The energy stored in food comes in large denominations. Lipids, carbohydrates and even proteins ingested or taken up by cells provides them with the energy they need for growth and metabolic activities. Before cells can access this energy, however, it must be broken down into "bite size" pieces. This is the role of the mitochondria.
In eukaryotic cells mitochondria are involved in the final stages of energy release from food molecules such as sugars. After being broken down to two-carbon fragments in the cytoplasm, the terminal products of catabolic processes such as glycolysis move inside the mitochondria organelles. In a further series of chemical breakdown reactions carbon dioxide is produced and energy trapped in high energy electrons. Finally the energy in these electrons is released in a stepwise chain reaction that transfers manageable packets of energy to molecules of ATP and the final waste product, water, is created.
Such complex activities are possible because of the structural organization of the mitochondrion. Each organelle has a double membrane; an outer membrane which is smooth and defines the shape of the organelle, and an inner membrane which is heavily folded into finger-like cristae producing a large surface area.
Within the central compartment of the inner mitochondrial membrane are the Krebs cycle enzymes that, in a cyclical series of chemical reactions, break down the two-carbon fragments to carbon dioxide. As part of these breakdown reactions high energy electrons are produced. Carried by modified nucleotide molecules, these electrons are transferred to a highly organized series of proteins, enzymes and electron carriers that are embedded in the inner mitochondrial membrane. In a stepwise, cascading fashion the electrons are serially transferred down an energy chain, loosing some of their energy at each step. This released energy is not lost, but used to pump hydrogen ions from one side of the membrane to the other.
When the electrons are at their lowest levels they are combined with oxygen and hydrogen ions to produce water. But, back in the mitochondrial membranes, the unbalanced gradient of hydrogen ions on either side of the mitochondrial membrane is producing an energy potential across the barrier. As the electrons flow back to the inner compartment, they pass through an enzyme/protein complex embedded within the mitochondrial inner membrane. Energy is once again moved, this time into a chemical reaction that forms ATP. This ATP molecule is the short term energy currency of cells and organisms.
All eukaryotic cells have mitochondria. Heart cells or other similar cells with high metabolic activity, have a lot of mitochondria to keep the cell provided with the ATP it needs for its specialized role. Other cells, such as lipid storage cells, still have mitochondria, but, because of their much lower levels of metabolic activity, they have fewer such organelles and produce less ATP.
Unlike other organelles within the cell, the mitochondria have their own DNA molecules and this genetic material is responsible for coding for some of the proteins and components used in the metabolic activities. Mitochondria also have their own distinct types of ribosomes and undergo what looks like a division process. At the time of sexual reproduction in animals, the fertilized zygote receives all its mitochondria from its mother. This 'maternal' inheritance of mitochondria and the DNA they carry makes it possible to trace backwards the genesis of our species to the 'original' woman on who's mitochondria we still all depend.