What is membrane fluidity
- Phospholipid bilayer needs to be ‘fluid’ to allow components to fulfil their physiological roles
- If membrane is too ‘gel like’ it will constrain movement and mobility of proteins and also restrict diffusion
- If membrane too ‘fluid’ then loses molecular organisation needed for membrane/protein function
- Need to enable membrane to remain fluid over a relatively wide temperature range
Modes of movement of individual lipid molecules
phospholipids are capable of a wide range of movement
- Flexion: changing the angle between hydrocarbon tails aka ‘wobbly bob’
- Rotation
- Lateral diffusion: movement throughout its own monolayer
- Flip-flop: highly energetic process where the phospholipid swaps to the other side of the bilayer. Rare
☞ all modes of movement are directly proportional to temperature and molecular mass
membrane proteins are a bit more restricted in terms of mobility, can only do: conformational change, rotation and lateral diffusion
What factors contribute to the fluidity of the membrane
- temperature is normally constant. Increased temperature → increased kinetic energy → increased membrane fluidity
- unsaturated fatty acids as C=C bonds introduce a ‘kink’ in the hydrocarbon tail, increasing the spacing between neighbouring phospholipids → increased membrane fluidity
- cholesterol acts as a stabilising agent due to it’s paradoxical effects on membrane fluidity. Its rigid (planar) sterol ring reduces fluidity whereas its flexible hydrocarbon tail increases fluidity
How does cholesterol stabilise the membrane
☞ cholesterol stabilises membrane by interaction with heads and tails
☞ cholesterol has rigid (planar) sterol ring that reduces fluidity
☞ cholesterol has a flexible hydrocarbon tail that increases fluidity
☞ it extends functioning temperature range by increasing membrane stability + fluidity
☞ acts as ‘buffering’ molecule preventing abrupt changes in membrane fluidity
at higher temps
- Small OH polar head interacts with acyl group of phospholipid head
- This limits movement
- Rigid steroid plates in hydrophobic region limits the movement of tails
- This works by dampening the interchain kinetic energy exchange between adjacent phospholipids → decreases fluidity
at lower temps
- Interferes with crystalline packing of saturated long-chain phospholipids
- This is due to angled stiff plates and stubby tail
What are lipid rafts
- In cell membranes, there are evidence of areas of organised/specialised distribution of lipids
- Includes increased cholesterol, sphingomyelin and glycolipids
- Reduced phosphatidyl choline + unsaturated long-chain phospholipids
- Increased tight packing + strong interactions between cholesterol, sphingomyelin + glycolipids
- More ordered + less fluid structures
- Provide more stable environment for signalling proteins
- Play a number of role (sep card)
What molecules are increased/decreased in lipid rafts
increased
- Cholesterol
- Sphingomyelin
- Glycolipids
decreased
- Phosphatidyl choline
- Unsaturated long-chain phospholipids
What are some possible roles of lipid rafts
- Play a role in stabilising and organising proteins: enhances efficiency for signalling
- Act as organising domains for receptors and signalling molecules
- Optimise kinetic interaction for signal transduction
Why does membrane fluidity matter (on a larger scale)
cell membranes need to transmit forces
- Throughout their own cell structures + in synchrony with other cells and tissues
- Force transmission through body parts, eg running – will transmit significant forces through bone, muscle and connective tissue etc
- If forces are excessive then damage occurs (eg sprains, fractures etc)
