Based on:- Mitochondria: The Dynamic Organelle
Eds:- Stephen Schaffer & M-Saadeh Suleiman – Springer (2007)
The primary function of mitochondria is to generate ATP that supplies cells with energy. Mitochondria are cell organelles comprising an outer and inner membrane, the latter enclosing a matrix space in which hydrogen and electrons are extracted. The inner membrane contains the respiratory chain; the energy from oxidation/reduction reactions is used to move protons out of the inner matrix and across the inner membrane. This establishes an electrochemical potential gradient across the membrane which allows the synthesize of ATP from ADP.
The respiratory chain is composed of proteins embedded in the membrane. An electrochemical driving force for protons across the inner membrane of the mitochondria supports the generation of ATP. Protons are extruded by an electron transport system, and then re-enter along an electrochemical gradient. This re-entry causes conversion of ADP into ATP.
Production of energy for the cell is still seen as the most important function of the mitochondria, but in recent years a number of other mitochondrial functions have become apparent. These include the generation of reactive oxygen species (ROS) that are important in cell signalling, modulation of calcium ions, cell protection and finally cell death.
The network of mitochondria is coming to be viewed as a signalling system within the cell. Signalling pathways are likely to involve mitochondrial production of reactive oxygen species, and mitochondrial ion channels may be involved in this process. Signal pathways are thought to be connected to ATP-sensitive potassium channels. The structure and organisation of proteins in both the inner and outer membranes of the mitochondria are considered central to these processes. Mitochondria are now known to provide small molecules for the maintenance of metabolic pathways and to regulate the constituents of the cytoplasmic fluid within the cell. This regulation is performed by the inner mitochondrial membrane. Mitochondria are dynamic in the sense that they can migrate and accumulate in response to stimuli, and are seen here as the organelles that respond to the environment and act as a hub for many cell functions.
Mitochondria also regulate the transport of positive ions, and their multiple potassium channels may be important for protecting cells. Selective uptake of positive potassium (K+) and positive calcium ions (Ca2+) has been demonstrated along with the exchange of sodium ions (Na+) for protons and there is a similar K+/protons exchange. However, the inner mitochondrial membrane is impermeable except for a few small molecules and ions, so as to preserve its proton gradient, which supports the production of ATP.