The structure in animal cells responsible for producing ATP is the mitochondrion. Often referred to as the "powerhouse of the cell," mitochondria are organelles that perform cellular respiration, converting biochemical energy from nutrients into ATP. This process involves several stages, including the Krebs cycle and the electron transport chain, which utilize oxygen to generate ATP efficiently. Mitochondria contain their own DNA, indicating their evolutionary origin and highlight their importance in energy metabolism, making them essential for cell survival and function.