MR 1 The Lipid Bilayer Flashcards
What is the general membrane composition?
40% lipid
60% protein
1-10% carbohydrate
20% water (membrane hydrated)
What is an amphipathic molecule
One containing both hydrophilic and hydrophobic moieties
What molecules can be phospholipid head groups?
Choline, amines, amino acids, sugars
How long are the most common fatty acid chains?
C16 and c18
What formation are double bonds in fatty acid chains?
Cis
What do the double bonds in fatty acid chains do?
Introduce a kink in the chain decreasing phospholipid packing
What is a plasmalogen?
Non classical phospholipid e.g sphingomyelin
Why is sphingomyelin special?
Only phospholipid not based on glycerol. In membrane resembles other phospholipids
What is a glycolipid?
Sugar containing lipid
Define cerebroside and ganglioside
Cerebroside head group is a sugar monomer
Ganglioside head group is a sugar oligosaccharide
What % of membrane lipid does cholesterol make up?
45%
What two structures can amphipathic molecules form in water?
Micelles and bilayers
What drives spontaneous bilayer formation in water?
Van der waals between hydrophobic tails
What stabilises the cooperative structure of bilayers?
Non covalent forces; electrostatic and h-bonding between hydrophilic moieties and interactions between hydrophilic groups and water
In what ways can lipids move in bilayers?
Intra-chain motion: kink formation in fatty acyl chains
Fast axial rotation
Fast lateral diffusion within plane of bilayer
Flip-flop- movement from one half of bilayer to other on one for one exchange basis(rare)
Name the modes of motion of membrane proteins
Conformational change
Rotational
Lateral
NO FLIP FLOP
Why don’t membrane proteins flip flop?
They have large hydrophilic moieties and large amounts of energy would be required for them to pass through the hydrophobic region of the bilayer
What restricts mobility of membrane proteins?
Lipid mediated effects- proteins spread out into fluid phase or cholesterol poor regions
Membrane protein associations
Association with extramembranous (peripheral) proteins e.g cytoskeleton
What are peripheral membrane proteins and how are they removed?
Bound to surface by electrostatic and hydrogen bonds (includes disulphide interactions)
Can be removed by changes in pH or ionic strength
What are integral membrane proteins and how are they removed?
Interact extensively with hydrophobic regions of bilayer.
Cannot be removed by changes in pH or ionic strength
Removed by detergents and organic solvents that compete for the non-polar interactions in the bilayer
How are membrane proteins translated so that they span the membrane of vesicles?
Using a stop transfer signal that is highly hydrophobic and remains in the ER so that the rest of the protein is translated outside of the ER in the cytoplasm
How long is the stop transfer signal?
18-20AAs which is the distance to span a phospholipid bilayer
What usually immediately follows a stop transfer signal?
A few basic (+ve) residues
What are hydropathy plots used for?
To see how many transmembrane regions a protein has
Why is asymmetrical orientation of proteins in membranes important?
Receptors for hydrophilic extracellular messengers like insulin must have recognition site facing towards extracellular space
How do fatty acids affect membrane fluidity?
Double bonds in chain reduce phospholipid packing increasing fluidity
How does cholesterol affect membrane fluidity?
Stabilises plasma membrane by H bonding to fatty acid chains
Abolishes endothermic phase transition of phospholipid bilayers
Reduces phospholipid packing, increasing fluidity
Reduces phospholipid chain motion decreasing fluidity
Explain the fluid mosaic model
Fluid- due to hydrophobic integral components such as lipids that move laterally throughout the membrane like a fluid
How is the shape of erythrocytes held?
Cytoskeleton of spectrin and actin
What proteins in the cytoskeleton are peripheral?
Actin Spectrin Ankyrin Band 4.1 Adducin
Which erythrocyte cytoskeleton proteins are integral?
Band 3
Glycophorin A
What causes hereditary spherocytosis?
In dominant form spectrin is depleted by 40-50% so cells round up and lysis increases as RBCs have reduced lifespan
What causes hereditary Elliptocytosis?
Spectrin molecules unable to form heterotetramers giving fragile elliptoid cells leading to haemolytic anaemia
