PSC2002/L03 Membrane Transport II Flashcards
Give 2 examples of P-type ATPases that phosphorylate themselves during pumping cycle.
Na+-K+ pumps
Ca2+ pumps in SR
Give 2 inhibitors of Na+-K+ pumps.
Ouabain (strophantus)
Cardiac glycosides from digitalis spp
Describe the structure of a Na+/K+ pump.
11 transmembrane domains
3 cation binding sites I, II, III
Actuator domain
Phosphorylation domain
Nucleotide domain
Ouabain extracellular domain
Describe why the Na+/K+ pump transports unequal numbers of ions.
2 K+ for 3 Na+
K+ ions larger
Na+ fits into domains more easily
Describe the mechanism of the Na+/K+ pump. (10)
Na+ enters channel
Conformational change moves N domain
N domain phosphorylates P domain (ATP hydrolysed)
Gate shut and open other side using energy
Na+ released
K+ enters and fills vacant pockets
Conformational change to release phosphate from P domain through actuator domain
Regenerated ATP binds N domain & energy used to open gate
K+ exits
Actuator domain resets and cycle repeats
What are the 5 phases of the Na+/K+ pump mechanism?
E1 -> E1Na -> E1-P -> E2-P -> E2K
What is the Gibs-Donnan effect?
Describes unequal distribution of permanent ions on either side of a membrane which occurs in presence of impermanent charged ions
Describe counteraction of the Gibbs-Donnan effect.
Negatively charged proteins & dissolved solutes hold onto positive ions in cell
= low water potential
Water enters cell
Na+/K+ ATPase lowers number of dissolved particles inside cell
Ouabain inhibited
Define secondary transporters. (3)
Use kinetic energy provided by electrochemical gradients
Transport one solute down a concentration gradient
Coupled to transport of another against concentration gradient
Define symporters/co-transporters.
Integral membrane proteins involved in transport of two or more molecules or ions in same direction
E.g., Na+/glucose transporter
Give 2 examples of symporters.
Amino acid transporter
Na+/phosphate transporter
Na/K/Cl transporter
Na+/HCO3 cotransporter
Cl-/K+ symporter
Define antiporters.
Integral membrane proteins involved in the transport of two or molecules or ions in opposite directions
Give an example of an antiporter.
Na+/H+ (NHE)
Na+/Ca2+ (NCX)
Describe ATP binding cassette transporters (ABC transporters).
Transport small molecules (can remove toxins)
Includes CFTR
Describe the structure of ABC transporters. (3)
Two membrane spanning domains
Two nucleotide binding domains
R - regulatory domain
What defines the probability of ABC-transporter opening? (2)
Presence of ATP in NBD and phosphorylation of R domain increases probability
Give an example of an atypical ABC-transporter.
CFTR
What is the difference between osmolarity and osmolality?
Osmolarity: total concentration of dissolved particles in a litre (osmol/L)
Osmolality: number of dissolved particles per unit mass (osmol/H2O)
Define osmotic pressure.
Pressure exerted by flow of water across membrane
Determined by solutes concentrations
Define tonicity.
Measure of the effect a solution has on cells placed in it and is driven by osmolarity
What occurs to a cell in:
a) Hypotonic
b) hypertonic
c) isotonic solution?
a) cell expands
b) cell shrinks
c) no change
What is the equation for Flux (flow)?
J = P./\C
J = flux
P = permeability
/\C = conc. difference
What is the equation for permeability?
P = Dk/x
D = diffusion coeficient
k = partition constant
x = distance
What is the equation for water flow?
Jv = Lp x /\P
Jv = water flow
Lp = hydraulic water pressure
/\P = pressure
Give an alternative equation for water flow.
Jv = Lp x o x RT x /\C
o = solute reflection coefficient
What is the membrane permeability to a compound if the solute reflection coefficient is:
a) 1
b) 0
c) 0.5?
a) Impermeable
b) Fully permeable
c) partially permeable
Define pores.
Tiny transient gaps in the bilayer
Each pore contains around 10 water molecules in single file
One pore per 1000 lipid molecules
Give 3 structures of aquaporins.
Hemipore 1
Hemipore 2
Hourglass
Which 2 kinds of aquaporins are regulated?
AQ2
AQ3
