Studies suggest that the movement of mitochondria, the energy generators of a cell, occurs along the microtubule network, which is akin to an intracellular highway system. The energy that mitochondria create is in the form of adenosine triphosphate, or ATP, which is produced by the conversion of oxygen and nutrients. This process, called aerobic respiration, is the reason why it is necessary for animals to breathe oxygen. Mitochondria and aerobic respiration make it possible for higher animals, which have very high energy demands, to exist. Without them, animal cells would only have the much less efficient process of anaerobic respiration to derive energy from. Mitochondrial dynamics were visualized in the digital video sequences presented in this section with mEmerald fused to a mitochondrial targeting signal. In addition, the cell nucleus labeled with a mCherry-H2B fusion.