Membrane-bound organelles that generate most of the cell's supply of chemical energy.Mitochondria are compartments inside almost every cell of the body that convert the chemical energy in food into a form the cell can spend. They are bounded by their own double membrane and contain their own small loop of DNA, separate from the DNA in the cell nucleus. A typical cell holds hundreds to thousands of them, and their number and shape change with the cell's energy demands.
The energy currency they produce is ATP, a molecule whose phosphate bonds store usable energy. When ATP is split, energy is released to drive muscle contraction, protein synthesis, ion pumping, and essentially every other energy-requiring process. Without a working mitochondrial population, a cell cannot maintain the ion gradients needed for nerve impulses or the assembly work needed to replace its parts, and it dies.
Beyond energy supply, mitochondria participate in building and breaking down molecules, in storing calcium, in generating signals that influence cell survival, and in a form of programmed cell death. They are also unusual in being inherited almost entirely from the mother, which makes their DNA a tool for tracing maternal ancestry. Their central role in energy production explains why tissues with the highest energy demand — brain, heart, muscle, liver — are the first to suffer when mitochondria malfunction.
The core process is oxidative phosphorylation. Food-derived molecules are broken down, and the energy released is used to pass electrons along a chain of protein complexes embedded in the inner mitochondrial membrane. At each step the electrons drop to a lower energy level, and the energy released is used to pump protons from one side of the membrane to the other. This creates a difference in proton concentration and charge across the membrane — an electrochemical gradient, often described as a battery being charged.
Protons then flow back across the membrane through a rotating molecular turbine, ATP synthase. The flow spins part of the enzyme, and that mechanical rotation drives the joining of ADP and phosphate into ATP. Oxygen's role is at the end of the electron chain: it accepts the spent electrons, and without it the chain backs up and the gradient collapses. The number of ATP molecules produced per molecule of glucose is far larger when this membrane-based route is used than when glucose is broken down without oxygen.
Mitochondrial biology is central to diagnosing inherited mitochondrial diseases, to understanding the tissue damage caused by oxygen deprivation in heart attack and stroke, and to research on aging and on drugs that target cellular energy metabolism.
Mitochondria carry their own DNA, and in humans it is inherited almost exclusively from the mother, so it traces an unbroken maternal lineage back through generations.