Week 1 (membrane transport) physiology Flashcards
Cell membranes and transport
cholesterol significance in cell membrane
Can stabilise lipid fluidity.
ATP-dependence of flip-flop motion
- Revealed to be active process
- Active in signalling events.
Lipid asymmetry across membrane (example)
Phosphatidyl-choline (PC) tends to stay on the cell surface, phosphatidyl-serine (PS) and PI is the opposite.
what facilitates lipid asymmetry
An enzyme, PLEP, (phospholipid exchange proteins) facilitates this non-random motion.
Flippase in cancer (multidrug resistance MDR)
- Flippases could also be a multidrug-transporter (like P-glycoprotein) that translocates drugs out
- They can also act as ATP-dependent efflux pumps (pump out non-polar drugs)
PS activity
It has -ve charge. Protein kinase C will bind to -ve charged proteins for activity.
It will also promote apoptosis when it is on the extracellular monolayer. (Due to possible inhibition of flippases)
how do drugs overcome the issue of MDR
- finding inhibitors of the efflux pumps
- evade pump detection
hydropathy of amino acids
- Determined by their polarity, what portion of the amino acids are in the aqueous phase, and how many in the oily phase.
- This is used to determine which amino acids are likely to be with the phospholipids, or extra/intracellular spaces.
Singer-Nicholson Model
Fluid Mosaic Model, proposes that biological membranes are composed of a fluid phospholipid bilayer interspersed with various proteins.
Why so many intracellular K+?
Cells that need protein synthesis need this.
Because there are cavities in enzymes that are negatively charged that need a cation to stabilise it. But Ca2+, H+, Mg2+ will disturb the conformation of the protein, but K+ won’t.
So high intracellular K+ is essential for protein synthesis(ribosomal subunit).
carnivore red blood cells (an exception?)
RBCs have high sodium intracellularly.
- high turnover rate (quick cell death)
- and don’t concern protein synthesis.
glycophorin A
The first integral membrane protein to be sequenced.
Hydropathy plot can localise potential alpha helical membrane spanning segments in glycophorin A.
Modelling a cell membrane?
- pure phospholipids
- mixed phospholipids
- add in non-transporter integral proteins
- add in non-functional transporter proteins
- biological membrane.
examples of (models) studying the cell membrane
- forming a single bilayer (allow the study of ion channels)
- using RBC, easy to burst, to study the cytoplasmic side of cells.
RBC can be turned inside out after burst (hypotonic lysis).
membrane permeability and oil solubility
This should be a linear relationship.
Diffusion coefficient vs oil solubility.
But water is an exception
Water (exception) - membrane permeability and oil solubility
Even though water is polar, its small size and shape is optimal to diffuse across the membrane.
water/cell swelling significance…
necrotic cell death
cell growth
water/cell shrinkage significance…
apoptosis - cell death
animal cell complication with inherent hypotonicity (protein intracellularly)
Have cytoskeletons to prevent cell swelling, actively pump out osmolytes.
is water permeation through cell membranes controlled?
Yes! This is shown by Mercury (Hg2+) blocking water permeation through the membrane.
- The more mercury, the less water diffusion.
Aquaporin and mercury
Aquaporins are water channels abundant in RBC, kidney tubules and other tissues.
There is a mercury binding site that inhibits aquaporin’s water permeability.
water movement across animal cell membranes
- Main regulated (ADH) way is aquaporin channels.
- There are no ATP-driven water pumps
- How is water actively controlled - via osmolytes.
Mutations/Alterations to aquaporin function
- Increased aquaporin 2 leads to more water retention.
- mutations of aquaporin can cause nephrogenic diabetes insipidus.
nephrogenic DI - no response to ADH.
Aquaporin 0,1,3,4,5 are around the eyeball.
ECM and cell volume (in situ cartilage cell)
A normal ECM can minimise cell swelling.
A weak ECM or absence of ECM will lead to cell swelling.
