Entropy and Life

Entropy is a measure of how many configurations could yield the same macrostate, and thus how probable the macrostate is. It can be a measure of information, or a measure of disorder in a physical system. But what about the entropy of biological systems?

Relatively few configurations yield life, compared to the many that don’t. Life is highly ordered, so living organisms should have much lower entropy than their non-living constituents. In fact, using energy to create and maintain order is one of the key signatures of life! One implication of life having low entropy is that life is improbable, which so far seems to be true based on the limited observations we have of other planets. But another implication is that living things act to reduce entropy locally, in the organism, there must be a corresponding increase in entropy somewhere else to offset that reduction. This is required because of the second law of thermodynamics, which says that by far the most statistically probable outcome is an entropy increase. But the second law applies to ‘closed systems’, which means a system that cannot exchange heat or energy with its surroundings. An organism that can interact with its surroundings can expel entropy via heat, to gain local order and reduce local entropy. Global disorder still increases, but for that organism, the ability to locally reduce entropy is literally a matter of life and death.

An obvious example of this principle is humans. Our human bodies are very highly ordered compared to inanimate things like air and water. Even compared to dirt, which has a whole ecosystem of microbiota and larger organisms like worms, a human represents many times more order. But this doesn’t contradict the second law, because the way we maintain life is to take in food and expel very disordered waste products. Humans can extract the chemical energy in the food and use it to maintain or decrease local entropy levels, and thus stay alive. Obviously other animals do this too, though they may eat different things than we do or digest them in different ways. And actually, the plants that we and other animals eat have done something similar, except that instead of getting chemical energy from combustion they are able to extract it directly from the sun’s light. Plants maintain their low entropy by releasing heat and high-entropy waste products, and anything that eats plants (or that eats something that eats plants) is converting solar energy into local order as well as expelled heat.

If you’re really feeling clever, you might ask, what about the planet Earth? If we’re receiving all this sunlight, and making life from it, shouldn’t there be a corresponding buildup of entropy on the planet, in the form of waste heat or some other disordering? Isn’t the Earth a closed system, isolated in space, whose order is constantly increasing?

But if a closed system is one which exchanges no heat with its surroundings, then the Earth doesn’t qualify because it is obviously exchanging heat with the Sun! The Earth receives a massive number of photons from the sun, which is where plants, and by extension the rest of us, get energy to create order. But in addition, the Earth is also radiating energy and heat into space, as all objects do. The incident energy from the sun is directional, high-energy, and highly ordered, but the energy the Earth radiates into space is in all directions, low-energy, and very disordered. That’s where the excess entropy is going!

Thus, life on our dear planet is not a violation of the second law of thermodynamics at all, because living organisms and even huge ecosystems are not closed systems. What’s more, the creation of order from chaos actually requires a net increase in entropy: it requires a reconfiguration of atoms and microstates, and the most likely outcome of any reconfiguration is an increase in entropy.  Many of chemical reactions necessary for life are entropically driven, where the outcome has many more available states than the inputs so the reaction is statistically favored to occur. Organisms that do work to create order must also create entropy, and the organisms most likely to survive are often those with the most clever control of entropy generation. So the proliferation of life is not threatened by entropy, as in the popular conception, but actually depends on entropy generation!