MAR Flashcards
What are the functions of biological membranes
- Isolate ionic gradients
- create a closed of chemical environment
- selective permeability
- communication by expressing ligands and receptors
- facilitate electrical transduction
Describe the approximate composition of membranes in terms of fat, protein and carbs.
40% lipid, 60% protein, 10% fat (doesnt add up but was what norman said)
What is the role of cholestrol in the plasma membrane
Hydrogen bonds to fats and rigid tail reduce movement of phospholipids and decrease fluidity.
what are the methods of movement of phospholipids
fast axial rotation, flip flop, flexion/intrachain motion, lateral diffusion.
what is the composition of a pure bilayer
phospholipids, or glycolipids (cerebrosides and gangliosides)
what are the characteristics of lipids in the bilayer?
16-18 carbons long, and can be unsaturated giving rise to a kink that can increase phospholipid spacing.
what is the evidence for proteins in the plasma membrane?
observational- Freeze fracture of crystals and then using osmium to create shadows in electron microscopy, also centrifuge and lysis of RBC and then SDS page.
indirect- Observation of concentration gradient and active transport and also specific cell responses.
What is the evidence for lateral diffusion of proteins and phospholipds?
Marked with specific antigens/tags and then observe how the patterns of the antigens/tags change around the bilayer.
what methods of movement are avaiiable to proteins?
rotation, lateral diffusion, conformational change. NOT FLIP FLOP
How are proteins kept in specific locations in the cell membranes?
some prefer cholestrol rich (signalling proteins), intracellular adhesion, extracellular tethering, aggregation.
What does a hydropathy plot reveal?
it shows the regions of a protein that are either hydrophobic or phydrophillic. Can be used to show the number of times that a protein crosses a membrane.
How can proteins be removed from the phopholipid bilayer?
the hydrophobic regions can be removed by biological detergents and the hydrophillic regions can be removed by altering the ionic bond strength by changing ionic changes and pH.
describe the role of spectrim and relate this to its structure. How does actin relate to this role?
Spectrim is a heterodimer that forms the cytoskeleton in each cell that attaches to proteins in the plasma membrane. Actin forms cross links enabling the spectrim to attach to the plasma membrane.
Describe two genetic diseases of spectrin
Spherocytosis- 50% deficiency in spectin levels
Eliptocytosis- defect in produced spectrin.
Describe how multiple membrane domains are produced in the ER
The first membrane spanning domain is produced through a 20 amino acid region being produced after the initial signal peptide. This region is hydrophobic and so remains in the membrane forcing the ribosome to detach. The next domains are created in pairs that are then brought intogether in tandem.
Describe how orientation of peptides is estabilshed
If the signal peptidase is cleaved during the process of translation then the C terminal will be external, if cleaved afterwards then the C terminal will be internal to the membrane
What can and what can’t pass through a phospholipid membrane?
Can- small uncharged particles (oxygen nitrogen, benzene and carbon dioxide.) and small polar molecules (water urea and glycerol)
Can’t: large polar molecules (sugars and amino acids) and charged particles (any ions).
Why are their processes to transport species across the cell membrane?
Metabolites in and waste out, Ionic gradients need to be estabilished and linked to this is both cell volume regulation and also electrical conduction.
What equation dictates the rate of transport of a substance across a cell membrane?
Rate=Pc(C1-C2)
Pc=permeability constant
C1=Outside concentration
C2=inside concentration
What is the difference between a pump and an exchanger?
Pumps= activity is coupled directly to the hydrolysis of ATP to ADP and use this energy directly to move the ions whereas Exchangers use already existing ionic concentration to facilate the movement of other ions.
What are the intracellular and Extracellular concentraions of K+, Na+,Ca2+, Cl-, and A-?
Intracellular: K+(155mm), A-(167mm), Na+(12mm), Ca2+(10^-4mm), Cl-(4mm)
Extracellular: A-(40mm), Na+(145mm),K+(5mm),Ca2+(1.5mm), Cl-(123mm)
Assuming permeability is not an issue, what dictates what direction ions will flow when placed on one side of a membrane?
The chemical gradient combined with the electrical gradient. Ions will want to increase entropy and so move to an area of lower concentration. They will also want to move to the area of most electrical attraction/least electrical repulsion. So a +ve ion will want to move to a negative location. This is united in the nernst equation that gives the Equilibrium potential for any ion (the goldmann equation gives the combined Eqs of multiple ions.
How can ion channels be activated?
They can be gated for ligands, physical pressure and voltages
What are the properties of ion channels?
Proteins, gated, selectively permeable, allow facilitated diffusion, saturatable (so have a Km and a Vmax).
Name the ion channels involved in Ca2+ influx/efflux, their location, stimuli and properties.
