The goal of our book was to describe the properties of cells and cellular structures that are commonly found in many different cell types from a physical perspective. For example, all eukaryotes contain structural filaments like actin, microtubules, and intermediate filaments, which are important for various cellular processes such as cell migration, endocytosis, cell division and others. Therefore we described in detail the mechanics and dynamics of these cytoscelletal components. Diffusion and the kinetics of various proteins is another example of a widespread mechanism used by cells. We also described the mechanics of the cell membrane, which undergoes transformations during processes like endocytosis or exocytosis. Similarly, almost all cells establish a membrane potential, something we have only briefly touched upon so far, and which we now want to briefly address. For a basic biological introduction to this field, you can refer to standard biology textbooks Alberts et al (Molekularbiologie der Zelle. Wiley, 2017) or more specialized books on electrophysiology Junge (Nerve and Muscle Excitation. Sinauer Associates, 1976); Rettinger et al (Elektrophysiologie. Springer Spektrum, 2018).