* Mitochondria are the powerhouses of the cell. They are responsible for producing ATP (adenosine triphosphate), the main energy currency of the cell.

* Active cells require more energy. Cells that are actively performing tasks like muscle contraction, protein synthesis, or signaling need a constant supply of ATP.

* More mitochondria = more ATP production. The more mitochondria a cell has, the more ATP it can produce, allowing it to meet its energy demands.

Examples:

* Muscle cells: Muscle cells that are actively contracting will have a high density of mitochondria to fuel their activity.

* Nerve cells: Nerve cells that are constantly transmitting signals require a lot of energy and thus have a large number of mitochondria.

* Liver cells: Liver cells, which perform a wide range of metabolic functions, also have a high number of mitochondria.

In contrast, inactive cells have fewer mitochondria because they have lower energy requirements.