I am very interested in biochemistry, and in particular the metabolism of the cell. Important molecules for cell metabolism are ATP (and ADP) and NAD+ (and NADH). ATP, ADP and NAD+ are ions: ATP and ADP have negative charge (as have the free phosphate ions) and NAD+ has positive charge. But the cell as a whole is neutral (at least I think so, and in any case, all the cells in an organism has no significant total charge). Therefore, threre must exist other ions (or perhaps free electrons) in the cell which neutralize these ions. Which are these ions? Which positive ions neutralize ATP, ADP and free phospate ions? Which negative ions (or is it free electrons) neutralize NAD+?