Module 2: Bio-membranes Flashcards
Effects of carbon chain length
The longer the chain length, the greater the melting point but the lower the solubility in H2O.
Effects of double bonds
the greater the number of double bonds, the lower the melting point
lead to unsaturated carbon chains and kinked
they stop close packing when unsaturated
Glycerophospholipids
2 fatty acids - Glycerol - phosphate -alcohol
fatty acids are often unsaturated
alcohol is the head group
Glycerophospholipid head groups
Serine, Ethanolamine, Choline, Glycerol, Inositol
Sphingolipids
Major membrane components
Derivatives of the amino alcohol sphingosine
N-acyl fatty-acyl derivatives of
sphingosine are called ceramides
Steroids
Mostly of eukaryotic origin
Most common is cholesterol (also known as sterol)
Cholesterol is a major component of the plasma membrane
Similar molecules in the
different kingdoms
sterols:
cholesterol - animal
ergesterol - fungal
stigmasterol - plant
Biological membranes
Define external boundaries of cells/
intracellular compartments (eukaryotic cells)
– Regulate traffic across this boundary
Functions
– Signal transduction
– Cell communication
– Complex reaction sequences
– Energy transduction
Special properties
– Flexible
– Self-sealing/can fuse
– Selectively permeable
– Two-dimensional
Membrane: fluid mosaic model
Lipid bilayer (~30-40 Å thick)
Lipids are in constant motion
* Free lateral diffusion
* Almost no unassisted flipping
Membrane proteins also diffuse
laterally
Lipid aggregates
in bilayers, the individual units are cylindrical (head = side-chain)
in micelles, individual units are wedge-shaped (head > side-chain)
also forms vesicles or liposome
Which structure forms is determined by
* Size of the fatty acyl chains
* Degree of saturation of the fatty acyl chains
* Size of hydrophilic head group
* Temperature
Stabilisation of bilayers (bonding)
ionic bonds between head groups
hydrogen bonds with water
van der waals interactions between fatty tails
Lipid mobility in bilayers
lipids have two main types of motion:
1. spinning without changing location
- rotation around their long axis
2. lateral diffusion - movement with the same leaflet
- phospholipids exchange positions with its neighbouring molecules
- VERY FAST!
this gives them a viscosity similar to olive oil (100x water)
transverse diffusion “flip flop”
very rare and takes a lot of energy - has a high energy barrier
affect of heat on membranes
heat disorders the interactions between the fatty tails to change the membrane from a gel to a fluid state
gel + heat = fluid
effects of fatty acids on membrane fluidity
long chain fatty acids - aggregate extensively to give low fluidity (gel-like state)
short chain fatty acids - have less surface area to aggregate and increase fluidity
unsaturated fatty acids - also aggregate less extensively and increase fluidity
Sphingomyelin and cholesterol
SM associates into a thicker, more gel-like bilayer than phospholipids
cholesterol increases thickness by ordering fatty acid tails, and stabilises head group interactions
what aspects of membrane function require curvature
- viral budding
- formation of vesicles e.g. at plasma membrane
- stability of curved structure e.g. microvilli in small intestine
proteins help stabilise curved membranes!
how does asymmetry arise?
lipids do not spontaneously flip from one leaflet to the other
specific enzymes can catalyse translocations
- lipids that are synthesised in one leaflet stay in that certain leaflet when moved e.g. in the cytosolic side of golgi moves to stay in cytosolic side of cell (plasma) membrane
BUT pc arrives at the plasma membrane on the cytosolic side but is transported to the other leaflet by “flippase” enzymes which require energy from ATP hydrolysis
Catalysed transbilayer translocations
Flippase:
moves PE + PS from outer to inner leaflets
Floppase:
moves phospholipids from inner to outer leaflets
Scramblase:
moves phospholipids in either direction toward equilibrium
note: all use ATP hydrolysis
membrane microdomains
micro-domains control lateral diffusion
e.g. stable associations of sphingolipids and cholesterol : lipid rafts
- can be disrupted by methyl-3-b-cyclodextrin (removes cholesterol from membranes) or antibiotic filipin (sequesters cholesterol)
- specific signalling proteins associated with lipid rafts
membrane proteins
different membranes have different compositions in terms of proteins:lipids ratio. some have more protein than lipid. e.g. bacterial, mitochondrial, chloroplast
membrane protein functions
transporters
receptors
adhesion molecules
lipid synthesis
energy transduction
types of membrane proteins
integral (transmembrane)
- firmly associated with membrane (need detergents etc to remove)
- span membrane (hydrophilic and hydrophobic interactions)
lipid-anchored
- protein covalently linked to one or more lipid molecules
- lipid embedded in membrane leaflet to anchor the protein
- polypeptide does not enter the bilayer
peripheral
- can be released with milder treatment
- does not contact hydrophobic core of bilayer
- forms hydrophilic interactions with membrane surface or other membrane proteins
