Final answer:
Muscle cells contain a high number of mitochondria because they require substantial energy to contract and perform their functions.
Step-by-step explanation:
A muscle cell expends significant energy to provide movement for the body. Consequently, it contains a high number of mitochondria, the organelles often referred to as the "powerhouses" or "energy factories" of the cell. Muscle cells need a lot of energy to contract, therefore they have a very high concentration of mitochondria, where adenosine triphosphate (ATP) is produced through cellular respiration. ATP is the cell's main energy-carrying molecule, necessary for muscle contraction and various cellular activities.