Membrane Bilayer: Structure and Dynamics Flashcards
What are the general functions of biological membranes (cells and organelles)?
- Continuous, highly selective permeability barrier
- Control of the enclosed chemical environment
- Communication
- Recognition - signalling molecules
- adhesion proteins
- immune surveillance - Signal generation in response to stimuli (electrical,chemical)
By dry weight, what is the most abundant components of membrane bilayer?
- 40% lipids
- 60% protein
- 1-10% carbohydrates
Membranes are hydrated structures, so, 20% of total weight is water.
What type of molecule is a membrane lipid?
- They are amphipathic molecule as they contain both hydrophilic and hydrophobic groups.
What are phospholipid made of?
What is phosphatidylcholine made of ?
- glycerol backbone with 2 fatty acids and phosphate head group attached to it
- it has a glycerol, which has a phosphate attached to it and then a choline head group attached to the phosphate.
Pg13
What are glycolipids?
- Sugar containing lipids
- cerebrosides- head group sugar monomer
- Gangliosides - head group oligosaccharide (sugar multimers)
How does a lipid Micelle and bilayer look?
- lipid Micelle and bilayer both have the hydrophilic head group facing outwards and the hydrophobic tails inwards.
- Micelle is a single layer and forms a circular shape in water.
- bilayer has two layers
What are the different types of phospholipid motion?
- flexion
- rotation
- flip flop (rare)
- lateral diffusion (due to vibrations)
Phospholipids don’t move directly in one line they move by Brownian motion (random movement)
Pg 27
What is the influence of cis double bonds in bilayer structure?
- The introduction of the kink increases the fluidity of the membrane by decreasing packing.
- They are important as it means proteins embedded in the membrane can undertake conformation change to bring about their function
- Also the presence of fatty acids with cis bonds beyond carbon 9 (position 9)in the chain cannot be synthesised in the human body so we need it in our diet.
What is the structure of cholesterol?
- It has a polar head group
- Rigid planar steroid ring structure
- Non-polar hydrocarbon tail.
Pg 39
How does the cholesterol insert into the phospholipid bilayer?
- the hydroxyl group of the cholesterol forms a hydrogen bond with the carboxyl oxygen of the carboxylic acid group of the fatty acid.
- locking cholesterol on an adjacent phospholipid
What is the paradoxical effect of Cholesterol?
- Reduced Phospholipid chain motion, reduced fluidity - by inserting the solid rigid structure of the cholesterol against a region of phospholipid which usually vibrate and flex. It reduces the possible mobility of part of the molecule, so if mobility is reduced the fluidity of the membrane is reduced.
- Reduced phospholipid packing, increases fluidity - by cholesterol attaching to a phospholipid molecule it prevents an adjacent phospholipid from packaging. So by reducing packing you’re increasing fluidity of the membrane.
What are lipid rafts ?
- They are dynamic cholesterol-rich structures, comprised of
- sphingolipids with saturated fatty acid chains
- tightly intercalated cholesterol
They can include or exclude membrane proteins.
Raft affinity for protein is modulated by intra or extracellular stimuli.
Proteins can move in and out.
What are the functions of Lipid rafts?
- Scaffolding proteins involved in signal transduction, e.g. receptors and signalling pathway components
- Cross linking of signalling receptors increase their affinity for rafts
- Partitioning changes their micro-environment, making new interactions possible, affecting their signalling
- Raft clustering may amplify signalling by bringing signalling components together.
Pg 36
