Week 3: Membrane Mobility Flashcards
What structure does biological memmbrane have?
Liquid crystal structure
Lipid crystal structure
Lipid are close packed and relatively high ordered - but free to move in some directions
What does cooling the membrane result in?
Phase transition
What does the barrier function of the cytoplasmic membrane depend on,
Physical state of lipid bilayers
At lower temperature what happens to the membrane lipid bilayers ?
Undergo a reversible change of from fluid to a non fluid array of fatty acyl chains
What does changing growth temperature trigger?
Cell responses that change the lipid composition of membranes
What is phase transition?
Temperature required to induce a change in lipid physical state from ordered gel phase to disordered liquid crystalline phase
What factors control membrane fluidity?
1) saturation/desaturation of fatty acids 2) cholesterol content 3) temperature
saturation/desaturation
Introduces bends - desaturated Lipids cannot pack so closely and do not form gels so easily Lipids with shorter chains are less stiff and less viscous Make tails more rigid - cannot rotate easily (membrane more fluid). Phase transition temperature is lowered
Cholesterol content
Disrupt packing of fatty acid chain and inhibit the formation of gel state Widens the range at which the membrane stays fluid. Bidirectional regulator At high temperature: stabilises the membrane and raises its melting point Lower temperature: it intercalates between phospholipids and prevent them from clustering together and stiffening
Temperature
Heats up the membrane Lipids require thermal energy when heated; energetic Lipids move around more, arranging and rearranging randomly, membrane more fluid
What does Cyanobacterium lack the genes for?
Coding for desaturates
Desaturases
Enzyme that introduce double bonds into fatty acids
Why is membrane fluidity important ?
1) it allows membrane proteins rapidly in the plane of bilayer 2) it permits the membrane Lipids and proteins to diffuse from sites where they are inserted into bilayer after their synthesis 3) it enables membrane to fuse with one another and mix their molecules 4) it ensures that membrane molecules are distributed evenly between daughter cell when a cell divides 5) necessary to allow diffusion and dynamic interaction of proteins in the membrane (cell signalling) 6) diffusion of other membrane compartment 7) membrane assembly
What can lipid bilayer exist as?
Relatively fluid state
How can phospholipid move?
Laterally within the same leaflet with considerable ease
How can the mobility of individual Lipids molecules within bilayer of plasma membrane be observed?
Under microscope by linking the polar heads of the lipid with gold particles or fluorescent compounds
What is the half-life of a phospholipid molecule moving across to the other leaflet measured?
Hours to days
What movement of phospholipid is more restricted?
Flip flop to other side of the membrane
How does flip flop occur?
The hydrophilic head group of lipid must pass through the interval hydrophobic sheet of membrane which is thermodynamically unfavourable?
What enzymes actively move certain phospholipids from one leaflet to the other?
Flippases
What does the enzyme establish?
Lipid asymmetry and reverse the slow rate of passive transmembrane movement
What are the heterogeneity in Membranes?
1) Asymmetric distribution of Lipids in the two bilayer leaflets 2) different lipid composition of different cellular membrane systems 3) patchy distribution of proteins and Lipids e.g. lipid rafts
What does the lipid bilayer consist of?
Two distinct leaflets that have a distinctly different lipid composition
What happens when intact human RBC are treated with lipid-digesting phospholipase?
80% of the phosphatidyl-choline of the membrane is hydrolysed Only 20%: phosphatidylethanolamjne and less than 10: phosphatidylserine are attacked
What does the outer leaflet have a high concentration and low concentration of?
High - phosphatidylcholine (PC) Low - PE and PS
What does the endoplasmic reticulum have high levels of?
Phosphatidylcholine (PC)
What if the intermediary between ER and PM?
Golgi complex
What does the artificial bilayer consist of?
Phosphatidylcholine and sphingomyelin
What was observed when studying artificial bilayer?
Cholesterol and sphingolipida tend to pack together to form micro domains that are more gelated and highly ordered
Where does cholesterol-rich microdomain tend to float?
More fluid and disordered environment of the artificial bilayer
What is lipid rafts?
Concentrate particular proteins thereby organising the membrane into functional compartments
What does lipid fast provide?
Favourable local environment for cell surface receptors to interact with other membrane proteins that transmit signal from extracellular surface to cell interior
Where is a GPI-anchored protein localised in?
Fat
What does the inner leaflet raft lipid consist of?
Cholesterol and glycerol
What are kinase involved in?
Cell signalling
What is the best evidence for lipid rafts?
Simple, artificial model Membranes
What are the methods for measuring mobility?
1) microscope 2) cell fusion 3) fluorescence recovery after photo bleaching (FRAP) 4) single particle trafficking
Cell fusion
Technique where two different types of cells(two different species) can be fused to produce a new cell with a common cytoplasm and single continuous plasma membrane
How was the effect of temperature on the diffusion of membrane proteins revealed?
Examining percentage of cells in which the proteins from two species have become intermixed after 49 Mins after fusion
FRAP
A membrane protein can be labelled using a specific probe (fluorescent antibody) Labelled cells are placed under the microscope and irradiated by a sharply focused laser beam that bleaches the fluorescent molecules Leaves a circular spot on the surface of cell that is largely devoid of fluorescence
What does the rate of fluorescence recovery provide?
Direct measure of the rate of diffusion of mobile molecules
What is single-particle trafficking?
Label individual membrane protein molecules or Lipids and follow their movement by high-speed video microscopy
What does computer analysis indicate?
Phospholipid diffuses freely within one compartment before it jumps the fence into neighbouring compartment
What are the movement of membrane proteins?
A) some membrane proteins move randomly throughout the membrane B) some membrane proteins fail to move and are immobilised C) some proteins nice in a highly directed manner toward one party of cell or another D) some membrane are crowded with proteins, the random movement of one molecule can be impeded by its neighbour E) membrane underlying skeleton forms network of fences around portions of membrane, create compartment that restrict the distance an integral protein can travel F) movement of transmembrane protein through bilayer is slowed by the presence of extracellular material that can entangle the external portion of protein molecule
