Energy metabolism is the term given to the process of producing energy (called ATP) from nutrients. Almost all cells in your body have tiny structures called mitochondria which are responsible for generating this energy which your cells need to function. All of the organs and tissues in your body are made up of cells and if the mitochondria do not function properly it means these cells do not have enough energy and this means the organs or tissues do not work properly. There are a huge variety of symptoms and severity within this group of disorders. Symptoms tend to be more severe in areas which require most energy, for example the brain or the heart, or muscles and in these cases, symptoms can be progressive.