To a chemist, redox merely refers to the transfer of electrons from one atom or molecule to another. One molecular part gains electrons and becomes reduced, and another part surrenders electrons and gets oxidized. So reduction is the opposite process of oxidation. To a non-scientist, oxidation is equated with “burning” phenomena. The wood in a campfire oxidizes, the iron on your balcony railing oxidizes (rusts), and the sugar and fat that you eat oxidizes (you burn it for energy). This is why the concept of reduction is so hard for people to understand. While all these familiar oxidations are taking place, oxygen is reducing. But since air is invisible, we do not talk about that part of the reaction, so it seems mysterious, or even techno-nerdish. But the electron equation is the transfer of electrons from the reduced wood, iron and food to oxygen.

This transfer of electrons is best not considered as a transfer of charge. Although it is on its most basic level, the resulting oxidized and reduced atoms regroup to minimize any charge at a molecular level. Take for example, the burning of gasoline in a car engine. The hydrocarbon fuel and the air it gets mixed with before it explodes can be thought of as carbon with charge zero, hydrogen with charge zero and oxygen with charge zero. After combustion, the carbon oxidizes to +4, the hydrogen oxidizes to +1 and the oxygen reduces to -2. But the +4 carbon atoms are not found in a charged molecule, they combine with two -2 oxygens to make CO2 (carbon dioxide), which is neutrally charged. Two +1 hydrogen atoms combine with one -2 oxygen atom to make water (H2O), which is also neutrally charged. But the shift of electrons (the redox reaction) has taken place, from carbon and hydrogen to oxygen, with a resounding release of energy. If you want to hear that energy, detatch your car's muffler. To a biologist, redox reactions are especially interesting. They represent the fundamental energy that supports life. Some kind of redox reaction is a necessary and essential aspect of living metabolic systems, without which death occurs.

The “energy” that drives life is actually a redox potential. The potential is the difference between one atom or molecule with a high redox potential and another one with a low redox potential. For us warm-blooded animals, the primary redox potential that keeps us alive and conscious is the potential between oxygen gas in the atomosphere and food. The fat-oxygen potential is quite high, almost to the level of hydrocarbon-air mixtures that power your automobile. The sugar-oxygen potential is roughly half as much.

But to microbes, the potential can be two different minerals, one of which comes out of a hydrothermal vent at the bottom of the ocean. It can be the difference between sugar and alcohol (the yeast-derived fermentation of beer and wine). It can be geothermal hydrogen and methane leaking up from the deepest reaches of the earth’s crust. It can be a truly amazing number of redox potentials. This makes the search for extraterrestrial life quite challenging. And it can even be an entropy gradient that drives a redox-potential reaction.

Life is a special case of redox study because living systems are not redox neutral. Living systems are highly reduced. They have a large supply of electrons in the form of fuels like fat, sugar and amino acids, and antioxidants like NADH, NADPH, glutathione, ascorbate (vitamin C) and vitamin E. Most vitamins are reduced in their active states. Take folic acid, which is activated by doubly reducing it to tetrahydrofolate. Tetrahydrobiopterin is another example. They can be obtained from foods in their reduced states, or in their oxidized state and subsequently reduced.

A substantial portion of the total energy of life is devoted to simple maintenance of a highly reduced state within a more oxidized environment. Before the hydrogen (hydride) fuel on NADH is used to make ATP to power enzymes, some of it is diverted to NADPH for recycling antioxidants. This diversion is especially important under conditions of extreme oxidative stress, like from oxidative stress from viral infections (eg. Covid) in the elderly.

The word “antioxidant” is a bit confusing because it encompasses both antioxidants and reducing agents. Most antioxidants are NOT reducing agents. The difference is that antioxidants are able to sacrifice electrons to highly oxidizing free radicals, whereas reducing agents are able to donate electrons to even mild oxidants. So the proper analogy is that antioxidants are like umbrellas protecting you from the heat of the sun, but they do not lower the ambient temperature of the air surrounding you. But reducing agents are like refrigerators, they can drop the ambient temperature. This temperature analogy is a good one for understanding the necessary and essential role of reduction in living systems. Life requires a cooler (reduced) temperature than the environment in which it lives.

When warm-blooded animals (people) cannot maintain their energy from aerobic (O2 + fuel >> CO2) redox potentials, disease manifests. It’s one thing to burn glucose to lactic acid on rare instances where the demands of outracing a bear require more energy than is aerobically available, but it’s an entirely different thing to be continuously relying on the much smaller redox potential of fuel-lactate compared to fuel-oxygen.



Stephen Fowkes on Facebook