Question

You are reading a paper describing measurements of the opening and closing of an ion channel over time. From these data, the authors calculated the fraction of time the channel was open, and called this the probability of being open (popen). They then used this probability to determine the energy difference between the open and closed states. However, they reported their results as a difference in free energy. Describe why they made the choice to report this as a free energy difference (instead of a potential energy difference). Answers can be qualitative (no derivations are necessary).

Answer 1

The probability of finding the system in open or closed states depends not only on the energy difference between open and closed states but also the degeneracy of open states versus degeneracy of closed states . Degeneracy of a state refers to the number of ways in which that state can occur. For example, if a state has higher energy but also larger degeneracy compared to another state, the probability of finding the system in higher energy state will be actually more than what follows from considering only the energy difference between the states.

Free energy takes into account not only the energy but also the degeneracy.