Final answer:
Mitochondria are known as the 'powerhouses' of the cell because they produce ATP, the main energy-carrying molecule, through the process of cellular respiration.
Step-by-step explanation:
The main source of energy in a cell is the mitochondria. Mitochondria are often referred to as the "powerhouses" or "energy factories" of a cell because they generate adenosine triphosphate (ATP), which is the cell's main energy-carrying molecule. The process of making ATP through the breakdown of glucose and other nutrients within the mitochondria is called cellular respiration. This process requires oxygen and produces carbon dioxide, which we exhale as a byproduct.