These are membrane-bound cell organelles that generate most of the chemical energy needed to power the cell's biochemical reactions.
Chemical energy produced by the mitochondria is stored in a small molecule called adenosine triphosphate (ATP).
Cells, which require more energy, possess more mitochondria such as muscles, liver, and brain cells.
Liver cells contain 1000-2000 mitochondria in each cell.
Heart, sperms, and muscle cells contain about 5,000 mitochondria per cell. These cells need more energy, so they contain more mitochondria than any other organ in the body.