The primary function of the mitochondria is to produce ATP, the molecule that the cell uses for energy when carrying out essential functions. The process by which glucose is converted to ATP in the mitochondria is known as cellular respiration and occurs in mitochondria of both plant and animal cells.
Mitochondria are comprised of two major membranes. The outer membrane controls which molecules are allowed to enter and exit the mitochondrion. The inner membrane contains specialized enzymes that work together the convert glucose to ATP, and it is the primary site of cellular respiration.
Mitochondria are found within all plant and animal cells. Cells that use more energy tend to have more mitochondria. If the energy needs of a cell increase, the mitochondria replicate to increase in number. For example, as an athlete begins to work his or her muscles harder, these muscle cells develop more mitochondria to keep up with their growing energy needs.
Mitochondria replicate independently of the rest of the cell, and they have their own DNA. For this reason, many scientists believe that mitochondria evolved from early prokaryotic bacteria that lived symbiotically inside of the cells of more complex organisms. The majority of proteins which mitochondria need to carry out cellular respiration are coded for in the mitochondrial DNA.