The number of mitochondria varies, from less than a hundred to several thousand, according to how much energy the cell needs. For example muscle cells need more energy than skin cells. Mitochondrial size also varies, from spheres of a few hundred nanometres in diameter to elongated shapes that can be 1 micrometre in diameter and as long as 7 micrometres. Bill.
Each cell can contain many mitochondria and each mitochondrion can contain many copies of its chromosomes. Mitochondria are considered instrumental to cells. They are generally known as and called "the powerhouse or power plant of the cell". This is because this is their main and fundamental task. They provide cells chemical energy that is necessary to perform the metabolic tasks related to life. Mitochondria are believed to have originated from remnants of the photosynthetic bacteria. These mitochondria avail of their protein from the genes that are encoded within the nuclear genome of a cell. Mitochondria are found in the cells of plants, animals and fungi. They have a double membrane. Mitochondrial functions include Steroid synthesis, Heme synthesis, Cellular proliferation, Apoptosis programmed cell death etc.