Oxygen is one of the most important molecules in biological systems and is required for life to sustain itself. This awesome ability of oxygen stems from its high electronegativity, even though it’s in its diatomic form. Electronegativity is the ability of an element to pull electrons towards itself kind of like a magnet. The stronger the magnet, the greater the force of attraction it will exert on surrounding metallic materials. It’s the same with electronegativity, the higher it is, the greater its force of attraction on surrounding molecules. Like in the candle example in the video, the flame produced stems from the oxygen, pulling the electrons from the paraffin towards itself. This caused the bonds to break in the paraffin, releasing energy in the process as light and heat.
In biological systems, the concept is the same, albeit in a more controlled manner. Living organisms use energy as a necessity, rather than all the energy being released all willy-nilly like in a combustion reaction. So in most complex living organisms, carbohydrates are the main source of energy. Carbohydrates are a great source of energy, due to the high amount of potential energy contained in its bonds. Just like the paraffin, carbohydrates are combustible and would burn in the presence of oxygen and energy would be released, but, in living organisms, this same process is done in steps by a series of controlled reactions, instead of being so straightforward; And instead of this energy being released in the form of light and heat, this energy is going to be used to form ATP (Adenosine triphosphate).
In the body, the process of making ATP is done by the electron transport chain. This process occurs on the inside of highly folded mitochondria, and the electrons are transported into the mitochondrial by the carrier molecule NAD (Nicotinamide adenine dinucleotide). Once inside of the mitochondria, the NADH+ goes towards a protein complex within the mitochondrial membrane. The protein complex uses the energy lost from the electron in the NADH+ to pump a proton, from the inner membrane to the outer membrane of the mitochondria. The electron travels down a chain of complexes, at each, the energy of the electron is decreasing until it’s in its lowest energy state. And it’s at that energy state where an oxygen molecule accepts low energy electrons to form water. Meanwhile, in the inter-membrane space, a proton gradient is being setup between the inner and outer membrane due to a difference in concentration by proton buildup. The only means of balancing the charge is for the proton to pass through the enzyme ATP synthase. As the ions flow through the ATP synthase channel, the energy they create while passing though is going to be used to bind a phosphate molecule to ADP to form ATP.
*Disclaimer: Although this video is GREAT for giving a person an introduction to the process, and would suit the needs of an A level student, a Tertiary-level student would need a bit…more. This video fails to mention the fact that before oxidative phosphorylation can occur (electron transport chain), substrate level phosphorylation has to happen (Glycolysis and TCA cycle).
Contributors: Le Frenchie, Roi (editor)