Assume that a yeast cell has a cytosolic K^+ concentration of 100 mM and is grow
ID: 166163 • Letter: A
Question
Assume that a yeast cell has a cytosolic K^+ concentration of 100 mM and is growing at 25 degree C in a medium with a concentration of 10 mM K^+. The membrane potential is -100 mV, inside negative with respect to the outside. There are a variety of K^+ channels in the plasma membrane of this yeast, some of which are ligand gated. Is K^+ "at a higher" free energy inside or outside of the cell or is it at equilibrium? In which direction would K^+ move in the presence of ligand (when the K^+ channels open)? Refer to your calculations to explain why you have chosen a particular direction of movement. How will this K^+ flux initially impact the membrane potential of the cell? Would this yeast cell likely have an active K^+ pump? If so, in which direction would this pump be actively transporting K^+? What is the equilibrium or Nernst potential for K^+ across this membrane? Please define equilibrium potential and show (derive) how the equation for equilibrium (Nernst) potential comes from the equation for Delta G_inside - outside.Explanation / Answer
Answer i:
K+ is at higher free energy inside the cell as it is hold up against its concentration gradient.
Answer ii:
Since cellular system are always out of equilibrium there will be net potential which will drive the movements of ions which can be calculated as follows:
Electrochemical Driving Force Equation Ef=Em-En
where; Em is membrane potential (-100mV in our case) and En is equilibrium chemical potential of cell (Note in this case Em not equal to En)
Nernst equilibrium potential En can be calculated using the following equation:
En=2.303 RT/nF log10(Cout/Cin)
Where R (universal gas constant) = 8.314472(15) J K1 mol1, F (Faraday constant) = 9.64853399(24)×104 C mol1, Cout = concentration of ions outside cell, Cin= concentration of ions outside cell.
Now plugging in all the concertation values we will have En = ~-25.69 mV
Thus; Ef will be -74.31 (Ef = -100 – (-25.69)).
Since this is a negative value, this shows that the K+ charge will move inside the cell against its concentration gradient.
Answer iii:
Since positive ion (K+) is moving inside the cell it will reduce the membrane potential initially until the equilibrium situation is reached. Each single ion (K+) from outside to inside which add one positive charge inside and at the same time increase the negative charge outside.
Answer iv:
The yeast will likely to have active K+ pump yes as it needed to maintain high K+ concentration inside the cell. The direction of K+ ion will be from outside to inside the cell against its concentration gradient.
Answer v:
Please refer to ii