Mitochondrial cell uniporters- through facilitated diffusion and is due to -ve charge of matrix.
PMCA- Uses ATP to pump Ca2+ out, high affinity low capacity, upregulated by calmodulin, a protein in the cell that can bind Ca2+.
NCX- Usually move Na+ in and Ca2+ out but when high Na+ in or when cell membrane is depolarised it can operate in reverse.
SERCA- On S/ER and pump Ca2+ into the SER using ATP.
IP3r-Bind IP3 released from PIP2.
AMPA/NMDA- bind glutamate allowing calcium influx.
VOCC- present at synapses and in transverse tubules to allow the depolarising of the membrane to activate them. Upregulated by phosphorylation from PKA and inhibited by beta-gamma subunit from μ-opiod receptor.
Ryanodine- activated by cystolic CA2+, also by ryanodine which is a plant alkaloid.
What is the role of the NA/K atpase?
3Na+out/2K+ in.
Contributes 10mv to the overal -ve P.D
Largest role is to create the Na and K ion gradients between inside and outside, this then allows them to diffuse down their gradients which like in the case of muscle contraction or nervous elctrical conduction requires changes in the P.d of the cell.
What exchangers are needed to regulate cell pH?
AE- Acidifies cell by removing (HCO3-) for a Cl-
NaBC- Alkali influx due to Na and HCO3- being contransported.
NHX- De-acidifies cell. Loss of H+ ions.
How is cell volume regulated? What are the mechanisms of this?
By controlling the levels of osmolytes within the cell. H2O is attracted to charged particles with high charge densities. These mechanisms must be electroneutral otherwise cell PD is affected. examples could be both K+ and Cl- influx/eflux, or H20 and CO2 being turned into H+ and HCO3- and vice versa.
Why is calcium conc so low in the cell and relatively high externally? What are the advantages/disadvatages of such a system?
CA2+ is toxic at high doses as reacts with phosphate to form an insoluble precipitate. It also activates many cellular processes in an uncontrolled manner so can cause cell death/apoptosis. The tight regulation allows Ca2+ to be used as a signalling molecule as a small change can have a huge effect due to the cell being sensitive to change. However this gradient is very energy exspensive to create.
What is the role of NCX in ischaimia?
No 02=No ATP, NO ATP=high inward NA+ gradient, so NCX could flow in reverse allowing Ca2+ in resulting in cell injury.
Describe bicarbonate reabsorption in the proximal tubule of the kidney.
NHX pulls NA in to cell and H into lumen. H reacts with HCO3 forming H2O and CO2. These can then diffuse into the cell. Carbonic anhydride acts again reforming H2CO3. H then goes out of NCX. HCO3 goes out of AE.
Describe the action of Loop diuretics
Block NKCC2 in thick ascending limb. Dont affect ROMK, NAKatpase, or KCLct.
More effective treatment for people with renal deficiency.
How does amiloride function?
Blocks ENaC in the distal convoluted tubule.
what are the resting potentials for smooth muscle cells, neurons, cardiac myocytes and skeletal muscle?
-50mV, -70mV, -80mV, -90mV.
How is the resting potential set up?
Na/K atpase increase K+ conc inside and Na+ outside. K+ channels are open allowig it to diffuse down its concentration gradient (leak channels) towards its equilibrium potential. The -ve membrane potential arises as the large -ve proteins in the cell cannot follow K+ outside the cell and so remain. This results in the membrane becoming polarised. There is a degree of “leak” from Na and Ca ion channels which prevent the cell membrane reaching the Ek.
In what ways can a membrane potential change? describe some of the mechanisms.
By changing permeability to particular ions depending on whether one wants to depolarise or hyperpolarise the cell membrane. This can be facilitated into both excitory and inhibitory mechanisms.
This can be subdivided into fast and slow action.
Fast actions are EPSP and IPSP. EPSP are caused by binding of Ach and glutamate allowing influx of Ca2+ or Na+. Glutamate binds to AMPA and NMDA in the CNS. EPSP are triggered by influx of Cl- or K+ efflux. The role of EPSPs is to increase the excitability of a cell and an IPSP decreases this excitability. The slow signal transduction pathways are through G-Proteins. These can either modulate ion channels directly are change cellular process and affect them indirectly.
How is it possible to investigate action potentials?
Voltage clamping- Fix the voltage of the membrane and observe current flow.
Patch clamping-looking at the effect on one particular ion channel.
Current clamping- Fix the current and look at how p.d is affected.
What is the equilibrium potential for Ca, Na, K and CL respectively?
dependent on ionic concs but Ca: 130 Na: 66 K: -90 cl: -96
Describe the influx/efflux of ions that contribute to the action potential.
