Session 1 Flashcards
What percentage of the membrane is protein and what percentage is lipid?
60% protein
40% lipid
Traces of carbs
What are the general functions of biological molecules?
Highly selective permeable barrier, control of enclosed chemical environment, communication, recognition, signal generation in response to stimuli
What is an amphipathic molecule?
Molecules which contain both hydrophobic and hydrophilic regions (moieties)
What are the predominant lipids in the membrane?
Phospholipids e.g. phosphatidylcholine
What are the different types of head groups?
Cholines, amines, amino acids and sugars
What variety of fatty acid chains are the most prevalent?
C16 & C18
Unsaturated fatty acid side chains in cis formation introduce a kink in the chain which reduces phospholipid packing
Describe plasmalogens
Only phospholipid not based on glycerol e.g. sphingomyelin
Describe glycolipids
Sugar containing lipids. Cerebrosides - head group sugar monomers. Gangliosides - head group oligosaccharides
Describe choesterol
A plasma membrane lipid which accounts for 45% of the total membrane lipid.
How are bilayers formed?
By congregation of amphipathic molecules. Occurs spontaneously due to van der Waals forces between hydrophobic tails. Co-operative structure is stabilised by non-covalent forces; electrostatic and H-bonding between hydrophilic moieties and interactions between hydrophilic groups and water
What are the four permitted modes of mobility of lipids in a lipid bilayer?
Intra-chain motion (kink formation in the fatty acyl chains)
Fast axial rotation
Fast lateral rotation
Flip-flop (movement of lipid molecules from one half of the bilayer to the other on a one for one exchange basis)
How does cholesterol affect the lipid bilayer?
Prevents crystalline formation of phospholipids which increases fluidity and prevents leakage of ions. It also decreases phospholipid chain movement which decreases fluidity.
What are the functions of the plasma membrane?
Interaction with the basement membrane, interaction with adjacent cells, absorption of body fluids, secretion, transport, synapses nerve junctions, electrical signal conduction and changing shape
What are the functions of membrane proteins?
Enzymes, transporters, pumps, ion channels, receptors and energy transducers.
What modes of motion are permitted for proteins?
Conformational changes, rotational and lateral
What is the mosaic model?
Model of the lipid bilayer. The lipid bilayer is associated with membrane proteins which may be deeply embedded in the bilayer (integral) or associated with the surface (peripheral)
How are peripheral membrane proteins bound to the surface of membranes?
By hydrogen bonds and electrostatic interactions. They can be removed by changes in pH or ionic strength
How are integral membrane proteins bound to the membrane?
They interact with the hydrophobic regions of the lipid bilayer. Cannot be removed by manipulation of pH or ionic strength, but require agents (detergents, organic solvents) that compete for the non-polar interactions in the bilayer
How are proteins orientated in the lipid bilayer?
Why is this important?
Asymmetrically.
Important for function because the recognition site needs to be directed towards the extracellular space in order to function.
What is the erythrocyte skeleton made of?
It’s a network of spectrin and actin molecules
Describe the structure of spectrin
Long, floppy rod-like molecule. Alpha and beta sub units wind together to form an antiparallel heterotetramer of A2B2. These rods are cross linked in to networks by short actin protofilaments
How is the spectrin-actin network attached to membrane?
Through adapter proteins
What is the function of the erythrocyte cytoskeleton?
What is the name of the defects by dysfunction of the erythrocyte cytoskeleton?
Needed to maintain the deformability necessary for erythrocytes to make their passage through capillary beds without lysing.
Haemolytic anaemias
What is hereditary spherocytosis?
What can it lead to?
Condition in which the spectrin levels in the erythrocyte cytoskeleton are depleted (by about 40-50%). Cells become round and more likely to lyse when passing through capillaries, thus they are cleared up by the spleen. This can lead to haemolytic anaemia.
What is hereditary elliptocytosis?
How can it be treated?
Condition in which spectrin molecules are unable to form heterotetramers, resulting in fragile, elliptoid cells.
Can be treated by cytochalasin drugs. These drugs cap the growing end of polymerising actin filaments which can alter the deformability of the erythrocyte.
Describe membrane protein synthesis
Signal sequence is recognised by SRP, which then binds. On ER the SRP is recognised by an SRP receptor or a docking protein. SRP is then released from signal sequence. Reacts with a signal sequence receptor within a protein translocator complex in the ER membrane. Stop transfer signal is used to stop the protein passing through
How are membrane proteins oriented?
Using signal peptidase to cleave parts of the protein
