Flashcards in Membranes and Receptors Deck (82):
What is an amphipathic molecule?
A molecule with both hydrophobic and hydrophilic moieties.
What is the typical composition of a membrane?
20% water when hydrated
Name the constituent parts of a phospholipid
Two fatty acids
Name two different types of head groups
What would you call a phospholipid where the head group is:
What is the effect of having unsaturated fatty acids?
The double bonds in the cis formation introduces a kink and reduces phospholipid packing
What's the significance of sphingomyelin?
Only phospholipid not based on glycerol
What are the two different types of glycolipids and what are their head groups?
Cerebrosides - head group is sugar monomer
Gangliosides - head group is oligosaccharide
What are the two different structures formed by amphipathic molecules in water?
Micelles and bilayers
How can lipids move in the bilayer?
Flexion (intrachain motion)
What are peripheral membrane proteins?
Bound to the surface of membranes by electrostatic attraction and hydrogen bonds. Can be removed using changes in pH or ionic strength.
What are integral membrane proteins?
Interact extensively with hydrophobic regions of the membrane
Cannot be removed by changes in pH or ionic strength
Require agents which compete for non polar interactions - eg. detergents, solvents
What evidence is there for membrane proteins?
Freeze fracture shows proteins on each fracture face.
How can membrane proteins move in the bilayer?
Why can membrane proteins not flip flop?
They have large hydrophilic moieties, so very large amounts of energy would be required.
What restrictions are there on membrane protein mobility?
Association with extra-membranous proteins - eg. cytoskeleton, other cells
Membrane protein associations - ie. aggregations
Lipid mediated effects - proteins tend to accumulate in cholesterol poor regions (signalling molecules are an exception to this)
How is a membrane protein oriented in synthesis?
ER signal sequence remains in ribosome
Synthesis continues into ER lumen until highly hydrophobic stop transfer signal is reached (18-20aa)
Rest of the protein is translated outside the ER in the cytoplasm
Signal peptidase cleaves signal peptide
How are proteins with multiple transmembrane domains formed?
There are multiple hydrophobic stop transfer sequences which are then inserted into the ER membrane in pairs.
What would you use to see how many transmembrane domains a proteins has?
What effects does cholesterol have on membrane fluidity?
Reduces phospholipid packaging because it gets in between phospholipids. This increases fluidity. Reduces phospholipid chain motion because it is a rigid molecule which binds to the fatty acid chains. This decreases fluidity. This acts as a buffer, keeping membrane fluidity stable when temperature changes.
What is a major function of the erythrocytes cytoskeleton?
To maintain the biconcave shape
What are the main constituents of the erythrocyte cytoskeleton? How does it attach to the membrane?
Actin bound to Band 4.1 which is bound to Glycophorin A
Spectrin bound to Ankyrin which is bound to Band 3.
Band 4.1 and Ankyrin are adaptor proteins.
Glycophorin A and Band 3 are intrinsic membrane proteins.
Name two types of haemolytic anaemias, their causes and their effects.
Hereditary spherocytosis - spectrin levels depleted 40-50%. This causes RBCs to round up and so lyse due to increased shear forces.
Hereditary elliptocytosis - defective spectrin unable to form heterotetramers, resulting in fragile elliptioid cells.
Discuss properties of solutes which affect their movement through membranes.
Hydrophobic molecules and small, uncharged, polar molecules can pass through the membrane. Large, uncharged, polar molecules and ions can't pass through the membrane.
What is passive diffusion?
Movement through the membrane dependant on permeability and concentration gradient.
What is facilitated diffusion?
Movement through a specific protein channel, increasing the permeability for that molecule. Doesn't require any energy.
What is active transport?
Using energy gained from the hydrolysis of ATP to transport ions or molecule against their electrochemical gradient.
Name the different types of transporters and their functions.
Uniport - transports single molecule in one direction
Symport - transports multiple molecules in the same direction
Antiport - transports multiple molecules in opposite directions
Give an example of a symporter.
Na+/glucose co transport in small intestine and kidney. Entry of Na+ allows for entry of glucose against concentration gradient without directly needing energy
What are the concentrations of Na+ inside and outside of the cell?
Inside - 12mM
Outside - 145mM
What are the concentrations of K+ inside and outside of the cell?
Inside - 155mM
Outside - 4mM
What are the concentrations of Cl- inside and outside of the cell?
Inside - 4.2mM
Outside - 123mM
What are the concentrations of Ca2+ inside and outside of the cell?
Inside - 0.1uM (10^-7)
Outside - 1.5mM
Describe the structure and function of the Na+/K+ ATPase
Uses ATP to pump 3Na+ out and 2K+ in
It's a P-type ATPase
Alpha subunit - binding sites for K+, Na+, ATP and ouabin (inhibits pump)
Beta subunit - glycoprotein which directs pump to surface
25% of BMR used by this pump
Sets up ion gradients which drives secondary active transport
How is the resting membrane potential set?
