Robert Altmann discovered mitochondria in 1886, thinking that he was seeing parasites. Altmann called his discovery "bioblasts" and posited that they were found in almost all cells. His ideas laid the groundwork for further discoveries by scientists.
Altmann was a German pathologist who theorized that these bioblasts were actually fundamental, living parts of the cell, but his ideas were criticized at first. He believed that bioblasts were cells that had, through a process called endosymbiosis, been absorbed into eukaryotic cells. With the invention of the electron microscope in 1962, scientists were able to study mitochondria further.
A mitochondrion is an organelle that produces energy for the cell by metabolizing ATP, or adenosine triphosphate. This process is vital to the functioning of the cell, such as movement and growth.