Energy change along the redox spectrum of organic compounds that contain oxygen drives oxidative metabolism. The successive changes that occur upon the glucose substrate through the various stages of oxidative metabolism (glycolysis, mobilization of pyruvate, the citric acid cycle) involve the oxidation-reduction series comprising alcohol, aldehydes & ketones, carboxylic acids, and carbon dioxide. Oxidative metabolism can be seen as a narrative of electron control in which electronegative oxygen is gaining control of bonding electrons that were under the control of carbon in the intial state. In this narrative of electron control, electronegative (electron greedy) oxygen begins with little electron control in forms such as oxygen gas or hydroxyl group but it ends in the reduced states of carbon dioxide or water, where it has increased its covalent bonds to less electronegative elements.

When oxygen forms a bond with a less electronegative element (which would be any on the periodic table except fluorine) the electrons are drawn in toward the powerful oxygen nucleus. Everybody learns that the electronegativity of oxygen leads to polar bonds, but what most students don’t seem to be learning is that this corresponds to an extra internal energy decrease beyond run-of-the-mill bond energy. Extra energy is lost when electronegative elements form bonds. In other words, polar bonds tend to be stronger than nonpolar bonds. The fact that polar bonds are exceptionally strong (requiring the input of large amounts of energy to break), is the reason that energy is liberated when they are formed in the oxidative metabolism of nutrient molecules. As the system moves from relatively weak C-H and O-O bonds to strong C=O and O-H bonds, internal energy decrease occurs, which translates to a free energy decrease. Oxidative metabolism harnesses the free energy decrease, coupling the process with ATP synthesis through some direct substrate level phosphorylation and but mostly chemiosmosis in the mitochondria.

Let's use a gravitational analogy. Picture a water wheel being used to power a simple crane which is stacking bricks. The water wheel couples the decrease in potential energy of water with the increase in potential energy of the bricks. In metabolism, electrons are 'falling' towards the oxygen nucleus as new bonds are formed giving oxygen greater access to pull the electrons inward towards itself. The 'fall of electrons' is analogous to water falling along the incline with gravity, and, furthermore, just as the water wheel couples the decrease in potential energy of water with the increase in potential energy of the bricks, the processes of metabolism couple the decrease in potential energy of electrons in metabolism with an increase of chemical energy elsewhere, in ATP formation, as the negatively charged phosphate group is pushed onto negatively charged ADP, a potential energy increase, like compressing an electrostatic spring. Of course, later, when an ATP formed in such way is encouraged to come apart under enzyme catalysis, the process will be coupled with one of a multitude of biological processes.

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