Na+/K+ ATPase sets up concentration gradient (only contributes -5mV)
K+ diffuses out of cell through K+ channel making the membrane potential approach Ek.
Other channels slightly leaky, making it slightly more positive.
Describe the plasma membrane calcium ATPase (PMCA)
Expels Ca2+ from the cell in exchange for H+ using ATP (antiporter)
It is high affinity and low capacity - removes residual calcium
Describe the sarco(endo)plasmic reticulum calcium ATPase (SERCA)
Accumulates calcium ions into the SR/ER in exchange for hydrogen ions using ATP
High affinity, low capacity - removes residual calcium
Describe the Na+/Ca2+ exchanger (NCX)
Secondary active transport - uses gradient from Na+/K+ ATPase
Expels 1Ca2+ for 3Na+ in.
Low affinity, high capacity - removes most Ca2+
Activity is membrane potential dependant - depolarised membrane reverses transport, ie. Ca2+ in, Na+ out
Describe the role of the NCX in ischaemia
ATP is depleted in ischaemia because of hypoxia Na+/K+ ATPase inhibited
Na+ accumulates in cell leading to membrane depolarisation
NCX reverses leading to Ca2+ accumulating in cell
High intracellular Ca2+ is toxic to cells because it activates lots of enzymes which shouldn't be active
Name two acid extruders and their function.
Na+/H+ exchanger (NHE) - 1 Na+ in, 1 H+ our via secondary active transport. Raises intracellular pH. Activated by growth factors. Inhibited by amiloride.
Sodium Bicarbonate co transporter (NBC) - Na+ & HCO3- in, H+ & Cl- out. Secondary active transport.
Name a base extruder and its function.
Anion exchanger (AE) - Cl- in, HCO3- out. Decreases pH.
How is intracellular pH regulated?
Acid and base extrudes compete with each other and reach a set point. Any drift from this equilibrium is corrected by increased activity in one group. Acidification activates NHE and NBC. Alkalinisation activates AE.
How is cell volume regulated?
Osmotically active ions (eg. Na+, K+, Cl-, organic osmolytes etc) are transported into or out of cells. Water follows these ions causing swelling or shrinking. Different cells use different combinations of transporters to bring balance to the force #Anakin
Describe bicarbonate reabsorption by the proximal convoluted tubule in the kidney.
NaHCO3 in lumen of tubule splits into Na+ and HCO3-
HCO3- combines with H+ to form H2CO3
Carbonic anhydrase splits H2CO3 into H2O and CO2 which are absorbed by the tubule cell and then recombined by carbonic anhydrase
H2CO3 splits into H+ and HCO3-
H+ is extruded into lumen by NHE under secondary active transport
Alkalinisation of cell activates AE, extruding the HCO3- into a capillary
What is the goal of renal anti hypertensive therapy?
Reduce reuptake of Na+ and other molecules so less water is reabsorbed by osmosis. This decreases blood volume, decreasing blood pressure.
Describe the action of loop diuretics.
Blocks Na+ reuptake in the thick ascending limb of the kidney. Insert diagram.
Describe the action of amiloride.
Blocks Na+ reuptake in the distal convoluted tubule (ENaC) and the proximal convoluted tubule (NHE)
Describe the action of spironolactone.
Aldosterone up regulates transporters in the kidney. Spironolactone is a glucocorticoid receptor antagonist and is used if aldosterone is high.
Describe the effect of a faulty CFTR protein in cystic fibrosis.
Faulty CFTR leads to accumulation of Cl- in the cell, causing cell swelling because water moves in by osmosis. This results in thick, viscous mucous.
Describe the role of CFTR in diarrhoea.
Cholera toxin permanently activates adenylyl cyclase, leading to high intracellular cAMP. This leads to phosphorylation of CFTR by PKA. This causes excessive Cl- transport into the lumen of the colon. Water follows, causing diarrhoea.
What is the RMP?
The potential across the membrane inside the cell relative to the extracellular solution.
Give some common ranges for RMP for:
a) nerve cells
b) smooth muscle
c) cardiac and skeletal muscle
a) -50 to -75mV
b) -50 mV
c) -80 to -90mV
What channels dominate the membrane ionic permeability at rest?
Open K+ channels
Define equilibrium potential and name the equation used to calculate it.
The membrane potential at which there is no net movement of the ion across the membrane, ie. concentration gradient equals electrical gradient.Nernst equation. Insert equation.
Define depolarisation and give an example of how this may occur.
Membrane potential becomes more positive as cell interior becomes less negative. Eg. Opening of sodium or calcium channels
Define hyperpolarisation and give an example of how this may occur.
Membrane potential becomes more negative (falls below RMP) as cell interior becomes more negative. Eg. Opening of chloride or voltage gated potassium channels.
Describe the difference between fast and slow synaptic transmission.
