Redox reactions are well known to anyone having taken basic chemistry. The concept is fairly straightforward—during a redox chemical reaction, due to changes in connectivity, and often, therefore, electronegativity, certain atoms will change their oxidation states. This can be worded to say that electron “ownership” changes. For example, in respiration, because the two atoms possess equal electronegativity in diatomic oxygen, the 4 double bonded electrons are shared equally. So each oxygen “owns” six electrons. In water, the oxygen now holds the bonding electrons closer to itself, because of its higher electronegativity relative to hydrogen. Thus, it could be said that oxygen has eight electrons, one more from each H-O bond.
So why redox? All living metabolisms deal with redox couplings.That is to say, biological energy generation deals heavily in redox chemistry. Living organisms move electron ownership between chemicals from unfavorable, high energy atomic configurations to favorable, stable ones. This process releases energy used to then power cellular function.
Given this conservation, surely there are reasons for the prevalence of redox in biology:
Being able to modify oxidation states opens up organisms to more energy and more variety. To explain this, we can say that there are more reactions one can do when shuffling electrons instead of just elements. The more degrees of thermodynamic freedom in a system, the more ways you can manipulate the elements contained within.
Non-redox reactions can only take so many forms. Requiring significant chemical remodeling, and usually resulting in a stable product/products that are difficult to further work with, non redox chemistry is not well suited for life. Life that involves countless interacting reactionary pathways.
How the biological electron gets around this problem is by being versatile. Electron ownership can be shuffled between any number of molecules, provided the final result is a favorable reaction. This means there are nearly endless electronic states able to be used by molecular interactions employing redox as a means of reaction power.
The utility this versatility affords can be seen in ATP biogenesis. ATP, to those not familiar, is the standardized molecular fuel of the cell (which, to drive the importance of the concept home, powers reactions by way of redox transfers itself). By and large, the majority of ATP in non-photoautotrophs comes from ATP synthase, powered by electron transport chains (ETCs). A familiar concept in modern biology, we will take a look at probably the best studied of all ETCs—that of the mitochondria.
Starting with a basal hydrocarbon, various “shuttles” strip electrons off the molecule, thereby oxidizing it in the process (eventually into CO2, the most biologically oxidized form of C). These electrons are passed off to mitochondrial transmembrane proteins. What follows is a number of electron transfers (and thus, redox) between these protein groups, and finally onto water. With each transfer, energy is generated to develop a transmembrane ion gradient, which then is processed through kinetic means into ATP.
a very simplified view of phosphorylation redox.
ATP production in this way can be thought of as analogous to combustion, with a carbonaceous compound split into water and carbon dioxide. However, unlike traditionally empirical combustion, the energy otherwise released as a flash of light and heat is split carefully and efficiently into small microtransactions. In this way, redox metabolism is exceedingly efficient. But this is just the surface.
The field of redox biology has a broad scope. There are, of course, the traditional metabolic approaches, looking at fermentation and photoautotrophy, both which make extensive use of redox. Other topics with a redox focus include radical biochemistry and even potential-dependent transcription. In the end, even studies not expressly studying redox chemistry still would do well to consider it, as it is the basis of much of biochemistry.