The majority of the energy in a cell is produced in the mitochondrion. Mitochondria are small, bean-shaped organelles that are found in almost all eukaryotic cells. They are often referred to as the "powerhouses" of the cell because they are responsible for producing the majority of the cell's energy currency, known as ATP (adenosine triphosphate).
Mitochondria generate ATP through a process called cellular respiration, which involves the breakdown of glucose and other molecules to release energy. This process occurs in several stages, including glycolysis, the Krebs cycle, and the electron transport chain. These processes take place in different parts of the mitochondrion and involve a series of chemical reactions that ultimately result in the production of ATP.
In summary, the mitochondrion is responsible for producing the majority of the energy in a cell through a process called cellular respiration.