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Flashcards in Session 2 Deck (10)
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1
Q

Describe the ionic differences in composition between intracellular and extracellular fluids

A

Na+ - 145mM OUT, 12mM IN
Cl- - 116mM OUT, 4.2mM IN
Ca2+ - 1.5mM OUT, 200nM IN
K+ - 4mM OUT, 140mM IN

2
Q

What are the roles of transport processes

A

Maintenance of ionic composition, maintenance of intracellular pH, regulation of cell volume, concentration of metabolic fuels and building blocks, removal of waste products, generation of ionic gradients

4
Q

Discuss the principles of solute movement across biological membranes

A
Hydrophobic molecules (O2, CO2, N2, benzene) and small, uncharged polar molecules (H20, urea, glycerol) diffuse across easily. 
Large, uncharged polar molecules (glucose, sucrose) and ions (H+, Na+, K+, Ca2+, Mg2+, Cl-, HCO3-) do not diffuse across.
5
Q

Distinguish passive diffusion, facilitated transport and active transport across membranes

A

Passive diffusion - dependent on permeability and concentration gradient, rate is proportional to concentration gradient
Facilitated transport - via protein pores, carrier molecules, ligand gated, voltage gated of gap junction
Active transport - allows the transport of ions or molecules against an unfavourable concentration and/or electrical gradient (energy directly or indirectly from ATP hydrolysis)

6
Q

Define the terms uniport, co-transport, symport and antiport

A

Uniport - one substance is transported from one side of the membrane to the other
Co-transport - more then one type of ion or molecule is transported per reaction cycle
Symport - in the same direction
Antiport - in the opposite direction

7
Q

Outline the major physiological roles of sodium-potassium ATPase (Na+-K+ ATPase, Na pump), plasma membrane Ca2+ ATPase (PMCA), sarcoendoplasmic reticulum ATPase (SERCA), sodium calcium exchange (NCX), sodium hydrogen exchange (NHE), anion exchange (AE)

A

Na+-K+ ATPase (primary active transporter):
P-type ATPase, 2 subunits (a - functional, B - directs pump to surface), forms Na+ and K+ gradients, generates gradient for other secondary active transporters (ion homeostasis, pH, cell volume), not primarily responsible for membrane potential, inhibited by Oubain
PMCA (primary active transporter):
Expels Ca2+ out of cell, high affinity, low capacity
SERCA (primary active transporter):
Accumulates Ca2+ into SR/ER, high affinity, low capacity
NCX (secondary active transporter):
Exchanges 3Na+ for 1Ca2+, role in expelling intracellular Ca2+, low affinity, high capacity
NHE:
Exchanges extracellular Na+ for intracellular H+, regulates pH, cell volume, activated by growth factors, inhibited by Amiloride
AE:
Cl-/HCO3- exchanger, base extruder - acidifies inside of cell

8
Q

Discuss how ion transporters work together in cell physiology

A

Cystic fibrosis:
Cl- enters via Na+/2Cl-/K+ symport, accumulates in cell due to faulty CFTR protein –> lack of osmotic gradient leads to retention of water in cell –> viscous mucus
Diarrhoea in cholera:
Cholera toxin stimulates protein kinase A which increases activity of CFTR, more water leaves cell due to increased osmotic gradient –> diarrhoea
Ischaemia:
NCX expels Ca2+ during cell recovery. When membrane is depolarised, NCX reversed. In ischaemia, ATP is depleted - Na+ pump inhibited. Na+ accumulates in cell –> membrane depolarisation. NCX reverses and Ca2+ accumulates in cell –> toxicity

9
Q

Discuss how ion transport contributes to control of resting intracellular Ca2+

A

Intracellular Ca2+ needs to be controlled to prevent calcium toxicity (stimulates other things in cell, insoluble calcium phosphate crystals)
Primary active transport:
PMCA, SERCA
Secondary active transport:
NCX
Facilitated transport:
Mitochondrial Ca2+ uniports buffer dangerous levels of Ca2+

10
Q

Discuss how ion transport contributes to control of cellular pH and cell volume regulation.

A

Cellular pH:
pH is held at a set point by different pumps
Acid extruders (alkalinisation) - NHE, NBC (sodium bicarbonate cotransporter)
Base extruders (acidification) - AE
Cellular volume:
Transport of osmotically active ions creates an osmotic gradient. Water follows the gradient –> cell shrinkage/swelling
NHE, NBC, AD

11
Q

Discuss how ion transport contributes to control of renal bicarbonate reabsorption and renal Na+ handling

A

All bicarbonate is reabsorbed by kidneys. Carbonic anhydrase enables H2CO3 H20 + CO2
Renal Anti-Hypertensive Therapy
Thick ascending limb - loop diuretics inhibit NKCC2
Distal convoluted tubule - thiazides inhibit NCCT, Amiloride inhibits Enac
Cortical collecting duct - Amiloride inhibits Enac, spironolactone is an antagonist of aldosterone receptor