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Coupled Flow
1. Consider the SGLT1 transporter that carries glucose from the intestinal lumen to the cytosol of the epithelial cell lining the intestine. Take the intracellular sodium concentration as 40mM. Given the following conditions, what would be the maximal glucose concentration that could be achieved in the cytosol of the epithelial cell?

Intestinal concentrations (mM) Transporter stoichiometry membrane potential
(mV) Intracellular glucose (maximal)
(mM)
Na+ glucose # Na+/#glucose
100 0.1 2 -70
200 0.1 2 -70
100 0.1 1 -70
100 0.1 2 0
100 1 2 -70

2. Consider an antiport in the plasma membrane of mammalian cells that exchanges 2 Na+ ions for 1 Ca++ion.
a) What equation describes the equilibrium in this case?
b) Given the following conditions: intracellular [Na+] is 40 mM; extracellular [Na+] is 140 mM; membrane potential is -70 mV
i) What energy change would take place (per mole) if one Na+ ion were to move from outside to inside the cell?…what about 2?
ii) What is the lowest intracellular [Ca++] that could be achieved by this transporter under the conditions given in (b)?
iii) Repeat (ii) except that the stoichiometry is reduced to 1:1.
iv) Repeat (ii) except that the membrane potential is reduced to -40 mV.
2. Surface Potential
Use an excel spreadsheet to solve this problem. Save the file as an Excel 2010 or earlier. If you have any problems with this contact me right away.
Consider a membrane with a surface potential of -100 mV. Assume the membrane is a flat sheet. The salt concentration of the medium is 50 mM KCl and 5 mM MgCl2. The pH of the medium is 7.0.
1. Plot the electrical potential, the [K+], the [Cl-], and the pH as a function of distance from the membrane.
2. Plot the charge density as a function of distance from the membrane.
3. Estimate the charge per unit area of the membrane from the charge density plot.

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