Mitochondria are like little batteries that generate energy in our cells. Each cell can contain many thousands of mitochondria and the number and size of mitochondria in a cell are largely dependent upon the function of each cell. For example, muscle cells need much more mitochondria to generate energy for us than the cells that are involved in our nerve transmissions. Mitochondria also contain their own DNA and this is genetically inherited from the mother.
Mitochondria provide energy to the cell from a process known as cellular respiration. This process gathers electrons from elements such as glucose to make molecules of adenosine triphosphate, or ATP. ATP molecules deliver just the right amount of energy for our needs and we can store energy in a range of ways, including as the storage form of carbohydrates (glycogen) or as fat beneath our skin (adipose tissue). If we require more energy for our needs, then the numbers of mitochondria can be increased to match our requirements. Our muscles, heart, lungs and brain cannot function efficiently without sufficient cellular energy.