of the membrane depends on the composition in terms of acyl chains
Butter = 51%
Olive oil = 14%
above the phase transition temperature they exist as a fluid (“liquid crystalline”)
changes affect activity of membrane proteins e.g. transport and catalytic activity
lipid bilayers below the phase transition temperature exist as a gel (“solid”)
cholesterol
present in eukaryotic membranes at 20-25% by mass
modulates / controls membrane fluidity
intercalates with acyl chains and reduces mobility - prevents extreme fluidity increases at high temperatures
disrupts ordered packing of extended acyl chains prevents extreme fluidity decreases at low temperatures
maintains fairly constant fluidity across temperature range
transverse diffusion (flip flop)
movement between leaflets
polar head has large solvation shell that must be shed to cross
massive energy requirement
about one-billionth the rate of lateral diffusion
lipid structures
phospholipids
glycerol backbone
two fatty acyl groups
phosphate group
head group
amphipathic lipids form various structures in aqueous environments
amphipathic: both hydrophilic and hydrophobic
micelles: polar head group forms outside, hydrophobic tails inside
bilayer membranes: two lipid sheets (monolayers or leaflets), polar heads on outside, hydrophobic tails inside, typically 4-6 nm thick
liposomes: aqueous environments enclosed by lipid bilayers, very useful experimental tools
lipid shapes
cone shaped lipids form micelles
lysophospholipids - one acyl chain
detergents
ionic salts of fatty acids
tubular shaped lipids form bilayers
phosphatidylcholine
digalactosyldiacylglycerol - two sugars
inverted cone shaped lipids form inverted tubular micelles:
phosphatidylethanolamine
monogalactosyldiacylglycerol - one sugar
properties of membranes
composition of membranes
proteins
50 – 75% by mass
carbohydrates
glycolipids and glycoproteins
lipids
25-50% by mass
phospholipids, gylcosphingolipids, cholesterol
fluid mosaic model
membranes are 2-D solutions of orientated lipids and globular proteins
lipid bilayer acts as a solvent for membrane proteins and as a permeability barrier
proposed by Singer and Nicolson in 1972
membrane lipids and proteins can undergo free diffusion within membrane
icebergs floating in a sea of fluid lipid
updates to the fluid mosaic model
up to 30% of the lipids in a membrane bilayer may not form bilayers by themselves
stabilised into a bilayer by the other 70% & proteins
cholesterol, phosphatidylethanolamines
chloroplast thylakoid membranes: 50% of polar lipid is monogalactosyldiacylglycerol (MGDG)
some membrane lipids show restricted movement within the membrane rather than free diffusion
some membrane proteins are immobile and may be tethered by cytoskeletal filaments
lipid : protein ratio differs between membranes
high in myelin membranes insulating nerve cells, 70 - 85% lipid, 15 – 30% protein
low in inner mitochondrial membranes. 23% lipid 77% protein
functions of membranes
form external boundaries of cells - impermeable to ions and most polar molecules. in order of increasing permeability: Na+, K+, Cl-, glucose, tryptophan, urea, glycerol, indole, water
separate compartments in cells
provide a physical support for catalytic proteins -
especially electron transport chains (respiration in mitochondria and photosynthesis in chloroplasts)
location of membranes
prokaryotes
some have a single membrane (G+), some have 2 (G-)
inner membrane
permeability barrier
outer membrane (gram negative bacteria) for protection :
fairly permeable to small molecules due to porins
region between membranes is called the periplasm
eukaryotes
single cell membrane / plasma membrane
internal compartments surrounded by specialised membranes (sometimes two: mitochondria, nucleus, endoplasmic reticulum)
membrane formation
spontaneous formation of lipid bilayers
driven by hydrophobic (water repulsive) interactions
van der Waals attractive forces between hydrocarbon chains
electrostatic forces & hydrogen bonding attractive forces between water and the polar head groups
growth of bimolecular sheet in water is fast and they can be large (human ovum ̴ 1mm diameter)
bilayers are self-sealing - a hole is energetically unfavourable, so no exposed hydrocarbon chains