Why do muscles contain mitochondria

The ATP is produced in the mitochondria using energy stored in food ... Some cells have more mitochondria than others. Your fat cells have many mitochondria because they store a lot of energy. Muscle cells have many mitochondria, which allows them to respond quickly to the need for doing work.

Because mitochondria have the function of undergoing respiration to produce energy for the cells in your body. Since muscle cells use more energy, there are more mitochondria associated with them.