Glucose breakdown begins in the cytosol, but the majority of the process occurs in the mitochondrion, the energy-harvesting organelle of the cell. In addition to participating in the breakdown of glucose (and making ATP in the process), the mitochondrion is also involved in breaking down fats and amino acids. All these fuels are processed in two major steps, termed the Krebs cycle and the electron transport chain. In the Krebs cycle, the carbon skeletons are broken apart to make CO2, while the hydrogen atoms are removed on special nucleotide carriers. In the electron transport chain, the hydrogens are stripped of their energy in a series of steps to make ATP, and in the end are reacted with oxygen to form water. The mitochondrion consumes virtually all the oxygen used by the cell. The mitochondrion also participates in many anabolic reactions, using the intermediates of the Krebs cycle as a source of carbon skeletons for creating and modifying nucleotides, amino acids, and other building blocks of the cell.
The mitochondrion is the descendant of a once free-living bacterium that took up residence inside an ancient cell, probably to take advantage of high-energy molecules the host could not metabolize. Mitochondria retain their own DNA on their own bacteria-like chromosome, although over time most of the original mitochondrion's genes were transferred to the host and now reside in the nucleus.