Week 4 MPW Lecture 10 Flashcards
What is a symporter?
A symporter is an integral membrane protein that moves a molecule against its concentration gradient using another molecules down gradient movement.
The signal for muscle contraction is opening of calcium channels, which releases calcium from extracellular space into the cell. Following muscle contraction, the calcium has to be pumped back out again, very rapidly in time for the next contraction. Two of the most common calcium pumps use co-transport of sodium and potassium to provide the driving force. Type A uses three sodium ions coming in for one calcium. Type B uses four sodium coming in and one potassium going out. Calculate the free energy needed to pump calcium out. For each pump, calculate the total free energy available from the sodium/potassium gradient, and hence comment on why the cell should have two different pumps.
For calcium being pumped out, the free energy from concentrations is positive (it is unfavourable because it is going against the concentration gradient). The free energy from membrane potential is also positive: this is a positive ion going from –ve charge to +ve charge. The free energy from concentrations is RT ln([out]/[in]) = 8.31x310xln(104) -1 free energy for pumping calcium out of the cell is a very large +41094 J mol which is +23727 J mol
The free energy from membrane potential is zFE = 2x96485x0.09 = +17367 J mol
quoted comes out negative, but as explained above this is unfavourable (it’s going out not in). Which means the total This is a large number, but I expect a large number because of the large concentration gradient.
a) For sodium coming in, we do the same kind of calculations as above. Both free energies are negative (favourable). The concentration free energy is 8.31x310xln(140/15)=5754 J mol-1, and as above is negative. The free energy from membrane potential is 1x96485x0.09=8684 J mol-1 (negative). So the total free energy for sodium coming in is negative -1 and the sum of these, ie -14438 J mol
b) The free energy for potassium going out comes from the concentration free energy (negative – concentration is higher inside), and the membrane potential free energy (positive – similar to calcium, it is a positive ion leaving a negative -1 membranepotential).Theconcentrationfreeenergyis8.31x310xln(150/4)=-9337Jmol .The membrane potential free energy is 96485x0.09=+8684 J mol -1 So the total free energy for potassium is the sum: -653 J mol. Not a lot!
c) For Pump A (three sodium in for one calcium out), the free energy for three sodium in is just three times the free -1
energy for one. In other words, free energy for sodium in = -43.3 kJ mol . And as we have seen, free energy for calcium -1
membrane potential stay roughly the same as in the table. out = +41.1 kJ mol
d) For Pump B (four sodiums in and one potassium out), the free energy for sodium is 4x(-14438) = -57.8 kJ mol-1, and the. So there is just enough free energy for this pump to work, as long as the concentrations and -1 not quite match, it’s because I used the un-rounded off figures.) This again compares to the free energy needed to free energy for potassium is -0.7 kJ mol. (If you are wondering why those numbers do -1 . pump calcium out of +41.1 kJ mol -1 , giving a total of -58.4 kJ mol. So this pump is easily strong enough, by a substantial 17 kJ mol
e) From which you would conclude that using three sodiums is only just enough so should work most of the time but may not quite be enough if concentrations have changed a bit. Whereas four sodiums is easily enough. It is not clear why it should bother to transport a potassium out, as this contributes very little to the free energy. Presumably it is about balancing the charge a bit.