Fast synaptic transmission is where the receptor is is also an ion channel, eg. nicotinic ACh
Slow synaptic transmission is where the receptor protein and the ion channel are separate proteins. These can be linked by either G-proteins or intracellular messengers. Eg. muscarinic receptor
What is an excitatory synapse?
Excitatory transmitters (eg. ACh, glutamate) open ligand gated channels causing membrane depolarisation (increased permeability to Na+, Ca2+). This is known as an excitatory post synaptic potential (EPSP). This has a longer time course then an action potential (roughly 20ms).
What is an inhibitory synapse?
Inhibitory transmitters (eg. Glycine, GABA) open ligand gated channels causing hyperpolarisation (increased permeability to K+, Cl-). This is known as an inhibitory post synaptic potential (IPSP) and lasts longer than an action potential (roughly 20ms).
What is an action potential?
Change in voltage across a membrane, dependant on ionic gradients and the relative permeability of the membrane. These are "all or nothing" and are propagated without a loss of amplitude.
Describe the steps leading to depolarisation during an action potential.
Once past threshold voltage, voltage gated Na+ channels open allowing an influx of Na+. This depolarises the membrane further causing more voltage gated Na+ channels to open.
Describe the process of repolarisation in an action potential.
During maintained depolarisation, voltage gated Na+ channels become inactivated (not closed) stopping further depolarisation. Voltage gated K+ channels are also opened leading to potassium efflux, decreasing the membrane potential.
What does "all or nothing" mean in relation to action potentials?
Unless the threshold voltage is reached, voltage gated Na+ channels won't open and there won't be any propagation of an action potential. If the threshold voltage is reached, the amplitude of the resultant action potential is the same each time, because you can't have half an action potential can you? You fat knobhead.
Define relative and absolute refractory periods.
Absolute - (nearly) all voltage gated Na+ channels are inactivated so excitability is 0
Relative - voltage gated Na+ channels are recovering from inactivation and excitability returns towards normal
Describe accommodation in relation to action potentials.
The longer the stimulus lasts, the larger the depolarisation necessary to initiate an action potential as voltage gated Na+ channels become inactivated. Insert diagram.
Describe the structure of voltage gated Na+/Ca2+ channels.
Four homologous repeats, each repeat has six transmembrane domains
One domain in each repeat is voltage sensitive
Function requires one subunit
Describe the structure of voltage gated K+ channels.
Each peptide has 6 transmembrane domains
1 domain in each peptide is voltage sensitive
Function requires 4 sub unitsLooks similar to voltage gated Na+/Ca2+ channels
What are the advantages to voltage gated K+ channels consisting of 4 sub units?
More than 4 sub units can be coded for, so different arrangements of these can provide K+ channels with slightly different functions, accounting for the variation seen in K+ channels.
How does the voltage sensitive domain of a voltage gated channel work?
Positively charged amino acids lie within the hydrophobic transmembrane region. Change in membrane potential will move the position of these positive residues, changing the confirmation of the protein.
Describe the action of local anaesthetics and give an example of one.
Local anaesthetics, like procaine, act by binding to and blocking Na+ channels, thereby stopping action potential generation. They are weak bases and cross the membrane in their unionised form. They block the Na+ channels when they're open and have a higher affinity to the inactivated state. This means they are use dependant.
In what order do local anaesthetics block conduction in nerve fibres? And what is the consequence of this?
Small myelinated axons
Non myelinated axons
Large myelinated axons
Because of this they affect sensory neurones before motor neurones.
Why is it dangerous to inject local anaesthetics into a blood vessel?
The local anaesthetic can travel to the heart if injected intravascularly, which can block the sodium channels in the heart leading to cardiac arrest.
Explain local circuit theory.
Depolarisation of a small region of membrane produces transmembrane currents in neighbouring regions. This opens further voltage gated Na+ channels causing the propagation of an action potential. The further the local current spreads, the higher the conduction velocity.
Name the three factors affecting conduction velocity.
Describe how membrane resistance affects conduction velocity.
High membrane resistance leads to high conduction velocity. See Ohm's Law.
Describe how axon diameter affects conduction velocity.
Large axon diameter leads to high conduction velocity, as larger diameter leads to lower cytoplasmic resistance, allowing local currents to travel further.
Describe how membrane capacitance affects conduction velocity.
Low membrane capacitance leads to high conduction velocity. Capacitance is the ability to store charge, so the lower the membrane capacitance, the quicker it will charge.
Describe how myelination increases conduction velocity.
Reduces membrane capacitance and increases membrane resistance. Also allows saltatory conduction.
Which axons are myelinated and why?
Motor neurones are myelinated as they are long, large diameter axons. Sensory are unmyelinated as they are much shorter. Insert diagram.
What is saltatory conduction?
Where the action potential jumps between Nodes of Ranvier as the myelin sheath acts as a good insulator, allowing local currents to trigger depolarisation in the next node. The nodes have a high density of voltage gated Na+ channels.
What forms the myelin sheath?
Schwann cells in the peripheral nervous system. Oligodendrocytes in the central nervous system.