Because they are the energy-producing organelles in plant and animal cells, a high number of mitochondria implies that the cell requires a great deal of energy to perform its specific function. Skeletal muscle cells, for example, have a large number of mitochondria because they are required to respond quickly when they are needed to do mechanical work. Fat cells also contain many mitochondria because their function is to store energy for when it is required by the body.
Some other examples of animal cells with a high number of mitochondria are the cells of the heart, kidneys and pancreas. The heart requires an amount of energy that will enable it to continue its work without interruption. The kidneys are involved in the process of excretion, and the pancreas is involved in the work of biosynthesis.
The number of mitochondria found in an animal cell can vary from one to several thousand depending upon the type of tissue and the organism. They can be described as "cellular power plants" that generate the majority of the cell's supply of adenosine triphosphate, or ATP, the molecule used as a chemical energy source. ATP is produced using the energy stored in food. The set of reactions that are used to generate ATP are referred to collectively as the Krebs Cycle.