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I read an interesting article in the journal Nature1 recently. The subject was a new field of study that I frankly have not heard about before, called “active matter.” It arose in physics during the mid-1990s and is heavily mathematical, but it has more to do with biology and phenomena like the movement of chemicals inside cells, birds in flocks, and fish in schools, than with anything we associate with math and physics.
You might think that cells are so small that chemicals inside them could move from one place to another by simple diffusion, the way salt or sugar dissolved in water finds its way to equilibrium—that is, to distribute the molecules equally throughout the dissolving medium. So, release a few molecules of protein or enzyme at one place in the cell, and they will eventually migrate all through the watery substance of the cytoplasm until they are evenly distributed. But this process is really too slow for a healthy cellular response.
What the physicists discovered is that relatively simple molecular structures—various kinds of microtubules that are found inside cells, certain proteins that can move along those tubules, and the cell’s energy molecule, adenosine triphosphate (ATP)—can combine to create a coordinated kind of movement: the proteins travel up and down the tubes, propelled by the energy in the ATP molecules. It makes no difference whether these substances are naturally derived or synthesized in a lab. They are certainly not living matter. And yet they move.
In earlier times, when natural scientists were just beginning to notice such things, to talk about living beings as compared to inanimate objects like rocks and dead bodies, people would discuss and ponder the “life force.” When scientists started experimenting with electricity in the 18th century, it became notional that this animating principle was somehow associated with electric discharges or current. Certainly, the Frankenstein story played up to this, when the macabre doctor brought the dead tissue of his monster alive with bolts of lighting. But now we talk about life more as an “organizing principle”—my favorite term is “temporary reversal of entropy”2—rather than any innate force in itself.
But it turns out that the idea of discarding the idea of a motivating force was wrong, or at least premature. There is a force to life, a power behind the animation, and that is the molecule adenosine triphosphate. Adenosine is a molecular structure called a purine—the amino acid known as adenine attached to a ribose sugar ring—and it also serves as one of the four nucleotide bases, A, in the DNA coding system. Phosphate is a phosphorus atom bound to four oxygen atoms by the sharing or trading of electrons. It is chemically reactive and forms the structure that connects the ribose rings in the DNA molecule. When you link three phosphate groups in a tail headed by a stable structure like adenosine, interesting things can happen.
Those three phosphates in adenosine triphosphate are somewhat unstable, and the last one in the string of three can easily become detached. When the bonds holding that third phosphate group are hydrolyzed—that is, when they react with water molecules, which are plentiful in the cell’s cytoplasm, to neutralize the bonding force—the break releases a bit of kinetic energy. This is the boost that pushes the proteins up or down in the microtubules—a simple chemical reaction.
On the glass slide where active matter researchers mix these molecules and watch what happens under the microscope, the adenosine triphosphate is quickly converted to adenosine diphosphate (ADP), all the available energy is eventually used up, and the reactions stop. On the other hand, in a living cell, which uses ATP as both a transport mechanism and as fuel for other chemical reactions, organelles called mitochondria work to convert the energy value in whatever food the organism has consumed into reassembling depleted ADP into the more potent ATP. Mitochondria are like small, single-purpose cells that have been adopted by and adapted to the host cell as an energy source. This adoption and differentiation was part of the great revolution that turned bacteria and other single-celled life forms into the more complex and more potent multi-celled animals and plants upon which evolution has continued to build to the present day.3
Aside from pushing proteins along microtubules, experiments with active matter at high densities can make bunched filaments of the protein actin dance and swirl. They can also make polymer spheres containing a particle of the mineral hematite clump and unclump spontaneously when placed in a hydrogen peroxide solution and exposed to blue light. All of these are bits of inanimate matter being pushed by chemical reactions in their environment.
The article in Nature goes on to link the study of active matter to the movements of living organisms: the swirling patterns of flocks of birds or schools of fish in motion. I’m not sure that I see the connection. According the the article, density is the secret. When a group of birds or fish is scattered and their density is low, they can move randomly. When they gather more closely and density increases, their movement becomes coordinated. Sure, I can understand that. Living organisms which have various sensing apparatus, a central nervous system, and the ability to move in a controlled fashion will make conscious decisions about their own direction rather than randomly bump into one another. This is why people crossing the main concourse of Grand Central Station at non-rush hours may move in all directions, but when the hall is packed with commuters, they tend to move in ribbons and streams and still they almost never bump into one another.
In the same way, densely packed fish in schools or birds flying in tightly grouped flocks must sense those other individuals around them—the fish feeling increased water pressure and microcurrents from those swimming ahead and on either side, the birds feeing air pressure and currents—and move accordingly. In the flocks and schools, motion and direction are a group choice, a collective decision, in the same way that waltz partners swirling around a dance floor, theater patrons heading toward the exits, hunting packs pursuing a prey animal, or people running from a fire do not need a leader or any outside direction to guide their motion. They move as a single mass because they have a common purpose.
I would imagine that the filaments and spheres packed on a slide are also moving under some contact—or contact avoidance—pressure related to water or solution density, reactive energy, or some other inanimate force. But here the analogy of active particles with conscious, living beings would seem to break down. Proteins driven up a tubule by ATP do not actually achieve consensus: they are still inanimate. A group of hounds chasing a fox, with a group of Englishmen and -women on horseback jostling along behind them have chosen their sport. They can agree to follow the dogs when they chase a fox but call them off if the dogs start chasing a rabbit or a deer. Sensation, awareness, and control really do matter at the sophisticated level of human beings.
I’m not sure how far the study of active matter will go, aside from chemical parlor tricks on microscope slides. One might extend the clumping, grouping, and swirling effects to larger-scale phenomena like lava streams, mud flows, and river formation. But in the end, physics is still one area of study and biology another—except perhaps at the most basic, chemical level of what happens inside a cell.
But there, in the murky distinction between the diffusions of salts and sugars in water and the reactions of proteins and fibers driven by the breaking of phosphate bonds, we come upon the first prime mover, the first stirrings of that old “life force,” and the start of the mystery of temporarily reversing entropy.
1. Gabriel Popkin, The Physics of Life, Nature, volume 259, January 7, 2016.
2. According to the Second Law of Thermodynamics, the amount of order in a closed system is always decreasing. You might temporarily gather energy together to create an ordered state, but in the release of that energy, some of it will always be lost—not destroyed, mind you, simply rendered unavailable—to the system. For example, the fuel you burn in a car’s engine creates turning force on the crankshaft, but it also creates heat, radiant energy, which does not go into the work of the car and is inevitably lost into the atmosphere. The original energy in that fuel was released millions of years ago by fusion reactions in the sun, a fraction of which was captured through photosynthesis by the algae in sea water, a fraction of which were consumed as food by plankton and converted into lipids, a fraction of which were trapped in the seabed when the plankton died and slowly turned into crude oil by pressure and geothermal heat. That might look like a gathering and concentrating of the sun’s nuclear energy, but the algae, the plankton, and the seabed are capturing and conserving only a tiny fraction of the 130 watts per square meter that fall on the Earth’s surface every minute. The rest is lost to the system, and that loss of energy, that state of increasing disorder, is called “entropy.”
3. See The First Great Revolution from December 6, 2015.