The most well recognized role of mitochondrion is as the cell’s powerhouse, or energy producer. It generates energy in the form of ATP using oxidative phosphorylation (OxPhos). Organs (heart is #1, and kidney is #2 in terms of ATP usage) that use a lot of oxygen, or have high energy demands are particularly rich in mitochondria. These organelles are constantly changing in size and number by the processes of fission (breaking apart) or fusion (putting together). This is called mitochondrial dynamics.