Function 

Mitochondria are vital to survival because they are involved not only in aerobic respiration but a number of critical functions.

Respiration - The Krebs/Citric Acid Cycle

• This is the second step in the production of energy rich ATP but the first to occur in the mitochondria, specifically, the matrix.
• Glucose and other compounds are broken down outside the mitochondria and then transported inside and converted to the starting substrate Acetyl-CoA. From 1 glucose molecule, 2 Acetyl-CoA molecules are derived so the cycle goes round twice.
• A molecule of Acteyl-CoA contains 2 carbon atoms and the main principle of Krebs is that carbon atoms are added and stripped with different steps. A 4 carbon oxaloacetic acid molecule combines with Acetyl-CoA to produce a 6 carbon citric acid molecule, the starting substrate.
• As the citric acid molecule moves around the cycle it undergoes a series of transforming reactions starting with the removal of 2 carboxyl groups as 2 CO₂ molecules.
• Further along the cycle, electron acceptors, notably NAD⁺ strip high energy electrons from the citric acid molecule becoming NADH. For each acetyl group that enters Krebs, 3 NADH molecules are produced and these then enter the next step in respiration, the electron transport chain. Electrons are also removed from the substrate by the acceptor Q which becomes QH₂.
• When the cycle finishes, the end product is the oxaloacetic acid molecule present at the start so the cycle continues.

Krebs cycle image courtesy of wikimedia. This image is in the public domain and thus free of any copyright restrictions. 

Respiration - The Electron Transport Chain (ETC)

The electron transport chain comprises of a series of protein complexes found on the inner mitochondrial membrane which transfer electrons donated by the electron donors produced in the citric acid cycle. This process is coupled with the pumping of hydrogen ions (H+) into the intermembrane space creating an electrochemical gradient across the membrane which the complex ATP synthase uses to produce ATP.

• First, NADH transfers 2 electrons to the complex, NADH dehydrogenase which pumps 1 hydrogen ion into the intermembrane space per electron.
• Next, the 2 electrons are transferred to the mobile complex ubiquinone which transports its cargo to a cytochrome BC1 complex.
• The cytochrome BC1 complex then transfers electrons to another mobile complex, cytochrome c, one at a time.
• At the same time, 1 hydrogen ion is pumped across the membrane for each electron transferred to cytochrome c.
• 4 electrons gather in the next complex, cytochrome c oxidase which combine with an oxygen molecule and 8 hydrogen ions. This creates 2 molecules of water and leaves 4 out of the 8 hydrogen ions free to be pumped into the intermembrane space.
• The concentration of hydrogen ions rises in the intermembrane space as electrons are transferred from complex to complex creating an electrochemical gradient. This gradient harbours the energy needed for the production of ATP from ADP and inorganic phosphate by the complex, ATP dehydrogenase.
• Watch the video for a graphical representation.

Video courtesy of Youtube, thus free of copyright. 

Heat Production

Sometimes protons can move back across the mitochondrial membrane into the matrix via facilitated diffusion and have no contribution to ATP production. The process, referred to as proton leak, results in the unharnessed energy contained within the proton gradient being converted to heat energy. The protein responsible for the transport of protons is called thermogenin which is found in high quantities in the brown fat type present in hibernating mammals and humans at birth.

 Thermal image courtesy of Flickr under the creative commons license.

Calcium Storage

Calcium must be kept at discrete concentrations inside a cell due to its involvement in most cellular processes and signalling pathways. Mitochondria can act as a mechanism of homeostasis by rapidly taking up calcium into the matrix through the calcium uniporter located on the inner mitochondrial membrane. This process reduces the concentration of free calcium in the cell cytosol when the level has suddenly increased saving the cell from damage. When calcium is needed for functions such as neurotransmission, the ions exit the mitochondria via sodium-calcium exchange proteins or via calcium-induced-calcium-release pathways.

Additional Functions

Mitochondria are involved in many other metabolic functions of which some are only present in specific cell types. Below is a list of the other tasks involving mitochondria.

• Regulation of the membrane potential
• Apoptosis (programmed cell death)
• Calcium signaling
• Regulation of cell proliferation
• Regulation of cellular metabolism
• Steroid synthesis.