Passive permeability cell membrane pathways
Down the concentration gradient
1. ion channels
2. gap junction
3. leaks
Transporters of cell membrane pathways
- Facilitated diffusion (down the conc gradient)
-
secondary active (up or down gradient)
Uses the energy of transport of another molecule. - primary active (up the gradient through ATP hydrolysis)
What’s important in basal leak rate
- hydrophobicity of molecule and membrane
- temperature
- shape of molecule
what could prevent leaks from cell membranes
cholesterol and low temperature
gap junction function
The channel could coordinate tissue activity
transfer small molecules and ions.
present in:
- non-excitable tissue (hepatocytes), transfer of small metabolites
- cardiac muscles: propagate electrical signals.
- even immune response: communication between immune cells
dye coupling - microinjection and gap junctions
dye transfer between cells through gap junctions
ion channels function
Hydrophilic gated channels:
1. ion selectivity.
2. can be desensitised or inactivated.
3. gated by voltage, ligands, membrane tension (pull open).
inhibitors of ion channels
TTX for Na+ channel
bee venom or TEA for K+ channel
mechanosensitive ion channels
With a glass pipette giving pressure:
stretching or sealing, stretching can activate or inactivate some ion channels.
Presence in mechano-transduction in connective tissues.
simple transporters - facilitated diffusion (characteristics)
IS substrate specific
- transporters have alternating binding sites.
- Saturates easily
- temperature sensitive (affects bilipid layer)
examples of simple transporters
GLUT - Na independent glucose exchange (uniport)
examples of secondary active transporters
(symport) SGLUT - NA dependent glucose exchange
antiport (bidirectional - eg. Na+/H+)
coupling ratio of sodium pump
3Na+, 2K+, 1 ATP
Is the sodium pump in charge of regulating the membrane potential?
No, if the pump is knocked out, the cell can still pass on signals for a few hours.
uniport transporters
independent transport of a compound like glucose or amino acid
symport transporters
using the chemical gradient of a compound (Na+) to bring along nother compound
antiport transporters
using the chemical gradient of a compound (Na+) and bring a compound from the opposite direction.
like Na+/H+
Cl-/HCO3-
uniport - ping pong model
External side of transporter has HIGH affinity, but internal has low affinity.
There’s no shuffling back and forth in the membrane, the conformational change is relatively slow.
bicarbonate/chloride ion facilitated diffusion (simple transporter)
- As an exception, it has very high diffusion rate (close to ion channel)
- facilitates bicarbonate transport out of the cells and out of the body.
Very important in respiration
What else do transporters do?
Important in cell structures.
To have a complex transporter structure, there must be a complex cytoskeleton to withstand it.
For example:
spectrin-actin complex is linked to band 3 anion transporter
the transporter can act as anchorage to cytoskeleton
Defect in band 3 leads to?
When the anion transporter has a defect, it actually causes an abnormal morphology of red blood cells.
ion channels at the node of ranvier
there is ion channel clustering at the node of ranvier to propagate action potentials and other functions.
they cluster due to cytoskeletons.
basic mechanism of secondary active transporters
- Build inward Na+ gradient using the active 3Na+/2K+ sodium pump (ATP hydrolysis)
- transport compounds using the sodium gradient.
example of the primary active transporter
Uses ATP hydrolysis as energy source:
- sodium/potassium pump.
- Calcium pump
specificity of passive permeability pathways
- leak: no specificity
- gap junctions: no specificity (size still matters)
- ion channels: specific (size still matters)
inhibition of passive permeability pathways
gap junction has a non-specific blocker - hexanol
only ion channels have inhibition via channel blockers or gates.
- TTX for Na+
- apamin for K+
transport kinetics of passive permeability pathways
all linear, no saturation
specificity, inhibition, and transport kinetics of simple transporters and active transporters
are specific, all show saturation, and have inhibition.
how to measure cell volume?
Fluorescence intensity - shrinks will get more intense, vice versa.
regulatory volume decrease
Osmolyte efflux due to cell swelling.
1. KCl co-transport
2. K+ and Cl+ coupled channels
3. osmolyte channels (amino acids)
regulatory volume increase
uptake of osmolytes:
1. NKCC cotransporter (Na,K, 2Cl)
2. NaCl cotransport
3. NHE (Na+/H+) coupled to Cl/HCO3- transport (bring in Na and Cl)
cell movement via iso-osmotic volume decrease/increase
at protrusion (cell swell), there will be iso-osmotic V increase
at retraction (cell shrink), there will be iso-osmotic V decrease
