Most of the ATP of a eukaryotic organism is made in the mitochondria during the last phase of cellular respiration that is called the electron transport chain (ETC).
A mitochondrion is an organelle within a eukaryotic cell. Cellular respiration occurs within the mitochondria. Cellular respiration is the process by which oxygen gas and the sugar glucose are converted into water, carbon dioxide gas, and an energy source known as adenosine triphosphate (ATP). Cells use ATP as an energy source.
Mitochondria have an inner and an outer membrane. There are three phases of cellular respiration. Each stage of cellular respiration utilizes a different part of the mitochondria.
The electron transport chain is the third and last phase of cellular respiration. The electron transport chain is located in the inner mitochondrial membrane. At the end of the electron transport chain, protons travel down their concentration gradient through an enzyme called ATP synthase. The protons cause the ATP synthase to spin. As it spins, the ATP synthase phosphorylates ADP into ATP. In this way, ATP is synthesized for the cell.