the predatory behavior of eukaryotic cells is yet another distinguishing feature of these organisms. mitochondria are present in each and every one of these cells, or they were formerly present. these minute bodies, which are found in the cytoplasm and are surrounded by two layers of membrane, are responsible for producing the majority of the atp that is used to power the activity of the cell. they do this by taking in oxygen and using the energy released during the oxidation of food components, such as carbohydrates. mitochondria are the size of small bacteria, and they possess their own genome in the form of a circular dna molecule, their own ribosomes that are distinct from those found in other parts of the eukaryotic cell, and their own transfer rnas. mitochondria are also the only organelles in the cell to have their own transfer rnas. it is now generally accepted that mitochondria originated from free-living, oxygen-metabolizing bacteria that were ingested by an ancestral cell that was unable to use oxygen in any other way (that is, was anaerobic). after evading digestion, these bacteria evolved in a symbiotic relationship with the cell that swallowed them and their progeny. in exchange for the power generation they provided for their hosts, they received protection and nutrition from their hosts. the beginning of this partnership between a primitive anaerobic predator cell and an aerobic bacterial cell is thought to have occurred approximately 1.5 billion years ago, when the atmosphere of the earth first began to contain a significant amount of oxygen.