Mitochondria are the main source of energy in animal cells. Basically, they take the energy stored in foods (sugar) and convert it into fuel that is more convenient for the cell to use. The more energy the cell requires, the more mitochondria it will have to fill that requirement. The mitochondria are the pink, sausage shaped organelles in the picture below. They are made of two membranes; a smooth outer membrane that covers the mitochondrion, and the inner membrane, which has many wrinkles in it to increase its surface area.