Different processes break down carbohydrates, fats, and
proteins into chemicals that are taken up by the citric-acid cycle
in the mitrochondria of cells. Mitochondria have their own
genomes, separate from the genomes of the cell nucleus, which are
similar to the genomes of bacteria. This supports the hypothesis
that mitochondria originated by incorporating bacterial cells.
See also in The book of science:
Readings on wikipedia: