Signalling Flashcards
What molecules are permeable/impermeable to the membrane?
What are the differences & similarities between a channel protein & a carrier protein?
Permeable: hydrophobic molecules (CO2, N2), water via osmosis (small, uncharged, polar)
Impermeable: ions, uncharged polar molecules (urea, glycerol, sucrose)
Partially permeable: glucose
Differences: channel protein has an aqueous pore, carrier protein has a solute-binding site & undergoes conformational change
Similarities: aid passive ion transport (doesn’t require energy) & ions move down the conc gradient
How is an electrochemical gradient established & how does it control the transport of ions? What causes ions to move across the membrane? What can impede ion transport?
What other gradient contributes to the transport of ions?
Membrane potential forms where one side of the membrane is slightly oppositely charged to the other.
Ions transported across the electrochemical gradient AND if they are moving towards an area of opposite charge (e.g +ve ion to -ve membrane)
Lack of a membrane potential (difference in electrical charge between inner & outer cell) & a reverse membrane potential impedes transport.
Concentration/ionic gradient & electrochemical charge/gradient affects transport.
How is an ion gradient generated?
How are ionic gradients used?
By active transport (use of energy from ATP hydrolysis)
Allows pump ions against the concentration gradient which generates ionic gradients.
Used by ion channels
Summarise the key steps of the Na+-K+ pump
How many sodium ions are transported out of the cell? How many potassium ions are transported into the cell?
What is the fundamental principle on how this ionic gradient is established?
Sodium ions bind to the P-type ATPase & ATP is hydrolysed causing a phosphate to bind onto the protein so it undergoes conformation change causing sodium to eject out of cell. This change causes a pocket to emerge for potassium so it binds. Protein de-phosphorylates & protein returns to original shape causing potassium to eject into cell.
3 Na+ out, 2 K+ in
Active transport: the use of ATP to produce an ionic gradient. Cycle of phosphorylation & de phosphorylation to transport ions across the membrane in both directions.
Calcium ions can be transported across the plasma membrane & the endoplasmic reticulum: what are the names of these protein channels?
What are transmembrane domains/regions in the plasma membrane protein? Why is this useful?
Summarise how the proteins pump calcium in or out of the membranes.
PMCA: plasma membrane calcium ATPase
SERCA: sarcoplasmic endoplasmic reticulum calcium ATPase (specialised for muscle cells)
Region with hydrophobic amino acid residues which help imbed the protein into the membrane. The inside of the lipid bilayer is hydrophobic while the outsides are hydrophilic.
The proteins work by calcium ions binding to the calcium-binding cavity & the phosphorylation of the protein allows conformational change for calcium to be ejected in or out of the cell.
- calcium is transported out of the membrane but into the ER.
What is the basis of secondary transport? How does it utilise active transport?
Summarise symport and antiport: how are they different or similar?
Secondary transport is the basis of using an already established ion gradient (from previous active transport) to transport other ions. Means that this process is not directly dependent on ATP, so it just uses carrier proteins.
Symport is when an ion/molecule transports another ion in the same direction across a membrane
Antiport is when an ion/molecule travels in the opposite direction to another ion against its gradient
Both are examples of co-transport & both use carrier proteins. However with symport, the co-transported ion travels down the gradient, whereas in antiport it travels against the gradient of the other ion.
How does NCX (sodium calcium exchanger) work as an antiporter? How many sodium/calcium ions are exchanged?
Does it require energy?
3 sodium ions move into the cell down a concentration gradient which has been generated by active transport with type P-ATPase & 1 ATP. 1 calcium ion uses this electrochemical gradient to transport them in the opposite direction (out of the cell)
The establishment of sodium’s electrochemical gradient requires 1 ATP, however the second gradient (in the opposite direction) for the calcium ion does not.
Summarise how a voltage-gated channel transport ions.
What does the simple ‘building block’ consist of? How large are the voltage-sensing & pore domains?
How do the voltage-gated channels differ with K+ and Na+?
When the membrane potential is reduced, the channel opens.
6 transmembrane regions from S1-S6. Voltage-sensing domain is S1-S4 and pore domain is S5-S6.
K+ channel has 4 of the building blocks in a tetrameric structure with a hole in the middle.
Na+ has 4 of the building blocks in 1 polypeptide chain which is wrapped in a four-fold symmetry, producing a hole in the centre.
What are the 2 structural features of the pore domain on the voltage-gated channel that facilitates K+ ion transport?
Why can Na+ not be transported through a K+ ion transport despite it being smaller?
K+ ions transported in their dehydrated form.
Selectivity filter: forms 4 bonds with oxygen in the domain’s subunits
Na+ ions can only form 2 bonds with oxygen in the domain’s subunits, therefore this is less energetically favourable & the selectivity filter prefers the stabilising the dehydrated K+
What region of the voltage-sensing domain of the voltage-gated channel is significant for sensing the voltage?
How does the voltage-sensing domain cause conformational change in the protein channel? In what position is the S4 protein when it is opened and closed?
In summary what drives the overall change of the protein channel?
The S4 region is responsible due to its regularly spaced positively charged arginine & lysine residues embedded in the hydrophobic environment. It senses the change in voltage (negative charge/electrons) that generates the membrane potential.
When the channel is closed, the S4 region is pointing down. When the membrane depolarises, the S4 region points up which drags the protein channel open.
It’s the residues that drive this conformational change of the channel.
What is the membrane potential?
How does it arise in 2 words?
What are the charges inside & outside of the resting potential?
The difference in voltage between the inside and the outside of the cell which includes the charge imbalances of all permeable ions.
Negative inside, positive outside
Ion gradients
Summarise the key points of producing K+ equilibrium potential.
What is an equilibrium potential of an ion?
With what equation can you calculate the equilibrium potential of a given ion?
When voltage = 0, a chemical gradient establishes where 2 K+ moves outside (from high to low conc) causing a voltage difference of -4
Voltage = -4, electrical gradient is established, causing a K+ to move inside at equilibrium, where the voltage difference is now -2.
The membrane is at its equilibrium potential
The potential generated when a given ion moves across a given membrane and is balanced by movement back into the cell.
Nernst equation
What is the membrane potential?
How does the resting membrane potential arise?
What ion’s equilibrium potential does it resemble?
What happens to the Na+ ion channels at the resting membrane potential?
The charge imbalance of all permeable ions, so the sum of all the different gradients & the permeabilities of all the ions.
The resting potential arises due to the influx of K+ out of the cell due to the leak channels.
It has a potential of -70 mV, similar to K+’s equilibrium potential of -90 mV.
Na+ ion channels are closed, so Na+ is impermeable to the membrane at rest.
How does the GHK equation more precisely calculate the membrane potential at a given time?
At resting potential, what can we assume about the permeabilities of the ions?
At action potential, what can we assume about the permeabilities of the ions?
How is the GHK equation simplified for the resting membrane potential?
It takes into account the permeability of Na+, K+ and Cl-
At the resting potential, permeabilities of Na+ and Cl- are 0 as only K+ is permeable to the membrane & move out of the cell.
At action potential, permeabilities of K+ and Cl- are cancelled as only Na+ is permeable to the membrane and moves into the cell.
It only uses the permeabilities of K+, so is the exact Nernst equation.
How does depolarisation occur in a neurone? Summarise in 2 steps.
How does this membrane potential change? To what ion’s equilibrium potential does it resemble now?
What happens to the Na+ voltage gated channels after depolarisation?
What does the charge go from before depolarisation to after?
- Voltage-gated Na+ channels respond to the change in voltage in the voltage-sensing domain causing them to open.
- Na+ passes into the cell causing an influx of positive charge & depolarisation of the membrane.
The membrane potential is now positive & resembles closely to sodium’s (+60 mV) from potassium’s (-90 mV)
They become inactivated to stop the ionic flow of Na+ so it doesn’t become too positive.
-70mV to +30mV
At what membrane potential to the K+ voltage-gated ion channels open?
How do these channels re-polarise the membrane in an action potential?
What is hyperpolarisation?
+30 mV (they open after a delay)
The membrane is more permeable to K+ & impermeable to Na+, so K+ flows out of the membrane to bring the membrane potential back to -70mV.
When the leak of K+ means the membrane potential goes more negative than -70mV briefly, but then the opening of some Na+ channels & closing of K+ channels brings this back to -70mV.
How does the action potential at the pre-synaptic cell carry on at the post-synaptic cell? Summarise in 3 key points.
In what way are the transmitters released?
What is the type of ion-channel on the post-synaptic membrane?
- Depolarisation opens the Ca2+ voltage-gated ion channels on the presynaptic cell, causing an influx of Ca2+ into presynaptic space.
- The influx stimulates the vesicles containing the transmitters to fuse with the presynaptic membrane.
- The transmitters are released and bind to the ion channels on the postsynaptic membrane, carrying on the action potential.
Quantal transmitter release (in packets)
Ligand-gated ion channels
How is an ionotropic glutamate receptor (iGluRs) similar structurally & different to that of a voltage-gated receptor? 3 points.
How is it activated?
Once open, what ions are now permeable to the post-synaptic membrane?
What happens to the membrane as a result?
What kind of synapse is this? Inhibitory or excitatory?
Has an inverted pore topology, and is a tetramer. But has a ligand-binding domain instead of a voltage-sensing domain.
Glutamate (transmitter) binds to the ligand-binding domain.
Na+ and Ca2+ flow into the cell (down a concentration and electrical gradient) - higher conc & more positive outside than inside.
Causes depolarisation of the membrane
Excitatory- electrical signalling is enhanced due to the excitation in the post-synaptic cell/depolarisation
