Final answer:
Mitochondria are organelles responsible for producing ATP, the cell's main energy-carrying molecule, through cellular respiration.
Step-by-step explanation:
Mitochondria are organelles found in most eukaryotic cells and are responsible for producing adenosine triphosphate (ATP), the cell's main energy-carrying molecule. ATP represents the short-term stored energy of the cell. Mitochondria generate ATP through cellular respiration, a process that uses the chemical energy from glucose and other nutrients. This process takes place in mitochondria and requires the presence of oxygen, producing carbon dioxide as a waste product. The high concentration of mitochondria in muscle cells is due to their high energy demand.