ICPP Flashcards
Describe main features of fluid mosaic model of the cell membrane
Amphipathic
Fluid - lipids are in motion within the bilayer
Affected by temp and molecular mass.
Mosaic - many membrane proteins embedded in bilayer
Lipid motions:
Intra chain
Fast axial rotation
Lateral diffusion
Recognise the main membrane lipids - 4
Recognise their head and tail
2 types of phospholipids:
1) Glycerophospholipids (normal). Can have different heads attached to phosphate group that are polar and charged.
2) Sphingomyelin - is a sphingosine combine with a fatty acid chain joined to a phosphate group. Can also have choline or ethanolamine attached.
Glycolipids - are sphingomyelin without the phosphate group. Have sugar group attached(head). Can be cerebrosides (monosaccharide) or gangliosides (oligosaccharide with Sialic acid residues).
Cholesterol - hydrophobic tail (steroid rings) and hydrophilic head (OH group).
Main lipid membrane properties
Glycerophospholipids- acyl chains can be saturated or not. Can have diff lengths. Thermodynamically most stable arrangement.
Sphingomyelin- mostly saturated acyl chain and unsaturated are trans so no kink so close packing.
Glycolipids - sugar groups extends extend reach of membrane so used for cell recognition and important in immune system for this.
Cholesterol - polar head binds to carbonyl group on fatty acid chain which limits movement and rigid steroid rings interfere with crystalline structure so reduce mobility and inc stability.
how do amphipathic molecules behave in aqueous
environments and how does this leads to the formation of lipid bilayers
The amphipathic molecules form one of two structures in water, micelles and bilayers.
The bilayer formation is spontaneous in water and is thermodynamically driven. Tail and head optimally energetically stable.
Extensive non- polar attractive forces between the hydrophobic tails and electrostatic and hydrogen bonding of head with water.
Discuss the influence of unsaturated fatty acids and
cholesterol on membrane fluidity
Adding unsaturated phospholipid goes from gel to more fluid as it cannot closely pack. Downwards shift of temp at which membrane is fluid.
Cholesterol allows fluid nature to be maintained over a wider temp range. Buffer.
At higher temp - Polar head interacts with acyl group of PL head – limits movement.
At lower temp - interferes with crystalline packing.
Understand how sphingolipids and cholesterol regions of
the membrane form rafts
Congregation of sphingolipids and cholesterol, which have reduced mobility compared to rest of membrane, allows selective hosting of specific proteins.
Inc tight packing. Can have GLs.
Reduced mobility of raft optimises signalling stability and limits random drift of proteins.
Act as organising domains for
receptors and signalling molecules
Optimise kinetic interaction for
signal transduction
Recognise the key role of hydration in maintenance of
physiological membrane structure and function
Allows it to form bilayer as it interacts with head.
Variation by 10% affects function.
Water interacts with polar regions stabilises the lipid head regions.
Understand the importance of flexibility and elasticity in the
cell membrane
Cell membranes need to be able to transmit forces:
Throughout own cell structures In synchrony with other cells tissues–organs – body
The elasticity of the bilayer is essential when forces act upon the membrane and distort its shape. When the force is removed it can then ‘bounce’ back into shape.
Appreciate Major Functions of the Cell Membrane
Continuous, highly selective permeability barrier
Control of the enclosed electrochemical environment
Communication
1 - Recognition 2 - Signalling
3 - Adhesion proteins 4- Immune Surveillance
Signal generation in response to stimuli (electrical, chemical)
Recognise proteins are flexible and elastic – a key
structural feature that enables function
Proteins direct forces throughout its chains and active sites to change structure to perform specific roles.
Peptide chains can flex through single bond structure.
Can store and release energy when acted upon by a force.
Changes in conformation require specific biological signal.
Phoshatidyl
Choline most common
Inositol - key signalling role
Know that hydration is essential to both protein structure and function
Need to incorporate water as an essential part of their biological structure.
Can enter hydrophilic crevices within protein if accessible
Water stays away from hydrophobic regions - this still
contributes to shaping of the functioning protein.
Recognise proteins exhibit varying degrees of mobility
within the membrane bilayer
Look at notion
Lateral diffusion
Rotational - F 1 /F0 ATPase
Conformational change
Understand conformational changes in proteins
underpin protein function
They are adopted following interactions with specific molecular or bioelectric signals.
