The number of mitochondria per cell varies greatly. In humans, for example, liver cells and muscle cells may contain hundreds to thousands of mitochondria, whereas erythrocytes (red blood cells) do not contain any mitochondria.
The mitochondrion is a membrane-bound organelle found in the cytoplasm of almost all eukaryotic cells (cells having well-defined nuclei such as plants, animals, fungi and other forms of life). Mitochondria are responsible for producing energy in the form of ATP, which is required by cells to function. They also generate heat and facilitate cell growth and death. Typically, mitochondria are oval in shape, and their sizes range from 0.5 to 10 ?m.