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

by Johnjoe McFadden

If Cairns’s results were right, then cells must also be capable of reversing the flow of genetic information, allowing the environmen...

Yet his results could not be accounted for by classical molecular biology: mutations should occur at the same rate irrespective of whether lactose was present or not.

However, if, as Löwdin argued, genes are essentially quantum information systems, then the presence of lactose would potentially constitute a quantum measurement as it would reveal whether or not the cell’s DNA had mutated: a quantum-level event dependent on the positions of single protons. Could quantum measurement account for the difference in mutation rates that Cairns observed?

This is taking place all the time: for example, reading of DNA by the protein synthesis machinery forces the proton to “make up its mind” on which side of the bond it is sitting—either in the normal (no growth) or in the tautomeric (growth) position; and mostly it will be found in the normal position.

But remember that the coin represents a quantum particle, a proton in the DNA strand; so even after measurement it is free to slip back into the quantum world to reestablish the original quantum superposition.

So after our coin has been tossed and landed on heads, it will be tossed again, and again and again.

Eventually, it will land on tails. In this state, the DNA may again be copied, but now it will make the active enzyme. In the absence of lactose, this will still not make any difference because, without lactos...

A return to the quantum superposition state will no longer be possible. The system will be irreversibly captured into the classical world as a mutant cell.

We can conceive of this as—only in the presence of lactose—taking those rare coins that fall on tails out of the box and placing them in another box, marked “mutants.”

Conventional explanations of adaptive mutations started to appear, which claimed to account for the raised mutation rates by a combination of several processes: a general increase in the mutation rate of all genes; cell death and release of the dead cells’ mutated DNA; and, finally, selective uptake and amplification of the mutated lactose gene by surviving cells that managed to incorporate it into their genome.

If quantum tunneling is involved in speeding up an enzyme reaction, then replacing a hydrogen nucleus (a single proton) with a deuterium nucleus (consisting of a proton and a neutron) should slow the reaction since quantum tunneling will be highly sensitive to doubling the mass of the particle trying to tunnel.

This then corresponds to what in quantum physics is called a double potential energy well and maps to the situation of a coding proton in the DNA strand, with the left-hand well in the diagram corresponding to the normal position of the proton, whereas the right-hand well corresponds to the rarer tautomeric position.

What the theoretical models do show, however, is that the action of the surrounding environment within the cell actively assists, rather than hinders, the tunneling process.

that stretch back to the dawn of life on our planet. Life could not have survived and evolved on earth if it hadn’t, billions of years ago, “discovered” the trick of encoding information in the quantum realm.*11

But what is consciousness?

it is aware and has experiences, concepts that just don’t seem to exist in the material world. And somehow this ethereal stuff of awareness and experience—our conscious mind—drives the material stuff of our brain to cause our actions (or at least, that’s our impression).

This puzzle, variously referred to as the mind–body problem or the hard problem of consciousness, is surely the deepest mystery of our entire existence.

Our aim is to try to pin down where consciousness makes its input in this causal chain so that we can then investigate whether quantum mechanics might have played a role in that event.

Myosin is an enzyme that uses chemical energy to power the contraction of muscles, essentially by causing the fibers to slide over one another.

Muscle cells have more sodium ions on the outside of their membrane than the inside, giving rise to a voltage difference across their membrane, a bit like a tiny battery. However, there are pores in these membranes called ion channels, which, if opened, allow the sodium ions into the cell. It was this electrical discharging process that triggered the artist’s muscle contraction.

The nerve cell, just like a muscle cell, normally has more positively charged sodium ions outside than inside. This difference is maintained by pumps that push positively charged sodium ions out of the cell through the nerve cell membrane. The excess of external positive charges provides a voltage difference across the cell membrane of about one-hundredth of a volt. Although this doesn’t sound like much, you have to remember that cell membranes are just a few nanometers thick, so it is a voltage across a very short distance. This means that we have an electrical gradient (what voltage actually is) across the cell membrane of a million volts per meter.

The head-end of the artist’s motor nerve, the body of the nerve cell, is connected to a cluster of structures called synapses (figure 8.2), which are kind of nerve-to-nerve junction boxes. