What is Oxidative Phosphorylation?
Living beings depend on a number of different energy sources. Utilizing such energy sources is a tightly regulated process and has been optimized in such a way that cells can utilize the energy whenever required.
All the intracellular processes require energy to proceed and an energy currency called adenosine triphosphate or ATP is what provides the said energy.
Oxidative phosphorylation, as the term suggests, is a process which involves oxygen utilization and phosphate group addition.
Hence, it is an essential process demonstrated by aerobic organisms. The oxygen which we breathe in is used in this process and phosphate group gets attached to adenosine diphosphate (ADP), which has two phosphate groups, to form ATP, which has three. The enzymes and compounds required to carry out oxidative phosphorylation takes place in the mitochondria in case of eukaryotes.
The inner and outer mitochondrial membranes play the major role in this process.
Significance of Oxidative Phosphorylation
Cellular metabolism is the process in which cells can breakdown different complex nutrient sources to ultimately gain ATP.
This process involves a number of pathways such as glycolysis (glucose breakdown), tricarboxylic (TCA) cycle and beta oxidation of fatty acids.
The resulting ATP, however, is generated when the certain products of the said metabolic pathways are fed into oxidative phosphorylation.
Hence, as a living system, it is important for the cell to ensure sustained synthesis of ATP. This process utilizes the high energy yielding capacities of NADH and FADH2.
Steps of Oxidative Phosphorylation
I. Feeding of Electrons:
Nicotinamide adenine dinucleotide (NAD/NADH) and flavin adenine dinucleotide (FADH2) are molecules which are generated by catabolic pathways.
These are electron rich sources which need to react with oxygen and break it down to form water.
A complete cycle of glycolysis releases 2 NADH, 3 NADH from the TCA cycle and 7 NADH from beta oxidation.
II. Passage Through the Electron Transport Chain (ETC):
In the inner membrane of the mitochondria, the electron donors pass through a series of complexes, termed as the electron transport chain.
NADH and FADH2 passes through these complexes and perform reducing reactions, all of which result in generation of electrons and release of energy.
Inside the ETC, for every pair of electrons generated, two protons are transferred outside the membrane, thereby generating a proton gradient.
Finally, the electrons react with molecular oxygen in the last complex of ETC and generate water.
III. ATP Synthesis via Chemiosmosis
Chemiosmosis refers to the flow of ions across a gradient. The resulting proton gradient causes an inflow of hydrogen ions from the outer matrix to the inter-mitochondrial space.
An enzyme called ATP synthase is present on the inner mitochondrial membrane.
The movement of H+ ions occurs only via one of the structures (which is a channel) of ATP synthase, and the other connected structure of this enzyme catalyzes the conversion of ADP to ATP.
The net gain is around 32 to 34 ATP molecules.
Oxidative Phosphorylation in Bacteria
This process takes place in the cell membrane of bacteria (due to the absence of specialised organelles).
Due to the fact that bacteria need to grow in heterogenous environment, they are able to generate a variety of electron donors/acceptors to feed into the electron transport chain.