Life on the Edge: The Coming of Age of Quantum Biology

by Johnjoe McFadden

So, the quantum wave function calculates the probability of detecting the atom at a specific location, were we to carry out a measurement of its position at that time.

If we place a detector on one of the slits, then we should expect equal probabilities: 50 percent of the time we will detect the atom at the left slit and 50 percent of time we will detect it at the right slit. But—and this is the important bit—if we don’t try to detect the atom at the level of the first screen then the wave function flows through both slits without collapsing. Thereafter, in quantum terms we can talk of a wave function describing a single atom that is in a superposition: of its being in two places at the same time, corresponding to its wave function going through both the left and right slits simultaneously.

On the other side of the slits, each separated piece of the wave function, one from the left and one from the right slit, spreads out again and both form sets of mathematical ripples that overlap, at some points reinforcing and at other points canceling each other’s amplitude. The combined effect is that the wave function now has the pattern characteristic of other wave phenomena, such as light. But bear in mind that this now complicated wave function is still describing only a single atom.

Remember that there are regions of the screen that atoms, fired one at a time, could reach with just one slit open but that were no longer reachable when both slits are open.

This only makes sense if each atom released from the atom gun is described by a wave function that can explore both paths simultaneously. The combined wave function with its regions of constructive and destructive interference cancels out the probability of the atom being found in some positions on the screen that it would reach if only one slit were open.

But more deeply, you can think of each atom in your body as being observed, or measured, by all the other atoms around it, so that any delicate quantum properties it might have are very quickly destroyed.

A measuring device that detects an electron has to be part of this macroworld. But how and why and when this measurement process takes place was never clarified by the founders of quantum mechanics.

In this way we can think of decoherence as the means by which all the material surrounding any given atom, say—what is referred to as its environment—is constantly measuring that atom and forcing it to behave like a classical particle.

Thus, “decoherence” is the physical process whereby coherence is lost and the quantum becomes classical.

The magnesium atom’s outermost electron is only loosely bound to the rest of the atom and can be knocked into the surrounding carbon cage by absorption of a photon of solar energy to leave a gap in what is now a positively charged atom.

This gap, or electron hole, can be thought of in a rather abstract way as a “thing” in itself: a positively charged hole. The idea is that we regard the rest of the magnesium atom as remaining neutral while we have created, through the absorption of the photon, a system consisting of the escaped negative electron and the positive hole it has left behind. This binary system is called an exciton (see figure 4.6) and can be thought of as a tiny battery with positive and negative poles capable of storing energy for later use.

Molecules neighboring the one that has absorbed the photon can themselves become excited, effectively inheriting the energy of the initially excited electron, which is then transferred to their own magnesium atom’s electron.

Until recently, it was thought that this energy-hopping from one chlorophyll molecule to another was haphazard, essentially adopting the search strategy of last resort, known as a random walk. This is sometimes referred to as a “drunken walk” because it resembles the path taken by an intoxicated drinker exiting a bar, wandering this way and that until he eventually finds his way home. But random walks are not a very efficient means of getting anywhere: if the drunk’s home is far away, he may well wake up the following morning in a bush on the other side of town. An object engaged in a random walk will tend to move away from its starting point by a distance proportional to the square root of the time taken. If in one minute a drunk has advanced by one meter, then after four minutes he will have advanced by two meters and after nine minutes, only three meters. Given this sluggish progress, it is not surprising that animals and microbes seldom use a random walk to find food or prey, only resorting to the strategy if no other options are available. Drop an ant onto unfamiliar ground and as soon as it encounters a scent, it will abandon a random walk and follow its nose.

In fact, the transfer of captured photon energy from a chlorophyll antenna molecule to the reaction center boasts the highest efficiency of any known natural or artificial reaction: close to 100 percent.

Under optimal conditions, nearly every energy parcel absorbed by a chlorophyll molecule makes it to the reaction center.

These oscillations are akin to the interference pattern of light and dark fringes in the two-slit experiment; or the quantum equivalent of the pulsating sound beats heard when tuning a musical instrument.

This “quantum beat” showed that the exciton wasn’t taking a single route through the chlorophyll maze but was instead following multiple routes simultaneously (figure 4.8). These alternative routes act a bit like the pulsed notes of the almost in-tune guitar: they generate beats when they are nearly the same length.

Our drunken walker will soon find himself overtaken, as the watery wave advances through the streets at a rate simply proportional to the time taken, not its square root.

Not only that, but because, like the superposed atom in the two-slit experiment, it travels by all possible routes simultaneously,

LHC2 is present in all higher plants and contains 50 percent of all the chlorophyll on the planet.

RuBisCO

The essential distinction lies in where we, and they, get the fundamental building blocks of life.

Both need carbon, but plants obtain it from air whereas we get it from organic sources, such as the plants themselves. Both need electrons to build biomolecules: we burn organic molecules to capture their electrons, while plants use light to burn water to capture its electrons. And both need energy: we scavenge it from the high-energy electrons that we obtain from our food by running them down respiratory energy hillsides; plants capture the energy of solar photons.

