Mitochondrial energy, conceptual image. Computer artwork of a glowing mitochondrion, representing the energy produced by these cell organelles. Mitochondria are a type of organelle found in the cytoplasm of eukaryotic cells. They oxidase sugars and fats to produce energy. Unlike other eukaryotic DNA, mtDNA is circular. Due to this it is thought that mitochondria were once free-living, but were incorporated into eukaryotic cells under a process known as endosymbiosis.