Slow depolarisation until threshold. Then large influx of Na+. Na+ influx stops and K+ efflux begins causing a depolarisation. Hyperpolarisation occurs and then the resting membrane is restored.
What is accomodation and how does it affect the action potential?
This is the slow inactivation of VONaC. This due to “noise” that is present that causes the cell to become desensitized to this stimulus so requires a larger stimulus to get a full blown action potential. Inactivation occurs when an individual channel is turned on the inactivation particle gets lodged in the pore stopping ion passing through.
Describe the structure of a VONaC.
4 subunits that come together that are one continuous polypeptide. In each subunit there is a +ve charged 4th unit of out 6.
Describe the structure of a Potassium channel
Similar to VONaC only each subunit is a separate polypeptides. These come together to form the pore.
What is the maximum conduction rate and relationship of V and diameter of an unmyelinated neurone? a myelinated neurone?
- 20m/s and V is proportional to root d.
- 120m/s and V is proportional to d.
What factors affect rate of electrical transmission?
Diameter of axon
Presence of myelin.
Why does myelin increase rate of conduction?
λ= root(Rm*r/2Ri) c=(Q/V) V=Ed
Myelin increase Rm and also d thefore λ will increase (x100) and also Vmax will increase. So the capacitance of the descrease (x100)
myelin also has small gaps called the nodes of ranvier which allows for saltatory conduction and due to the increased length constant this is possible.
what can decrease the amount of Rm?
more ion channels open in the membrane
Describe a competitive and a depolarising channel blocker.
- Tubocuraraine: binds competively to a NAchR in the place of Ach and so stops channel activation.
- Succuinylcholine: Mimics Ach and binds to receptor but it cant get broken down. It leads to an initial influx and then a sustained inactivation of the membrane ion channels.
Describe mysathenia gravis
Caused by antibodies binding to NAchR which result in their demise. So a quantum of Ach has a smaller effect and so a larger release is required to have a normal effect. This manifests as muscular weakness and easy fatigability. Also it can be observered that it results in smaller MEPPs.
What is a MEPP? hypotheticall how could myathenia gravis, botulism, hypocalcimia, and pesticide poisoning affect them?
Mini end plate potential. Triggered by the release of one vesicle of Ach. smaller, less frequent, no change, elongated?
What is the sequale of action at the NMJ?
Action potential reaches the synapse
VOCC open allowing an influx of Ca2+ into the cell
This binds to synaptotagmin.
This protein then moves the vesicles down to the cell membrane.
Vesicle binds to snare complex which opens a pore. This allows the diffusion of the Ach into the synaptic cleft. Ach then diffuses across the cleft to open Nachr. Ach-esterase breaks down the Ach to choline and acetate.
Describe the NAchr
5 subunits. 2 alpha sub units bind the Ach and undergo a conformational change.
Pore is not very selective and allows Na and K to diffuse across.
What medication can be used to block VOCC?
L-type: DHP and are found in muscle neurones and in the heart.
N-type: omega connotoxins found in the CNS
How is the SER refilled with Ca after it has been depleted?
in muscle the majority of ca efflux is then sequestered again. In other cells, STIM is a protein embedded in the membrane that undergoes a conformational change when it doesn’t bind Ca. This can then act on ORAI which causes an influx of Ca into the cell.
How does the cell buffer calcium levels to stop levels rising to high systemically thorough the cell?
through the use of calcium binding proteins i.e calbinding and calsequestrian. However if these are saturated then buffers will have a reduced effect.
How is the IP3 receptor activated?
GPCR activated on the surface that is coupled to a G protein containing alpha-Q. This activates phopholipase C that cleaves PIP2 into IP3 and DAG. IP3 then binds to the IP3r on the S/ER and causes a release of Ca2+.
What is the role of mitochondria in the regulation of cellular calcium?
Has a calcium uniporter that takes up calcium from the cystol at high concentrations. These are placed strategically in the cell so mitochondria are found in places where the cystolic concentrations may locally be very high.
What are the different methods of using proteins to effect the intracellular physiology due to a ligand?
Proteins with intergral ezymatic activity
Ions channels
Coupled effector and Receptor (GPCRs
Nuclear DNA receptors.
Why does evolution favour GPCRs?
Due to a defect in one doesn’t affect the function of the other. So function is less likely to be lost entirely. This cannot occur in ion channels or proteins with intrinsic enzymatic activity.
What is the difference between a receptor and an acceptor?
Receptors are switched off until the ligand binds however acceptors undergo allosteric upregulation as they are active in the absence of the ligand.
What are the types of signalling molecule?
Hormone, paracrine, neurotransmitter, cell surface bound signalling ligands.