These conformational states contribute to conveying further signals within the cell for example via G-proteins. They can also contribute to directed molecular movements as part of a larger task eg actin/myosin mediated contraction.
Recognise the different categories of the relationship of
proteins with the membrane
Look at notion
2 major groups - peripheral and integral
Peripheral proteins- external and internal membrane face
Integral:
Allow molecules to interact eg receptor kinases, transporters etc.
Not removed by ionic change.
non-polar bonding occurs within the hydrophobic interior of the lipid bilayer. Peptide chains on external and internal membrane face have
mainly polar residuals to interact with polar head, water, ions, peripheral proteins etc.
internal:
provides flexibility and elasticity of whole cell membrane
enzymes and important regulatory subunits of receptors, ion channels and transporters.
Disperses forces throughout network to protect cell integrity from mechanical disruption
External:
Essential for transmission of mechanical force generated
enzymes, antigens and adhesive molecules that attach the extracellular matrix.
Lipid Anchored Proteins
covalent link via fatty acid/lipid group within bilayer
Functional protein is outside membrane – can move laterally eg G proteins
Describe restrictions on protein movement in the
membrane - exemplified by integral and peripheral
protein cell - cell interaction
Aggregation - lipid rafts
Tethering - Extracellular and Intracellular - integrin
Cell – Cell Interaction - cadherins
Describe cytoskeletal elements in RBCs - 3 groups, 7 proteins
Look at notion
1.Transmembrane proteins
2. Intermediate Anchoring Proteins connecting 1 & 3
3. Long flexible/elastic force carrying proteins
1 -2 -3 interact via electrostatic non-covalent bonds
Trans - band 3.1 and glycophorin
Band 3.1 - essential transporter in carriage of CO2
Glycophorin - antigen – reduces adhesive/friction forces
Also shape biconcave and enable forces to be distributed through whole structure.
Intermediate- Ankyrin
Anchors Band 3.1 to Spectrin
Adducin – Actin – Band 4.1
Band 4.1 anchors Glycophorin Adducin Stabilises Assembly
Interact via polar bonds
Flexible - spectrin - double helix - Transmits forces throughout whole RBC
Describe how cytoskeleton imparts great structural
flexibility exemplified by RBCs
RBC has immense flexibility and elasticity • (ie stores Potential Energy - Releases Kinetic energy) due to weak but large number of electrostatic forces.
• Cortical cytoskeleton extends lifespan ≈ 120 days.
Example loss of cytoskeletal integrity in Disease and
Drug Therapy
Cytoskeleletal protein mutations - loss of RBC structural integrity
Hereditary Spherocytosis - rigid, Forces cause pinched off microvesicles - loss of O2
carrying capacity
Cytochalasin in chemotherapy Inhibits polymerisation of actin filaments in cancer cells so incorrect anchoring of spectrin with glycophorin in RBCs
Cortical cytoskeleton
Cell cortex - layer of actin beneath membrane
Ses 2
What is the membrane to permeable to? How is it measured.
Look at notion for coefficient diagram
extremely permeable to small uncharged gas molecules such as O2, CO2 and N2.
Then Small Uncharged Polar Molecules - This includes water, urea and NH3.
Large Polar Molecules - This includes sugars and amino acids.
Inorganic/Organic Ions
Permeability coefficient scale is cm/sec
Describe the general factors in Fick’s Law determining the rate of passive diffusion for lipid soluble solutes. No need to know equation.
J = P (C1-C2)
Diffusion coefficient = factors affecting permeability x diff in concs
What are impermeable solutes selectively transported via?
membrane proteins classed as: Pores; Channels; Carriers
Describe Pores Channels and Carriers
Look at notion for diagram of mechanism and differences between them.
Pores - AQPs are always open so controlled add or removal from membrane by hormones.
+ charged residues in the pore prevent the movement of H+.
Selectivity means ion gradients are not disrupted.
Integral membrane proteins and bi- directional.
Channels - 1 gate, need specific signal to open, allow ions through, depends on electrochemical gradient. Ping pong transport.
They consist of a number of transmembrane subunits.
