“The force that drives the flower drives my green age”
At the dawn of modern chemistry, in the 18th century, the prevailing view was that there was something special about living tissue compared to inert matter. In conventional accounts of the development of chemistry, this view – known as “vitalism” – is widely believed to have been definitively killed by Wöhler’s synthesis of urea from inorganic starting materials in 1828, opening the way to a purely mechanical concept of biology, full of pumps and levers.
And yet, there is something special about a swimming bacteria, a crawling amoeba, a growing plant, a muscle, a heart, a brain. We know now that what’s special about these forms of living matter isn’t some occult life-force; it’s a continuous input of free energy. These systems are driven systems, sustained far from equilibrium by this constant free energy input. In soft matter physics, we call this kind of matter active matter. This encompasses not just biological tissues, but increasingly, synthetic analogues.
Active matter is characterised by the free energy input being, in some sense, internal, rather than external. The fluid in a stirred tank or a heated pan is not at equilibrium, and this continuous free energy input can create considerable structure – for example convection cells, shear banding, or indeed, on a large scale, the wind patterns in a tropical storm. Yet we don’t refer to these systems as active matter. In a muscle, or the active gel of an amoeba’s cytoplasm, the free energy is being converted internally. Active matter is characterised by a hierarchical structure, and the free energy is deployed at the scale of the sub-units of the structure.
It’s important to stress that what we’re talking about here is free energy – that fraction of total energy that can be converted into useful work, given the requirement of the second law of thermodynamics that the total entropy of the universe can never decrease. Active systems typically operate at constant temperature, so the total energy that enters must be balanced by the energy that leaves. The inputs – in the form of light and chemicals in a high free energy state – have a lower entropy than the heat and waste products that leave the system.
Active matter exports entropy, and this allows it to generate order. This is what makes all the marvellous complexity and order of life consistent with the second law of thermodynamics.
We can see how this works at the level of the whole earth. The earth is constantly receiving energy from the sun, in the form of the high energy photons characteristic of a white-hot object with a temperature of about 6000 K. But, given that the earth is not getting (much [1]) hotter, it must be re-radiating the same amount of energy into outer space. Since the earth is much cooler than the sun, this radiation is in the form of many low energy photons, in the infra-red, which carry away much more entropy than is brought by the fewer, higher energy photons arriving in the sun. The earth exports entropy, and this allows it to generate order.
As in the macrocosm, so in the microcosm. Photosynthesising bacteria – cyanobacteria – and the chloroplasts in plants harvest high energy photons from the sun, using their free energy to split water molecules. The resulting hydrogen ions are pumped across membranes, and the free energy thus stored is used to synthesise the universal biological free energy vector ATP. Almost [2] all other organisms, directly or indirectly, exploit the free energy that cyanobacteria and plants have captured from the sun.
What do biological systems do with this constant input of free energy? It allows them to export entropy, and thus create a little oasis of order amidst the increasing disorder of the universe as a whole. We can roughly divide their entropy-defying activities into three categories:
- Construction, Assembly and Growth. The molecular components of life – proteins, lipids, nucleic acids and polysaccharides – generally have a higher free energy than their building blocks. So making the molecules of life needs to take place through the coupling of “uphill” reactions, that need an input of free energy, with the “downhill”, free energy releasing, reaction of ATP hydrolysis. Then these molecular components need to be assembled to produce the functional structures of cell biology. This usually involves the shepherding of the molecules to the right places, so that the self-assembly mechanisms of local free energy minimisation can produce functional structures.
- Motility. Only in the smallest and simplest cells is diffusion sufficient to move molecules to where they are needed, so mechanisms for active transport are a precondition for the evolution of size and complexity. At the level of whole cells, many bacteria can swim towards food sources and away from toxic chemicals. In multicellular organisms like ourselves cell motility is involved in the creation and repair of tissues, while molecular scale motors permit the muscular contractions that underlie wriggling, walking, running and swimming in animals of all sorts.
- Information processing. The human brain accounts for a disproportionate amount of the energy we use; there’s a deep relationship between information processing and entropy, which means that computation necessarily uses free energy. But an organism doesn’t need a nervous system to do information processing; the basic unit of biological computing is the molecule. Many bacteria are able to sense their environment and respond accordingly, and these kinds of capabilities underly the much greater complexity of cell-signalling in multi-cellular organisms.
Active matter, then, incorporates molecular scale components that use free energy – usually in the form of chemical fuels like ATP – to accomplish these goals. What are the physical principles that underlie how they work?
Biological active matter is soft matter, in the sense that it operates in an environment dominated by Brownian motion, and interaction energies comparable to thermal energy. Molecules are moving around by diffusion, weak interactions bring components together, and thermal energy breaks them apart.
There’s an important difference between active matter and soft matter at equilibrium, though. At equilibrium, every possible interaction can happen in reverse, with the same probability. This “principle of detailed balance” is broken in active matter. In biological systems, the origin of broken detailed balance arises because the concentration of the free energy vector ATP is clamped at a high, and out of equilibrium, value. It’s the resulting directionality of time which underlies the apparently purposeful nature of what active matter enables. It permits the construction of complex functional structures, and, by in effect rectifying Brownian motion, allows directional motion.
These are molecular machines – devices that convert chemical free energy into useful work – but they are machines that don’t depend on mechanism as we understand it macroscopically. It’s not Newton’s laws, (or, indeed, the Schrödinger equation), that governs the behaviour of these “soft machines”. Inertia is essentially negligible, there’s constant agitation from Brownian motion, and weak forces leading to components constantly sticking and unsticking to each other.
In biology, we see these principles at work in the molecular motors that make our muscles work, and in the active gels that allow amoeba to propel themselves by oozing along surfaces. We’re now starting to see synthetic examples, too, in the form of self-propelled colloid particles and synthetic molecular motors made using supramolecular chemistry.
Understanding the principles of active matter gives us a new insight into what makes living matter different. There is a difference between the matter of life and death, but we don’t need any occult vital forces to explain it. Living matter is active matter – it uses a constant supply of free energy, it constantly exports entropy, and it creates its own order. Without a constant flux of free energy, the second law of thermodynamics drives everything to equilibrium, and equilibrium is death.
[1] The fact that the atmosphere is less transparent in to outgoing low energy photons than to incoming high energy photons means that at steady state the earth is warmer than it would be if it were a pure “black body” – this is the greenhouse effect. As currently the concentration of greenhouse gases in the atmosphere is currently increasing, largely as a result of human action, the steady state temperature of the earth is increasing.
[2] A few ecosystems – notably those around deep-sea hydrothermal vents – rely on chemical sources of energy rather than the sun.
