I’m having an epiphany – the result of pondering the origin-of-life and the role of carbon, hydrogen, and oxygen. While we think of carbon-based chemistry (or “organic chemistry”) as the basis of life, perhaps C is just a “carrier”. Its job is to ferry electron transfer down a redox gradient from H to O, with the end result being water (H2O). But I’m telling the story backwards having started with the conclusion. Let’s rewind.
Hydrogen is the primordial and most abundant element in the universe. We’ll ignore helium for the moment because it is chemically inert. Then comes oxygen followed by carbon, at least in our part of the galaxy, the Milky Way.
Hydrogen, with a single proton in its nucleus, holds on tightly to its electron but is not shy about giving it away or sharing it with a neighbor – especially one that loves holding on tightly to electrons. Who’s that neighbor? Oxygen, of course! If you’ve taken a chemistry class, you’ve likely heard about electronegativity – the ability of an atom to pull electrons to itself in a shared chemical bond. Oxygen is one of the most electronegative elements and is “electron-greedy”. Hydrogen, on the other hand, is much less electronegative and willingly gives up its electron to oxygen. The opposite – oxygen donating an electron to hydrogen – almost never happens without a large influx of energy.
Thus, the natural state of affairs (or “thermodynamic spontaneity”) is in one direction. Electrons flow from hydrogen to oxygen. This is the basis of redox (or reduction-oxidation) reactions transforming “chemical” energy into useful forms that humans can work with. A simple chemical reaction (that my students see over and over again) is:
2 H2 + O2 –> 2 H2O (Equation 1)
It’s also known as the rocket fuel reaction because you get the most bang for your buck if you have to carry both your vehicular fuel and oxidant. The energy released in this chemical reaction powers the motive force of your vehicle. You’ve broken the weaker chemical bonds of “higher energy” less stable molecules (H2 and O2) and created “lower energy” more stable molecules of H2O with stronger chemical bonds.
What happens when the fuel runs out and you’re just left with a whole bunch of (chemically very stable) water molecules? Well, nothing. You’re dead in the water.
Here’s where carbon comes in. Being a potential four-armed tetrapod, it can make connections to many other atoms in diverse ways. Carbon forms relatively strong chemical bonds, enough to keep it stable potentially for long periods of time, but not so strong to be completely chemically inert. That allows it to play its major role in living systems and show up in a diverse array of molecules – C is for Carrier!
Molecular oxygen (O2) was not abundant on the early Earth. It’s relatively reactive, chemically speaking. Instead oxygen was likely “trapped” in the form of CO2, a rather stable molecule with strong chemical bonds. However, the reaction of CO2 and H2 to produce water and organic molecules is thermodynamically favorable, the more H2O you produce and the more H2 you consume. A representative reaction is below.
6 CO2 + 12 H2 –> (CH2O)6 + 6 H2O (Equation 2)
The molecule (CH2O)6 could also be written as C6H12O6 which some of you might recognize as the chemical formula of a sugar such as glucose. We call these molecules carbohydrates, “carbo” for C and “hydrate” for H2O. Think of it simply as biomass.
But if you went further and made more water, you could do something such as
6 CO2 + 18 H2 –> (CH2)6 + 12 H2O (Equation 3)
The molecule (CH2)6 is a hydrocarbon, containing only hydrogen and carbon. It’s a hexane or hexene because there are six carbons. These are excellent fuels. The octane in your automobile is a hydrocarbon. (And yes, octane is eight-carbon molecule – look, you can learn organic chemistry!)
As fuels, hydrocarbons and carbohydrates can be burned for energy – a thermodynamically favorable reaction. For example,
(CH2O)6 + 6 O2 –> 6 CO2 + 6 H2O (Equation 4)
Molecular oxygen is particularly good for fuel-burning because you break its weaker chemical bonds to form the strong chemical bonds of water. Notice that CO2 has been recycled in this process.
But where did O2 come from in the first place? We’ve said that on the early Earth it was likely trapped as CO2. Here’s what I think happened. There are plenty of different alternatives to Equations 2 and 3 that allow the production of a rich array of molecules containing different ratios and connections among carbon, hydrogen, and oxygen. Small energetic differences allowed for a large variety of chemical reactions where molecules were transformed into other molecules. Eventually cycles of chemical reactions emerged and energetically sustained by connecting H to O making H2O. This is the origin of metabolism. Why cycles? Because they persist. And what persists, persists; what doesn’t, doesn’t.
These cycling reactions are thought to take place in deep ocean hydrothermal vents that have the right chemical substances and environmental conditions for the redox reactions to proceed. But there’s another huge energy source available to planet Earth. The Sun! But how would you trap that solar energy and utilize it? In a story that we haven’t quite figured out, but we know what happens today, photosynthesis emerged. It can be encapsulated by the equation below.
6 CO2 + 6 H2O + photons –> (CH2O)6 + 6 O2 (Equation 5)
Without the photons of light, the reaction would be energetically very unfavorable because you’d be attempting to transform H2O back to oxygen and breaking the very strong O–H bonds. The evolution of light-trapping structures is an amazing story in itself, but if you look closely at the nitty-gritty of photosynthesis, it’s ultimately a story about the movement of electrons. Redox reactions drive biochemistry and the engines of life. Our current biosphere operates on the almost symmetric Equations 4 and 5. We now, just by living, transform energy from the sun into other uses while dissipating lots of heat in the process and increasing overall entropy. But if you strip out the carrier carbon and lay the chemistry bare, it’s about H and O and H-two-O.
P.S. To read more about these wonderful elements, I recommend Hydrogen by John Rigden and Oxygen by Nick Lane (book covers shown above). I read both these books before I started blogging, and they have prime space on by current bookshelf. They’re both excellent reads. As for carbon, I’ve more recently enjoyed A Symphony in C by Robert Hazen.
Less than a month after this epiphany, I read a paper from twenty years ago that discusses this same idea except more comprehensively and with much erudition. I'd like to say "Great minds think alike" but maybe I'm just slow.
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