Yesterday, in my General Chemistry class, we discussed using bond energies to calculate the change in enthalpy of a chemical reaction. Breaking bonds is endothermic and requires energy input into the system. Conversely, making bonds is exothermic and energy is released from the system. Chemical reactions almost always involve both the making and breaking of bonds. Therefore, whether the overall chemical reaction will be endothermic or exothermic will depend on whether the bonds being broken are stronger (or weaker) than the bonds being formed.
One example I showed was ATP hydrolysis. The reaction is marginally exothermic. Even though the same types of bonds were being made and broken, the bond energies are slightly different in different chemical structures. That’s the beauty of chemistry – a subtle interplay between structure and energetics! The purpose of this example was to counter the conceptually wrong mind-worm students acquire where they tell me that “breaking bonds releases energy”. This usually comes from a simplified misunderstanding of something they hear in a biology class.
Towards the end of class, I couldn’t resist connecting bond energies to the origin of carbon-based life on Earth. The students had previously worked a problem on the strength of the O–H bond in water and the corresponding wavelength of a photon that matched the bond energy. Referring to the solar spectrum and ultraviolet light, I speculated about how adenine may have been important as a photon absorber prior to its role in the universal energy transduction of living systems. I mused about water-splitting, the invention of photosynthesis and suitable molecular pigments (conjugated pi-systems!) that may have arisen through chemical evolution. I didn’t say anything about such pigments dissipating thermal energy and seemingly “wasting” it.
This brings us to today’s question: Why do animals exist on Planet Earth?
This morning, I went down a rabbit-hole reading several articles by Karo Michaelian. It all started with “The Pigment World: Life’s Origins as Photo-Dissipating Pigments” (Life 2024, 14, 912). He makes the provocative claim that animals essentially “provide a specialized gardening service to the plants and cyanobacteria, catalyzing their absorption and dissipation of sunlight in the presence of water, promoting photon dissipation, the water cycle, and entropy production.” That’s a mouthful. We’ll break it down momentarily, but essentially the claim is that animals help to move molecules around, spreading them far and wide so that more and more photons can be absorbed and that energy dissipated. It’s the second law of thermodynamics in action at the level of the biosphere. And what’s the stuff we’re moving around? Pigment molecules!
It's an interesting argument. He begins with the argument that many leaves absorb photons in the ultraviolet and visible range before dropping off significantly at the infrared boundary. Leaves look green to us because red and blue light are absorbed more than green. Photosynthesis however only makes use of a narrow regime of red light, yet leaves strongly absorb in the ultraviolet and in the (blue) visible range. Plants evolved to absorb photons which are hardly absorbed by water, and apparently “fill even small photon niches left by water over all incident wavelengths”. That’s rather curious. Also, the albedo in life-rich ecosystems (jungles and forests) is considerably lower than in sandy deserts which reflect much more of the incident light. Additionally, “the albedo of water bodies is also reduced by a concentrated surface microlayer of cyanobacteria”. What happens to this absorbed energy? It is converted to heat – essentially chopping up a smaller number of high-energy photons into a large number of low-energy photons. It’s the second law of thermodynamics: energy is being dissipated and entropy increases mightily!
The evolution of these absorbing pigments in plants may have been primarily to increase transpiration. Photosynthesis is a secondary player in this regard. That’s a shocker to me. I’ve always considered the oxygenating of the atmosphere via photosynthesis to be a driver for the complexity of life – which it is – but I hadn’t thought of it as a byproduct to mostly increase heat dissipation via transpiration. In the first week of class, I told students about water’s high heat capacity and its suitability as a calorimeter. In a couple of weeks after we get through entropy, we’ll be looking at the change in enthalpy and entropy of vaporization as liquid water turns into gas. Water is an excellent dissipator that helps drive the second law of thermodynamics, but does so if there’s more of it in Earth’s water cycle. Transpiration puts more water into the cycle!
What do animals do? They help disperse the pollen or seed of plants. They help bring nutrients to plants through poo or death. As heterotrophs disperse organic matter, they disperse the pigment molecules. More opportunities for absorbing photons. More dissipation to high entropy heat. We animals are the gardeners, helping the second law to roll along. Humans in particular have come up with alternative ways to tap photons with inorganic materials, but that’s a recent phenomenon. The organic pigments have been at it far longer than we have. All this makes me wonder if the reason why photosynthesis is so inefficient is because life isn’t optimizing for capturing energy from photons in that way; rather it is optimizing for seemingly wasteful heat dissipation. The second law rules!
The tropics are rife with life. Is it because they receive the most photons? Why are there so many insects there? They’re a key part of the gardening crew. Why are there larger animals further away from the equator? The gardening crew is mostly about seed dispersal and larger creatures roam far and wide to stay alive in a less energy-rich environment. Michaelian argues that his proposal cuts to the heart of the source of evolution – the second law, a physical imperative. It cuts through the Gordian knot of biological relativity. It gets around the problem of extending the ecosystem to include more and more of its environment until it becomes an organism of sample size one where Darwinian evolution becomes nonsense. It’s an intriguing argument.
A linchpin of the argument is the chemical evolution of pigment molecules that absorb well in the UV-C range eventually transforming into the “broadband pigment world of today”. A specific detailed example looks at the oligomerization of HCN into adenine (C5H5N5) and relies on physics-based arguments about the dissipative process after a UV-C photon is absorbed. In particular, it hinges on the photoexcited pigment rapidly reach the conical intersection that shunts it towards a particular product. There is some hand-waving about how this opens up producing a broader spectrum of molecules capable of absorbing a larger range of wavelengths in the uv-vis range. Analogies are made to how thermal convection cells arise as forces come into “balance”. Stationary states, autocatalytic cycles, and other such features are invoked. And finally, once the ozone layer built up, access to UV-C is now much reduced and therefore we’re unlikely to see life originate again from scratch on our planet. (Also, the heterotrophs will chomp up anything they can!)
The final kicker? If UV-C is crucial to the origin and evolution of carbon-based life, then you’re unlikely to see life evolve on systems powered by M-type red dwarf stars. That’s not good news for astrobiologists who have become increasingly interested in such systems as providing suitable cradles for life. UV-C, primarily thought of as destroyer now also takes on the role of creator – Brahma and Shiva, two-in-one, with Vishnu in between as preserver while the photon flux from our sun lasts.
The bottom-line of how the second law and chemistry intersect? In Michaelian’s words (from a different article): “All material will dissipatively structure, depending on the strength of the atomic bonding and appropriate wavelength region.” Funny how a first-day class exercise of connecting bond energies to photon energies might turn out to be the foundation of everything we see in our solar system be it on our living planet or our seemingly dead neighbors. I haven’t yet wrapped my head around all of this. In the meantime, I’ll just keep on being a gardener and cultivator of my students’ understanding of chemistry and its wonders. And I won’t look at a plant in the same way again!
