Membrane Bi-layer Flashcards
What is the composition of the membrane?
40% lipid
60% protein
1-10% carbohydrate
Give some of the functions of the biological membrane
- Continuous, highly selective permeability barrier
- Allows control of the enclosed chemical environment
- Communication
- Recognition of signalling molecules, immune surveillance and adhesion proteins
- Signal generation in response to stimuli (electrical or chemical)
What does it mean if a molecule is amphipathic?
It has hydrophilic and hydrophobic regions
How are phospholipids named?
Phosphatidyl-X
Where ‘x’ is whatever the head group is
What type of bond introduces a kink in the fatty acid chain of the phospholipid?
A cis bond
How is a glycolipids formed?
When a phosphocholine moiety is replaced with a sugar on a phospholipid
What is the difference between cerebrosides and gangliosides?
They are both types of glycolipid
Cerebroside - head group is a sugar monomer
Ganglioside - head group is an oligosaccharide
How is the lipid bilayer formed?
It is spontaneous in water
Driven by Van der Walls attractive forced between hydrophobic tails
What forces is the bilayer stabilised by?
Electrostatic and hydrogen bonding between hydrophilic moieties
Interactions between hydrophilic groups and water
What percentage does cholesterol make up of the total membrane lipid?
45%
What is the structure of cholesterol?
A polar head group
A rigid, planar steroid ring structure
A non-polar, hydrocarbon tail
What effects does cholesterol have on the bilayer?
It reduces phospholipid packing, increasing fluidity
It reduces phospholipid chain motion, reducing fluidity
What could form a head group in a phospholipid?
Choline, amine, amino acid, sugars
What movements can phospholipids do?
Lateral diffusion
Flexion
Rotation
Flip flop
What effect do cis double bonds have on the bilayer?
Reduce packing
What functional evidence is there for proteins in the bilayer?
They carry out facilitated diffusion, produce ion gradients and there is specificity to cell responses
What biochemical evidence is there for proteins in the membrane?
Freeze fracture
Membrane fractionation and gel electrophoresis
What movements can proteins carry out?
Conformational change
Rotation
Lateral diffusion
What movement can proteins NOT carry out?
Flip flop
How is membrane protein mobility restricted by other proteins?
Aggregates with other proteins
Tethered to basement membrane or internal structures eg cytoskeleton
What are the lipid mediated effects in protein mobility restriction?
Most proteins tend to go to cholesterol-poor regions in the membrane
How do peripheral membrane proteins associate with the bilayer?
Electrostatic and hydrogen bond interactions
How can peripheral proteins be removed from the bilayer?
Change to pH
How do integral membrane proteins interact with the bilayer?
Interact with hydrophobic regions
How are integral proteins removed?
Need detergents or organic solvents that compete for the non-polar interactions in the bilayer
Give an example of when asymmetrical protein orientation is important
Receptor for a hydrophilic extracellular molecule such as insulin needs to have its recognition site facing towards the extracellular space for it to function.
Give the structure of spectrin
α and β subunits wind together to form an anti-parallel heterodimer.
Two heterodimers form a head-to-head association to form a heterotetramer of α2β2.
Give properties of spectrin
Long and floppy
Describe how actin and spectrin are in the cytoskeleton
Rods of spectrin are cross-linked by short, actin profilaments.
Band 4.1 and adducin molecules form interactions towards the ends of the spectrin rods.
How is the actin-spectrin network attached to the membrane?
Through adapter proteins:
Ankyrin (band 4.9)
Band 4.1
Why is the erythrocyte cytoskeleton important?
Allows erythrocytes to pass through very narrow capillaries without lysis.
How does dominant hereditary spherocytosis lead to haemolytic anaemia?
Spectrin levels are depleted by 40-50%.
Cells are more round and become less resistance to lysis.
Cleared by the spleen so shortened life
Bone marrow is unable to compensate enough for shortened life span -> haemolytic anaemia.
What type of backbone are phospholipids based on?
Glycerol
Which carbon atoms of glycerol are fatty acid side chains attached to?
C1 and C2
Where are phospholipids synthesised?
In the ER
What are two common carbon chain lengthens in the fatty acid side chains of phospholipids?
C16 and C18
Which bonds can peripheral membrane proteins form with the membrane?
Electrostatic interactions
Hydrogen bonds
Disulphide bonds
Name some peripheral membrane proteins of the erythrocyte cytoskeleton
Spectrin Actin Adducin Band 4.1 Ankyrin
Name two integral membrane proteins of the erythrocyte skeleton
Band 3 - anion exchanger
Glycophorin A
What length are membrane spanning domains of membrane proteins normally?
Between 18 and 22
Which secondary structure are membrane spanning domains normally?
Alpha-helix
