Membrane dynamic- mobility and heterogeneity
Learning objectives:
1. Understand the constraints to mobility of membrane components
2. Understand the factors that control membrane fluidity
3. Understand the ways in which different parts of the same membrane can vary in lipid composition
4. Understand different ways by which membrane dynamics can be measured
Membrane fluidity
Biological membranes normally have a liquid crystal structure
- Can be observed in a simplified form in artificial lipid bilayers
- Crystal means that there is a certain amount of order
- Liquid crystal means that it is not a solid rather it is a fluid.
Therefore, taking a snapshot at any instant would be predictable
If take a video, will see rapid movement
- Liquid crystal structure means that the lipids are close-packed and relatively highly-ordered, but still free to
move in some directions
Membranes stay as liquid crystal structure at a certain temperature range
Cooling the membrane down (10 degrees lower than body temperature ) results in a phase transition
- Below the phase transition temperature the membrane becomes a crystalline gel and very little lipid
movement is possible (rigid and immobile)
How organisms regulate membrane fluidity
Organisms try to keep their lipid membranes in the liquid crystalline state
Changing growth temperature triggers cell responses that change lipid composition of membranes.
- E.g. cells respond to lowering growth temperature by making their membrane more fluid.
-
Does not occur in humans, as are large, complex organisms that can regulate body temperature.
Membrane fluidity is controlled by factors including:
1. Saturation or desaturation of fatty acids
- Desaturation means the introduction of C-C double bonds in the fatty acid tails (makes rigid fatty acid tails.
The more rigid the FA tails, the more fluid the membrane)
- These introduce bends thus desaturated lipids cannot pack closely and do not form gels- the phase-
transition temperature is lowered.
2. Cholesterol control
- Cholesterol molecule disrupt packing of fatty acid chains
- Inhibit formation of the gel state.
- Cholesterol increases mechanical strength
- Humans can live without cell wall due to cholesterol
Mutant studies have shown the importance of membrane fluidity
Nario murata identified genes in cyanobacterium that encode for enzymes called desaturases
Mutants of a cyanobacterium that lack the genes coding for desaturases grow normally at warm temperature but
slowly at low temperature.
- Desaturases are enzymes that introduce double bonds into fatty acids tails
Why is membrane fluidity important?
1. Allow diffusion and dynamic interaction of proteins in the membrane e.g. cell signalling.
2. Diffusion of other membrane components is vital
3. Essential for membrane assembly
, Movement of phospholipids in membrane with timescale
1. Flex
- ~10-9 sec
2. Lateral shift- swap places on the same plane
- ~10-6 sec
3. Transverse diffusion (flip-flop)- move from 1 leaflet of the bilayer to the other, which requires a large amount of
energy as breaking favourable interactions.
- ~10-5 sec
Heterogeneity in membranes
Heterogeneity = the state of being diverse in character or content
Types of heterogeneity
1. Asymmetric distribution of lipids in the 2 bilayer leaflets
- Image showing % membrane lipid in erythrocyte plasma membrane
- The energy barrier for cholesterol to undergo transverse diffusion is small as predominantly a hydrophobic
structure
Learning objectives:
1. Understand the constraints to mobility of membrane components
2. Understand the factors that control membrane fluidity
3. Understand the ways in which different parts of the same membrane can vary in lipid composition
4. Understand different ways by which membrane dynamics can be measured
Membrane fluidity
Biological membranes normally have a liquid crystal structure
- Can be observed in a simplified form in artificial lipid bilayers
- Crystal means that there is a certain amount of order
- Liquid crystal means that it is not a solid rather it is a fluid.
Therefore, taking a snapshot at any instant would be predictable
If take a video, will see rapid movement
- Liquid crystal structure means that the lipids are close-packed and relatively highly-ordered, but still free to
move in some directions
Membranes stay as liquid crystal structure at a certain temperature range
Cooling the membrane down (10 degrees lower than body temperature ) results in a phase transition
- Below the phase transition temperature the membrane becomes a crystalline gel and very little lipid
movement is possible (rigid and immobile)
How organisms regulate membrane fluidity
Organisms try to keep their lipid membranes in the liquid crystalline state
Changing growth temperature triggers cell responses that change lipid composition of membranes.
- E.g. cells respond to lowering growth temperature by making their membrane more fluid.
-
Does not occur in humans, as are large, complex organisms that can regulate body temperature.
Membrane fluidity is controlled by factors including:
1. Saturation or desaturation of fatty acids
- Desaturation means the introduction of C-C double bonds in the fatty acid tails (makes rigid fatty acid tails.
The more rigid the FA tails, the more fluid the membrane)
- These introduce bends thus desaturated lipids cannot pack closely and do not form gels- the phase-
transition temperature is lowered.
2. Cholesterol control
- Cholesterol molecule disrupt packing of fatty acid chains
- Inhibit formation of the gel state.
- Cholesterol increases mechanical strength
- Humans can live without cell wall due to cholesterol
Mutant studies have shown the importance of membrane fluidity
Nario murata identified genes in cyanobacterium that encode for enzymes called desaturases
Mutants of a cyanobacterium that lack the genes coding for desaturases grow normally at warm temperature but
slowly at low temperature.
- Desaturases are enzymes that introduce double bonds into fatty acids tails
Why is membrane fluidity important?
1. Allow diffusion and dynamic interaction of proteins in the membrane e.g. cell signalling.
2. Diffusion of other membrane components is vital
3. Essential for membrane assembly
, Movement of phospholipids in membrane with timescale
1. Flex
- ~10-9 sec
2. Lateral shift- swap places on the same plane
- ~10-6 sec
3. Transverse diffusion (flip-flop)- move from 1 leaflet of the bilayer to the other, which requires a large amount of
energy as breaking favourable interactions.
- ~10-5 sec
Heterogeneity in membranes
Heterogeneity = the state of being diverse in character or content
Types of heterogeneity
1. Asymmetric distribution of lipids in the 2 bilayer leaflets
- Image showing % membrane lipid in erythrocyte plasma membrane
- The energy barrier for cholesterol to undergo transverse diffusion is small as predominantly a hydrophobic
structure
