Production of ATP by oxidative phosphorylation

Describe the processes involved in the production of ATP by oxidative phosphorylation. 

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Adenosine triphosphate (ATP) is a nucleotide triphosphate made up of ribose sugar, the nitrogenous base adenine and three phosphate groups. Commonly referred to as the “energy currency of life”, it is critical in transporting energy within cells as part of metabolism. This energy is used in numerous vital cellular processes such as motility and cell division, making ATP a key component of life itself. ATP is produced in the mitochondrion or cytosol of a cell through three processes: glycolysis, the citric acid cycle and oxidative phosphorylation, the latter of which will be the subject of this essay.

Oxidative phosphorylation takes place in cell mitochondria, is the final phase of cellular respiration and produces a greater amount of ATP than both glycolysis and the citric acid cycle. NADH and FADH2 molecules (produced by glycolysis and the citric acid cycle respectively) donate electrons to the electron transport chain. The chain itself is a sequence of electron carriers, which accept electrons from less electronegative carriers then pass said electrons on to more electronegative carriers along a “downhill” slope.

At the end of this chain, oxygen combines with these electrons as well as hydrogen ions to create water, thereby using the chemical energy of moving electrons to drive oxidative phosphorylation. The chain itself does not directly generate ATP, it’s function is to ease a large drop in free energy, breaking said drop down into stages which release energy more gradually. As such, while oxidative phosphorylation produces more ATP than glycolysis and the citric acid cycle, it requires these earlier phases in order to power it’s own reactions. Likewise, while the electron transport chain does not produce ATP, it serves a crucial role in it’s creation, providing the initial energy required for the second stage of oxidative phosphorylation.

While the first stage of oxidative phosphorylation could be said to create ATP indirectly, the second stage, chemiosmosis does directly synthesise ATP. The protein complex ATP synthase is the crucial component of chemiosmosis, as it is the enzyme which directly creates ATP using ADP and organic phosphate.

ATP synthase acts like an ion pump operating in reverse, that is, it utilises the differing concentrations of H+ on both sides of the inner mitochondrial membrane as a power source. As H+ ions flow down their concentration gradient, they attach to binding sites in the rotor of ATP synthase, firstly causing the rotor to turn, which then causes an internal rod to revolve which activates catalytic sites within the catalytic knob. These catalytic sites can then produce ATP from ADP and inorganic phosphate.

In summary, oxidative phosphorylation is heavily involved in the production of ATP, generating much more ATP than glycolysis and the citric acid cycle combined. Oxidative phosphorylation as a whole consists of two processes; the electron transport chain, which provides and maintains the H+ gradient necessary for the next phase; chemiosmosis, which uses ATP synthase in combination with the Hydrogen ions to directly produce ATP. Oxidative Phosphorylation also fundamentally relies on the NADH and FADH2 generated by the earlier phases of respiration to function.

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