After a busy last week, I now have time to resume my reading of Into the Cool while I enjoy Spring Break! This morning’s chapter was “Thermodynamics and Life”, a mere seventeen pages, and it took me an hour and a half. Not because the writing was particularly dense, but I kept stopping to wonder and ponder after reading a paragraph or two. I was particularly slow reading the four pages describing the work and thoughts of one Alfred Lotka.
Today I’ll be quoting a fair bit from Dorion Sagan and Eric Schneider, the authors of Into the Cool. What a fantastic book. I think today’s is my fourth blog post this month on some aspect of the book. Funny story. I borrowed Into the Cool from the library some ten years ago, read a little but didn’t appreciate it thinking it was fringe-ish science, and returned the book. How wrong I was. It’s a marvelous book!
Let’s look at Lotka. He is “best known for the Lotka-Volterra equations used to simulate populations of predation and prey… [which] led to population dynamics, today a subfield of ecology… But Lotka also developed such equations to model observations about life as an energy-driven autocatalytic process… Lotka’s most frequently cited work gives a nod to Boltzmann’s view that life’s evolutionary struggle is for available entropy. Lotka argues that selective advantages accrue to those living beings best able to capture and store energy.”
One aspect that has confused me thus far in my reading of non-equilibrium thermodynamics is the existence of two seemingly mutually exclusive principles: Maximum energy flux and minimum entropy production. Having been immersed in equilibrium thermodynamics (maximum entropy production) including teaching it for twenty years makes it a challenge for me to conceptualize what’s going on. Sagan and Schneider are particularly helpful in clearing up the paradox.
“Maximum power principles state that those organisms or ecosystems that can most efficiently convert energy into biomass (including seeds and spores) enjoy an evolutionary advantage over their neighbors. Individuals and populations that fail to maintain or expand their systems’ energy flow head for the exits of extinction… Lotka wrote that the energy flux through a system will maximize but only insofar as such maximization is compatible with the constraints to which the system is subject… Modern discussions of Lotka’s [most-cited work] almost never mention Lotka’s ideas on minimum entropy production.”
The key here is that power is energy flow per unit time. I’m so used to picturing energy flow down a gradient almost exclusively as gravitational potential energy and falling objects down a height, a length unit. I puzzled over this alluding to it in my most recent blog post about degrading a gradient, but I think the fog is clearing. The relationship of length, time, and amount of material flowing, across some gradient, is starting to make more sense although I haven’t quite grasped it yet. Hence, all those pauses as I was reading the chapter, and as I’m writing this post. I’ll turn to Sagan and Schneider again.
“Biological systems do attempt to capture and degrade as much high-quality energy as completely as possible; yet, at the same time, green life captures solar energy, intercepting it from falling to ground state and maximum entropy production. Life passes around that stripped-off photon, trading immediate entropy production for getting the most out of energy over time. The second law does not say that systems come to equilibrium as fast as possible. Life defers, delays the immediate fall of free energy to ground state, trapping and rerouting it. This, the essence of metabolism, allows life to preserve itself as a degrading system.”
I’d been talking about this in vaguer terms in my G-Chem classes this semester, discussing entropy in terms of heat dissipation and “quality” of energy. Photons are high-grade energy that can be used (while degraded). Heat is low-grade energy which can hardly be degraded anymore. There’s no more entropy to produce in the equilibrium state. But if energy is continuously flowing, non-equilibrium thermodynamics moves you towards steady state – it’s equilibrium-like in the sense that concentrations of intermediates are hardly changing with time, and entropy production is minimized in a sense, but energy is still flowing maximally over time. As much as the system allows. Until a dam breaks and that energy can be rerouted for increased flow. That’s where kinetics come in. What is a catalyst? It lowers the activation barrier by providing an alternate pathway. Or so my G-Chem students learn. By doing this life can wring out useful energy to do work losing free energy in tiny steps rather than blowing it all up in a single fiery explosion.
Sagan and Schneider summarize it by rewriting Lotka. “Evolution and ecosystems will… maximize, on one hand, the energy intake of organic nature from the sun, and on the other, minimize the outgo of free energy by dissipative processes in living and decaying material. The net effect is to optimize in this sense the energy flux through the system of organic matter… But when organisms through their own evolution come upon a new energy source, there may be a period of danger associated with experimentation and rapid spread. The new energy forms, useful as they are, have not yet been integrated into stable modes of survival… Having lived through the global depression, two world wars, and the state-orchestrated development and deployment of nuclear warheads, Lotka towards the end of 1945 wonders whether, with higher energy flow, humans will become even more addicted to the energy capture and degradation business. Considering luxury products he notes that the desire for such things… is not, like the biological appetite for food, in principle limited. He interprets luxury as new forms into which excess energy can flow.”
I admit to not having thought about the difference between commodities and luxuries in this way. Sagan and Schneider make connections to a related difference natural and sexual selection. They also make an argument for why we observe convergence in biological evolution – the constraints are energetic and entropic. There’s an interplay or balance between rapid spread and adapting to shifting environmental conditions. Replicate identical copies quickly when you can, but at the same time albeit at a slower pace, Diversify! Or wait out harsher conditions as a seed or spore. “Go with the flow… but if there is no flow, hunker down and wait for the next good gradient.”
The final paragraph of this chapter sums up the story well in evocative prose:
“Quickly growing systems – ones that through evolution, technology, or both, tap into previously unrecognized or untapped gradients – may spread like wildfire. But, like raging flames, they rob themselves of their own resources. Slow growers, by contrast, display an innate ingenuity; they make up in longevity and cunning what they lack in rapid gradient destruction, dissipation, and entropy production. They gratify nature not instantly, but enduringly. There are many ways to skin a cat, whether Schrodinger’s new cat of the role thermodynamics plays in living systems or Blake’s feline of energy and fearful symmetry.”
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