Main types are ligand gated ion channels(receptor proteins) or VG.
Both facilitated diffusion.
Carrier/transporters - Like enzymes get saturated. 2 gates. Facilitated or active transport. Sequential binding and conformational changes.
Uniporter(1 substance in 1 direction), symporter(2 or more in same direction) or antiporters(2 or more in opp).
All have hydrophilic centres and central aqueous core.
Understand the basic principles of: Passive and Facilitated Diffusion, and Active Transport
Passive - ficks law, conc gradient is energy source
Facilitated- same but needs to travel through protein
Active transport- ATP is energy source. Against gradient.
Distinguish between Primary and Secondary Active Transport
Primary - Whenever ATP is used directly to transport a solute AGAINST ITS ELECTROCHEMICAL GRADIENT
Secondary- Whenever an electrochemical gradient set up by an ATPase - such as that for Na+ - is then used to transport another ion or solute AGAINST its Gradient
Compare the primary active transport mechanisms of the main ATPases and their roles in regulating ion flow. Look at ATP synthase videos.
3 main types- P, F and V
P-type = pump:
Na+/K+ ATPase
need ATP hydrolysis (ATP phosphorylates aspartate) and has sequential changes in conformation and affinity.
3 Na+ in and 2 K+ out. Electrogenic antiporter.
Ca2+ ATPase are PMCAs.
ATP hydrolysis to release Ca2+ and H+ out.
Needs Mg2+ as co-factor.
In sarcoplasmic reticulum - SERCAs - Ca2+ in, H+ out.
K+/H+ ATPase in parietal cells in stomach. K+ in and H+ out.
F1/F0 ATPase - ATP synthase
Describe the Symporters and Antiporters that serve as Secondary Active Transporters using electrochemical gradients as the energy source - look at notion
Both electro-neutral
Symporters:
SGLTs - there is 1-5, use 1Na+ for 1 glucose(SGLT 2) or 2Na+(SGLT1).
Antiporters:
NCX
NHE - Exchanges extracellular Na+ for intracellular H+
• Electroneutral
• Regulates pHin
• Regulates cell volume
• Activated by growth factors
• Inhibited by amiloride (a potassium sparing diuretic)
Describe how ions and sugars are transported across membranes using:
Facilitated diffusion
Active transport
What is a semi-permeable membrane?
A layer through which only allowed substances can pass
Physiological roles of transporters
Maintenance of ionic composition
• Maintenance of intracellular pH
• Regulation of cell volume
• Concentration of metabolic fuels and building blocks
• The expulsion of metabolic waste products and toxic substances
• The generation of ion gradients necessary for the electrical excitability
of nerve and muscle (excitable tissues)
Sodium pump functions
Drives Secondary Active transport
Control of pHi i
– Regulation of cell volume and [Ca2+]i H+ or Ca2+
– Absorption of Na+ in epithelia – Nutrient uptake, e.g. glucose or
glucose, amino acids from the small intestine
• K+ diffusion through ‘leaky’ channels is mainly responsible for resting
membrane potential (-70 to -90 mV)
Ses 3
Recall the pH scale
normal pH in tissues, plasma and cytoplasm
Is there variation- notion
0.3 inc halves H+
– log10 [H+]
Human tissue survival is pH 6.8 - 7.8. Plasma pH is 7.35-7.45.
Cytoplasmic pH is 7.2, but varies widely across other organelles
Why is pH control important - 1 general and 2 clinical
pH changes can cause a change in net electrical charge on proteins and other biological molecules. This disrupts electrostatic interactions and hydrogen bonding so alters protein structure and function. Alters binding of substrates and
ligands.
R-COOH —> R-COO- + H+
R-NH3+ —> R-NH2 + H+ •
Clinical:
Tissue ischaemia – e.g. cardiac ischaemia and stroke
• Reduction in blood flow decreases O2 supply
• Switch to anaerobic glycolysis leads to cytoplasmic acidification • Over-activation of Na-H exchanger 1 leads to
intracellular Na overload and consequently Ca overload via Na-Ca exchanger
• Altered cellular function eg arrhythmias, apoptosis or necrosis
Dents disease
characterised by proximal tubule
dysfunction and progressive renal failure.
