2. Cell Membrane potential

Question 1

Describe in your own words what you understand by the term “Equilibrium potential”

Answer 1

The equilibrium potential is essentially when the concentration gradient moving an ion out/in of a cell reach equal level with the electrical gradient bringing the ion into/out of the cell thus the ion levels stay constant.
- Forces: Concentration gradient and charge gradient

Question 2

What contribution to membrane potential is made by the potassium ion?

Answer 2

The predicted membrane potential (Nearnst potential) for potassium in isolation is -90 mV (the membrane potential at which a ratio of 150 intracellular potassium are electrically balanced against 5 extracellular potassium)

Question 3

What contribution to membrane potential is made by the sodium ion?

Answer 3

The predicted membrane potential (Nearnst potential) for sodium in isolation is +60 mV (the membrane potential at which a ratio of 15 intracellular sodium are electrically balanced against 150 extracellular sodium)

Question 4

What contribution to membrane potential is made by the sodium-potassium pump?

Answer 4

This essentially is working to maintain the imbalance of sodium and potassium inside and outside of the cell. Also electrogenic and contributing 20% of the membrane’s potential

Question 5

Why is the membrane potential closer to EK+ rather than ENa+ ?

Answer 5

Because the membrane is far more permeable to K+ over Na+ (50-75 x more so), however if K+ were acting in isolation the membrane potential would be -90 mV, however, although K+ plays a far greater role in the potential, Na+ does exert some influence; therefore, with Na+’s impact the membrane potential lies around -70 MV. This lies much closer to potassium’s ideal value of - 90 mV than sodium’s ideal value of +60 mV, demonstrating the relative permeabilities of the two cations.

Question 6

How is the Nernst equation useful for experimental purposes when investigating the distribution of ions across the cell membrane?

Answer 6

• Describes the equilibrium state where the tendancy for an ion to move down its concentration gradient in one direction is balanced by its tendancry to move down its electrical gradient in the opposite direction.
• Nernst potential basically states two main things; That a relatively small electrical potential difference can balance a large concntration difference across a membrane, and that the existance of a concentration gradient is not evidence that work has to be done to move a particular ionic species.

Experimentially:

• If the measured and predicted (nernst) values are the same, then an equilibrium situation equists.
• If there is a marked difference, then the system is NOT in equilibrium and energy must be expended to maintain the non-equilibrium state.