Mitochondria generate most of the energy used by cells through oxygen dependent processes. They convert nutrients into adenosine triphosphate, the molecule that powers cellular work. When mitochondria become dysfunctional, this conversion becomes less efficient. Energy output declines, forcing cells to rely on less productive pathways. More fuel is required to produce the same amount of energy, increasing metabolic strain. High demand tissues such as muscle, brain, and heart are affected first.