• Due to mutations in CLC5 (which is 2Cl-/H+ exchanger)
and defects in endocytosis – due in part to impaired acidification.
Why is cell cytoplasm acidic, how is pH buffered - notion
And why is this not enough
- The electrochemical gradient favours inward movement of H+ and outward movement of HCO3-.
This creates an transmembrane pH gradient and an electrical gradient. - Also during metabolism CO2
production causes proton
generation:
CO2 + H2O —> H2CO3 —> H+ + HCO3-
- In anaerobic glycolysis glucose is metabolised to lactic acid
Buffers only reduce the impact of acute changes to intracellular pH and are insufficient on their own. Regulated transport processes are required to prevent H+ accumulation and acidosis.
Describe how cytoplasmic pH is primarily regulated by
the ensemble activity of Na+/H+ exchanger and the Cl-/HCO3- exchanger - notion for graph
NHE - most cells - alkalises
• Exchanges extracellular Na+
for intracellular H+
• Electroneutral 1:1
• Regulates pH in H+
• Regulates cell volume
• Activated by growth factors
• Inhibited by amiloride
Acidification activates activates NHE and NBC.
AE - most cells - band 3 - acidifies
Cl- in and HCO3- out
Activated by alkalisation.
pH is held at the set point. Any drift away from this pH is corrected by the increased activity of either the Na+/H+ or Cl-/HCO3- exchangers.
Why is cell vol regulation important
Excessive swelling jeopardizes membrane integrity.
Swelling or shrinking can interfere with cell cytoskeleton.
Cellular functions depend on correct hydration of proteins.
H2O is able to passively diffuse across plasma membranes – so intracellular volume is sensitive to extracellular osmolarity
Outline the response of cells to moderate osmotic stress
and describe mechanisms of regulating cell volume - notion for particular transporters
• No standard method for cell volume regulation
• Different cell types use particular combinations of transporters to achieve the regulation they need
• Transport of osmotically ‘active’ ions, • e.g. Na+, K+, Cl- or organic osmolytes
(amino acids) • Water follows • Net extrusion of solutes – cell shrinkage
• Net influx solutes – cell swelling
Describe how organic anion transporters (OATs) and
organic cation transporters (OCTs) mediate the cellular
uptake of organic anions and cations.
SLCs are secondary active or facilitated transporters.
The SLC22 subfamily consists of
split into 2 main subfamilies OATs and OCTs.
OATs - Natural substrates eg vitamins, prostaglandins.
Common drugs - antibiotics, antivirals, diuretics, NSAIDs.
OCTs (eg dopamine, serotonin, histamine, choline) - 1-3, passive carriers - facilitated diffusion, uses electrochemical gradient.
Both commonly localised on epithelial and endothelials cells and regulate solute transport between fluid compartments.
Describe the role of OATs and OCTs in drug transport,
elimination and toxicity.
Many drugs are hydrophilic.
For drugs to reach their target, carriers are required at each membrane boundary from site of administration.
OAT/OCT expression profiles can influence drug side effects and toxicity. They are uptake transporters so used by drugs.
Eg OAT/OCT drug transport in the kidney shortens plasma half-life (e.g. penicillin).
Probenecid used to extend half life of penicillin by blocking OAT so less elimination.
Give specific examples of drug uptake and transport by
OATs and OCTs in the kidney.
Describe how OCTs and OATs may influence responses
to drugs.
Metformin – first line drug for treating type 2 diabetes
Key site is the liver -to
decrease hepatic glucose production.
Some patients express variant alleles for OCT1 that reduce OCT1 activity.
In these patients Hepatic uptake of metformin is reduced and blood glucose lowering effects are reduced.
Discuss the role of OCTs in cytotoxicity associated with
kidney damage, hearing loss and sensory neuropathy. Notion
Cisplatin is Important for treatment of various metastatic and inoperable tumours.
Toxicity is due to cisplatin being an excellent OCT2 substrate but a poor substrate of either MATE1 or MATE2-K
Cisplatin accumulates in proximal tubule cell to cause cell death - kidney damage.
Kills hair cells and accumulates in neurones - sensory neuropathy.
Ses 4
Outline what a membrane potential is
Membrane potential is the difference of electrical charge
that exists across the plasma membrane.
