During an experiment a neuropsychologist measures the membrane potential of a squid giant axon in the presence of a bathing solution of known composition. He then removes the cytoplasm and replaces it with a synthetic solution. At the same time he alters the external medium from that used before and again measures the membrane potential. From the above info and the info given below calculate the following values...
First measurement outside Inside
(control) Second measurement outside Inside (experimental)
K+ 20mM ? 150mM 15mM
Na+ 180 ? 20 200
potential -42.5 mVolts +18.06 mVolts
Cl- distributes itself across the cell membrane according to the membrane potential. All other anions are impermeable.
(a) What is the permeability ratio for the movement of Na+ compared to the movement of K+ at rest (Pna/Pk)?
(b) What is the equilibrium potential of Na+ under the conditions of the second measurement?
(c) What is the equilibrium potential of K+ under the conditions of the second measurement?
(d) What is the Cl- outside/ Cl- inside ratio under the conditions of the second measurement?
(e) Assuming that K+i= 10 x Na+ i, what is the concentration of Na+ inside the axon during the initial (control) measurements?
(f) With the same assumptions as in (e) above, what is the concentration of K+ inside the axon during the initial (control) measurements?
The Goldman equation is used to perform calculations using data from Membrane Potential Measurements of Axon of Giant Squid. Permeability ratio and ion concentrations are calculated.