The discovery of quantum coherence in warm, wet, turbulent systems such as plants and microbes has come as a huge shock to quantum physicists, and a great deal of research is now focused on working out precisely how living systems protect, and utilize, their delicate quantum coherent states.

Consider again those quantum beats that Greg Engel first saw in his FMO complex data, which show that particles move within living cells as waves.

The

familiar statistical laws and Newtonian laws are, ultimately, quantum laws that have been filtered through a decoherence lens that screens out the weird stuff (which is why quantum phenomena appear weird to us).

Life seems to bridge the quantum and classical worlds, perched on the quantum edge.

But when its mate was eaten by a passing eel, the ovaries that had lain dormant in its body for several years matured, its testes ceased to function and the male clownfish became the queen female ready to mate with the next male in the pecking order.

The unusual capability of the dominant male to change sex on the death of the queen fish, a capability known as protandrous hermaphroditism, may be an adaptation to life in the dangerous reef, as it allows the colony to survive the demise of the single reproductive female without ever having to leave the protection of the host anemone.

Sharks, two-thirds of whose brains is devoted to olfaction, can famously smell a drop of blood from more than a kilometer away.

The olfactory sense is perhaps even more remarkable on land because the volume of the atmosphere, in which odorants are diluted, is even vaster than that of the ocean.

Dogs can “see” our olfactory fingerprint as easily as we can see the color of a person’s shirt.

A bear’s sense of smell is over seven times as sensitive as even a bloodhound’s; and it can smell a carcass 20 kilometers away.

The sense of smell is so important to animal survival that behavioral responses to odors appear to be hard-wired in a number of species.

Experiments with Orkney Island voles demonstrated that they avoided traps baited with the secretions of predatory stoats, even though stoats have been absent from the island for five thousand years!

The initiating event in the detection of an odor, such as that of an orange, a coral reef, a mate, a predator or prey is now understood to be the binding of a single molecule of odorant to a single olfactory receptor on the surface of the brush end of one of those broom-like olfactory neurons.

For example, many molecules that have very different shapes, such as the compounds in figure 5.2 a–d, smell the same—in this case, they all smell musky.*7 Conversely, compounds that have very similar structures (such as compounds e and f in the figure) often have very different smells—in this case compound f smells like urine whereas e has no smell at all.

But Dyson noticed that odor groups (chemicals that smell the same) were often composed of compounds that incorporated the same chemical groups,

Dyson went on to propose that what the nose detects is not the shape of an entire molecule but rather a different physical feature, namely the frequency at which the molecular bonds between its atoms vibrate.

In inelastic scattering, photons similarly lose energy to the molecular bonds they bump into, causing them to vibrate; the scattered light therefore emerges with less energy. Reducing the energy of light lowers its frequency and shifts its color toward the blue end of the spectrum, providing Raman with his “wonderful blue opalescence.”

But whereas the physics of how the eye detects the vibration frequency of light and the ear records the vibration frequency of air are pretty well understood, no one had any idea, until recently, how the nose might detect the frequency of a molecular vibration.

inelastic electron tunneling spectroscopy (IETS). In IETS, two metal plates are placed very close to each other, separated by a tiny gap. If a voltage is applied between the plates, electrons will gather on one plate, making it negatively charged (the donor), and will experience an attractive force from the other, positively charged, plate (the acceptor). Considered classically, the electrons lack the energy to jump across the insulating gap between the plates; but electrons are quantum objects and, if the gap is small enough, they can quantum tunnel across from donor to acceptor. This process is called elastic tunneling because the electrons do not gain or lose energy in the process.

If a chemical is placed in the gap between the plates, then an electron can tunnel across so long as it is able to donate its excess energy to the chemical—which it can do so long as the molecules in the gap have bonds capable of vibrating at just the right frequency, corresponding to that of the dumped energy.

Having passed on their excess energy in this way, these “inelastically” tunneling electrons arrive on the acceptor plate with slightly lower energy; so by analyzing the energy differences between electrons leaving the donor site and arriving at the acceptor site, inelastic electron tunneling spectroscopy probes the nature of a chemical’s molecular bonds.

In IETS, the electron only pops off the donor site if the chemical between the two plates has a bond similarly tuned to just the right frequency for it to make the jump. In effect, the tunneling electron loses energy by plucking a molecular bond on its quantum journey across the plates.

However, if the receptor captures an odorant molecule that possesses a bond tuned to just the right vibrational frequency, then the electron can pop from donor to acceptor via tunneling while simultaneously transferring just the right amount of energy to the odorant, effectively plucking one of its molecular bonds.

Humans, fruit flies, anemonefish and a host of other animals are probably harnessing the ability of an electron to vanish from one point in space and instantly materialize in another so that they can capture that “message from a material reality” and find food, or a mate—or their way home.

This magnetized bubble, the “magnetosphere,” protects all life on earth, because without it the solar wind—the stream of energetic particles emitted from the sun—would have long ago eroded our atmosphere.

The precise origin of this magnetism is complicated, but it is thought to be due to what is known as a geo-dynamo effect, whereby electric currents are generated by the circulation of liquid metals in the earth’s core, which in turn generates a magnetic field.