Photosynthesis is as biological as you get, but it relies on some serious quantum physics
Quantum theory doesn't just apply to physics - it's behind natural things like photosynthesis, homing pigeons and possibly consciousness itself.
Because of the way we are taught science, it is tempting to divide the subject up into tight compartments. Physics is about how stuff behaves, while biology explains the living side of nature. (As someone with a physics background, I might cruelly say that chemistry is the clean-up operation for the bits in between that neither of the other subjects wants.) But these labels and divisions are arbitrary and human-imposed. Quantum theory has no intention of staying confined in the box labelled physics.
Indeed, we are discovering an increasing range of quantum processes — like quantum tunnelling and entanglement — cropping up in nature, where they might not have been expected before.
One of the most dramatic and important biological processes that is likely to involve high-level quantum effects is photosynthesis, the process by which plants convert light into energy.
Photosynthesis: let there be (coherent) light
Any interaction between light and matter is quantum mechanical, just as anything involving an atom or electron is, but recent studies of photosynthesis have shown that quantum physics probably has a more functional role.
The physics and chemistry involved in photosynthesis is convoluted, with a whole chain of reactions taking place. First the light bumps up the energy levels of electrons in special coloured molecules like the green chlorophyll in a plant. This energy is converted to chemical form by the photosynthetic reaction centre, which produces oxygen and incorporates carbon into the plant.
One of the steps of this intricate process is the fastest known chemical reaction in existence, taking place in a trillionth of a second. The oddities of the quantum world come into play in the energy's journey from that first excitation of an electron in chlorophyll to its arrival in the reaction centre, where it gets to work converting carbon dioxide to sugars (and releasing some oxygen in the bargain).
The way the energy passes from molecule to molecule on the way in is a result of quantum particles behaving like waves. The energised wave of the first excited electron extends into the next molecule, passing on the excitation, and so on. What's more, these waves don't seem to take a random drunkard's walk, but rather they overlap, coming into coherence — the state where the waves all ripple together.
This coherent behaviour had been postulated for a while, and there was some weak evidence of it existing from large plant samples, but in 2013 researchers in Spain and Glasgow discovered it at the molecular level, training lasers on single light-processing molecules to observe the detailed workings of the reaction centres that convert photons to chemical energy.
Experiments on light-harvesting purple bacteria also showed that the ability of the quantum particle to probabilistically explore all routes and find the best path meant that the connections change as parts of the organism move, constantly tuning the process, meaning that the conversion can reach levels of around 90 per cent efficiency, far higher than a solar cell (and possibly with implications for photovoltaic cell development in the future).
The pigeon's spinning, entangled compass
A rather less certain but fascinating possibility is that a quantum effect is behind one of the marvels of nature — the way that birds like homing pigeons can navigate, apparently by picking up the Earth's magnetic field, using a built-in compass. This mysterious ability has been linked to magnetic particles in their beaks, but there is also evidence that may be stronger that the process is triggered by light hitting the retina of the bird's eye. (In fact three mechanisms have been proposed, and it is entirely possible pigeons use some combination of them.)
When light hits the receptor in the bird's eye, it is used to split a molecule to form two free radicals. These are very reactive molecules that have an unpaired electron (it's free radicals that are restrained by antioxidants from causing cell damage). These electrons can act as tiny magnetic compasses, with their quantum property called spin influenced by a magnetic field.
Typically one radical will be closer to the nearest atomic nucleus, and hence feels the magnetic field less than the other. This difference between the two gives the chemicals a different level of reactivity, making it possible for the bird to get some feedback from the interaction, perhaps by the synthesis of a chemical in the retina. The two unpaired electrons are created entangled, linked to each other in a quantum fashion, and this could help amplify the effect.
I think, therefore I'm quantum
The most extreme — and most contentious — overlap between quantum theory and biology is the idea that consciousness itself is a quantum phenomenon. Although there is no direct evidence to base this theory on, some have suggested that it is not possible to explain the phenomenon of the conscious mind using conventional classical physics, and that it needs quantum effects like entanglement to make it possible.
One suggestion, with the clumsy name of 'orchestrated objective reduction', comes from physicist Roger Penrose and medical doctor Stuart Hameroff. Penrose proposed that the brain is capable of computation that would be impossible using conventional mechanisms, with the probabilistic nature at the heart of quantum theory explaining this extra capability. Hameroff, an anaesthetist, suggested that the cytoskeleton, the structure that supports the neurons in the brain, and in particular microtubules (thin polymers that form part of the cytoskeleton) could act as quantum systems, where electrons tunnel between the microtubules.
The idea that consciousness involves quantum effects does not seem to stretch the bounds of probability to too great an extent, though as yet the jury is out. We just don't understand what consciousness is, or the mechanism behind it, well enough to explore how much it could depend on quantum effects.
But given that atoms and light are governed by quantum theory, and pretty well everything in our natural world is either atoms or light, it is inevitable that quantum processes will rule in nature.
