A structure in the cytoplasm of all cells except bacteria in which food molecules (sugars, fatty acids, and amino acids) are broken down in the presence of oxygen and converted to energy in the form of ATP. Mitochondria have an inner and outer membrane. The inner membrane has many twists and folds (called cristae), which increase the surface area available to proteins and their associative reactions. The inner membrane encloses a liquid containing DNA, RNA, small ribosomes, and solutes. The DNA in mitochondria is genetically distinct from that in the cell nucleus, and mitochondria can manufacture some of their own proteins independent of the rest of the cell. Each cell can contain thousands of mitochondria, which move about producing ATP in response to the cell's need for chemical energy. It is thought that mitochondria originated as separate, single-celled organisms that became so symbiotic with their hosts as to be indispensible. Mitochondrial DNA is thus considered a remnant of a past existence as a separate organism.