All living cells carry out many energy-requiring processes such as growth and division, movement, and uptake of nutrients from their environment. To achieve this, they utilize energy stored in the form of a high intracellular ratio of the adenine nucleotides, adenosine triphosphate (ATP) and adenosine diphosphate (ADP). These compounds can be likened to the chemicals in an electrical cell or battery. The oxidation of food molecules such as glucose or fats (catabolism) (and/or photosynthesis in a photosynthetic organism) “charges up” the battery by converting ADP to ATP, whereas energy-requiring processes tend to “flatten” the battery by converting ATP back to ADP again. For living cells to function smoothly, it is essential that these two processes remain in balance. In eukaryotic cells, one of the principal systems carrying out this task is the adenosine monophosphate (AMP)–activated protein kinase, often referred to by its acronym AMPK.