1.2 - Membrane Fluidity

Question 1

What is membrane fluidity

Answer 1

• 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

Question 2

Modes of movement of individual lipid molecules

Answer 2

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

Question 3

What factors contribute to the fluidity of the membrane

Answer 3

• 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

Question 4

How does cholesterol stabilise the membrane

Answer 4

☞ 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

Question 5

What are lipid rafts

Answer 5

• 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)

Question 6

What molecules are increased/decreased in lipid rafts

Answer 6

increased
- Cholesterol
- Sphingomyelin
- Glycolipids

decreased
- Phosphatidyl choline
- Unsaturated long-chain phospholipids

Question 7

What are some possible roles of lipid rafts

Answer 7

• 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

Question 8

Why does membrane fluidity matter (on a larger scale)

Answer 8

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)