It is always expressed as the potential inside the cell relative to the extracellular solution.
Membrane potential of cells
Smooth muscle myocytes = -50mV
Neurones = -70 mV
Cardiac myocytes = -80 mV
Skeletal muscle myocytes = -90 mV
how the resting
membrane potential of a cell may be measured
Zero electrode outside cell
Then put microelectrode into cell
Membrane potential dips and voltmeter has negative reading
Therefore, membrane potential is more negative compared to outside cell.
explain how the selective permeability of cell membranes
arises
Enabled by the phospholipid bilayer (prevents passive diffusion of ions) and ion channel properties.
They are:
Selective for ion species
Gating - open or close due to conformational change
Ion flow - down electrochemical gradient
Ion selectivity of channels and the types of channel that are open makes the whole cell membrane selectively permeable to ions
Describe how the resting potential is set up given the
distribution of ions across cell membranes.
Resting membrane potential arises because it is selectively permeable to K+.
K+ ions diffuse down their chemical gradient until the chemical gradient is at equilibrium with the electrical gradient.
At this membrane potential there is no net movement of the ion (electrochemical gradient is zero), but a voltage has been generated.
Explain the term equilibrium potential for a given ion
Ions will diffuse down their chemical gradient until the chemical gradient is at equilibrium with the electrical gradient.
This is equilibrium potential for the ion.
Nernst equation notion
The Nernst equation allows you to calculate the membrane potential at which an ion will be in equilibrium, given the extracellular and intracellular ion concentrations.
How do membrane potentials change
Make membrane more permeable to other ions eg Na+ which changes it to +60mV
Describe how Na+/K+ ATPase maintains Ion Gradients set
up by the Cell
over time, leakage of ions in real cells will lead to a slow collapse of ion gradients. We therefore need to invest energy into maintaining the RMP, which is the job of the Na+/K+ ATPase pump.
Define depolarization, hyperpolarization and repolarisation, and explain
the mechanisms that may lead to each of these
Depolarization - A decrease in the size of the membrane potential from its normal value.
Opening Na+ or Ca2+ channels causes it.
Hyperpolarization - An increase in the size of the membrane potential from its normal value.
Caused by opening K+ channels.
Repolarisation - returning to RMP
outline some of the
roles of the membrane potential in signalling within and
between cells.
Nicotinic Acetylcholine Receptors 1. Have an intrinsic ion channel 2. Opened by binding of acetylcholine
3. Channel lets Na+/Ca2+ in and K+ out, but anions do not move
4. Moves the membrane potential towards 0 mV,
intermediate between ENa
and EK
Gated channels
- Ligand Gating
• The channel opens or closes in response to binding of a
chemical ligand - Voltage Gating
Channel opens or closes in response to changes in membrane potential - Mechanical Gating
• Channel opens or closes in response to membrane
deformation
• E.g. Channels in mechanoreceptors: carotid sinus stretch receptors, hair cells
Chemical synapses - fast, (excitatory, inhibitory) and slow notion for slow explanation
Chemical synapses occur between: nerve cell – nerve cell
nerve cell – muscle cell
nerve cell – gland cell
sensory cell – nerve cell
Fast - receptor protein is also an ion channel. Receptors make synapses excitatory or inhibitory.
Excitatory- when transmitters bind to ligand-gated channels which cause membrane depolarization.
Can be permeable to Na+, Ca2+, sometimes cations in general (nAChR).
Resulting depolarization is called an Excitatory post-synaptic potential (EPSP).
Inhibitory - ligand gated channels cause hyperpolarisation.
Permeable to K+ or Cl-.
Inhibitory post-synaptic potential (IPSP).
Slow - receptor and channel are separate proteins.
Direct G-protein gating - localised and rapid. When transmitters binds to receptor parts of G protein break off and bind to channel to open.
Gating via an intracellular messenger
factors that can influence membrane potential
- Changes in ion concentration
• Most important is extracellular K+ concentration (3.5-5.5 mM normal)
• Sometimes altered in clinical situations
• Can alter membrane excitability, e.g. in heart - Electrogenic pumps - Na+/K+ pump
One positive charge is moved out for each cycle.
Active transport of ions maintains ionic gradients and prevents collapse of RMP from passive movement of ions through channels.
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