If it weren't for the mitochondria, heterotrophic cells could only obtain their ATP from anaerobic glycolysis, but with them they can completely degrade glucose in the presence of oxygen to convert it into carbon dioxide and water, obtaining much more energy.
The number of mitochondria depends on the energy needs of the cell, being located in areas where a significant consumption of ATP is required.
The chondriome is the set of all the mitochondria in a cell.
Structure of the mitochondria
The mitochondria are cylindrical and are bounded by a double membrane, the outer mitochondrial membrane, which separates the hialoplasma, and the inner mitochondrial membrane, having a folds inward increasing its surface, the mitochondrial cristae. These membranes leave two compartments: the intermembrane space, between both membranes, and the mitochondrial matrix, the space that surrounds the inner mitochondrial membrane.