Final answer:
Mitochondria are organelles responsible for cellular respiration and ATP production in eukaryotic cells. They are often referred to as the "powerhouses" of the cell.
Step-by-step explanation:
A mitochondrion is a membrane-enclosed organelle found in eukaryotic cells that is responsible for cellular respiration. It is often referred to as the "powerhouse" of the cell because it produces ATP, the cell's main energy source. The number of mitochondria in a cell varies depending on the cell's energy needs, with highly active cells such as muscle cells having a larger number of mitochondria.