M&R Session 2: Permeability and transporters Flashcards
Name molecules that can passively diffuse across biological membranes
Water (osmotic gradient: up concentration gradient of a solute)
Carbon dioxide
Oxygen
Urea
Ethanol
Small hydrophobic molecules e.g. steroids, benzene
What are aquaporins?
Specific water tubules found in the proximal kidney tubule
What is the role in membrane proteins in mediating movement of hydrophilic molecules?
Maintain intracellular pH and ionic composition
Regulate cell volume
Extrusion of waste from metabolism and toxins
Generate ionic gradients for electrical excitability
What is facilitated diffusion? (form of passive transport)
Ion channels that increase permeability for a polar substance, e.g. in erythrocyte membranes Cl- can move through due to the presence of band 3 protein which is an anion exchanger (low permeability in phosphatidylserine membrane)
Saturable-each carrier molecule can only interact with small number of molecules at a time, and there’s a finite number of carriers in each membrane
Some of these pores are gated
Ligand-gated ion channels?
Facilitated diffusion. Open or close by conformational change when ligand binds
E.g. nAChR (NA+ flows in on ACh binding), ATP-sensitive K+ channel (K+ kept inside when ATP binds).
Voltage-gated ion channels?
Facilitated diffusion. Open or close in response to potential difference across membrane.
E.g. Na+ channel
What are connexins?
Gap junction proteins involved in facilitated diffusion pore gating. Closed when cellular Ca2+ goes above 10um.
What is the purpose of active transport?
To overcome unfavourable chemical or electrical gradients. The movement of ions is coupled to a thermodynamically-favourable reaction
Primary active transport?
Free energy directly from activity of ATP-dependent pumps (ATPases)
Secondary active transport?
Free energy indirectly from other sources, e.g. the gradient of other substances (often Na+), light and high potential electrons. Often at expense of ATP hydrolysis, primary energy source is used indirectly
Uniporter?
One species transporter to other side of membrane
Symporter?
A cotransporter in which one the transfer of one molecule depends on the simultaneous transfer of another in the same direction
Antiporter?
A cotransporter in which one the transfer of one molecule depends on the simultaneous transfer of another in the opposite direction
Gradient of Na+ across membrane?
Inwards
Inside conc: 12mM
Outside conc: 145mM
Gradient of K+ across membrane?
Outwards
Inside conc: 155mM
Outside conc: 4mM
Gradient of Cl- across membrane?
Inwards
Inside conc: 4mM
Outside conc: 120mM
Gradient of Ca2+ across membrane?
Inwards
Inside conc: 10-7 mM
Outside conc: 1.5mM
What is the Na+-glucose?
Symporter
Moves Na+ and glucose in
Na+/K+-ATPase
Primary active transport. Antiport
3 Na+ out for 2 K+ in per ATP hydrolysed
Found in all cells to maintain Na+ and K+ gradients (NOT RMP)
Drives secondary active transport for pH control, cell volume regulation, control of Ca2+, absorption of Na+ in epithelia and nutrient uptake
P-type ATPase as ATP phosphorylates aspartate
Plasma membrane Ca2+ ATPase (PMCA)?
Primary active transport
Expels Ca2+ in order to maintain higher Ca2+ outside than in
High affinity, low capacity (removes residual Ca2+)
Sarcoplasmic reticulum Ca2+-ATPase (SERCA)?
Primary active transport
Found on ER
High affinity, low capacity (removes residual Ca2+)
Ca2+ moved into SR for storage. Important in signalling
Na+-Ca2+ exchanger (NCX)?
Secondary AT, antiport
3 Na+ out, 1 Ca2+ in … Can antiport either way depending on gradient
electrogenic as current flows in direction of Na+ gradient
Low affinity, high capacity (removes majority of Ca2+)
Membrane-potential dependent: inhibited in depolarisation and activity reverses [reperfusion ischaemia injury in MI]
Mitochondrial Ca2+ uniporters?
Facilitated transport
Operate at high [Ca2+]in to buffer potentially damaging levels
Passive transporter of potassium ions?
Voltage-operated K+ channel
Na+/H+ exchanger (NHE)?
Secondary active transport
Uses Na+ moved out by Na+ pump to drive Na+ in and H+ out
Electroneutral: 1 Na+ in for 1 H+ out
Found in most cells, regulates pH, cell volume and cell growth
Activated by growth factors
Inhibited by amiloride
Na+-bicarbonate-chloride cotransporter (NBC)?
H+ moves out, Na+ moves in, Cl- moves out; allowing HCO3- in (or reverse)
Alkalinises cell and regulates volume in some cells
Electroneutral, coupled to AE
Anion exchanger (AE)?
HCO3- out, Cl- in through band 3 protein
Acidifies cell and regulates volume
Na+-HCO3- cotransport?
Some cells. Symporter
1 Na+ and 3HCO3- in
Alkalinises cell
How is cellular [Ca2+] controlled by membrane proteins?
PMCA SERCA Facilitated diffusion channels NCX Mitochondrial uniports
How is cellular pH controlled by membrane proteins?
First buffering capacity but when exceeded needs transporters. Na+-K+-ATPase used a lot as maintains Na+ gradient
Acidification: opposed by NHE (expels H+) or inward movement of bicarbonate ions (e.g. NBC)
Alkalisation: opposed by expelling bicarbonate via AE
How might a cell prevent cell shrinking?
Influx of osmotically-active ions such as Na+, K+ and Cl-, therefore water will also enter
How might a cell prevent cell swelling?
Efflux of osmotically-active ions such as Na+, K+, Cl-, so water will also leave. Able to do this passively because the Na+/K+ ATPase maintains a low [Na+]in and provides a driving force for the movement of these ions to resist swelling
What effect does ion transport to change cytplasmic pH have on the membrane potential?
No effect: transport is electroneutral so osmotic strength of cytoplasm can be varied without affecting the MP
State the approximate fluid volumes for a typical 70kg person
Approx. 42L total aqueous volume
28L intracellular
14L extracellular: 3L plasma, 10.5L interstitial fluid, 0.5L transcellular fluid
Man ~60% water, women ~45-50% water (as more fat)
How do the kidneys reabsorb bicarbonate?
Normally: proximal tubule reabsorbs all bicarbonate and retains for pH buffers
Uses NHE, Na+ pump and carbonic anhydrase
What is the clinical relevance of renal Na+ handling?
Transport mechanisms can be blocked to treat hypertension, by diuretics (as decreased water will decrease blood pressure)
How do loop diuretics reduce hypertension?
Block the Na+/K+/2Cl- symporter in the thick ascending limb
Partial inhibition of sodium uptake to reduce circulating blood pressure
How do thiazides treat hypertension?
Block the NCCT (Na+/Cl- symporter) in the distal convoluted tubule
How does spironolactone reduce hypertension?
Blocks ROMK and Na+/K+ ATPase in the cortical collecting duct
Stops aldosterone binding
How does amiloride treat hypertension?
Blocks ENaC (epithelial Na+ channel) in the cortical collecting duct
What is the action of aldosterone in the kidney?
Upregulates transporters causing increased sodium and water retention
What is the function of ADH in the kidney?
Stimulates the formation of aquaporins so water is taken up more efficiently
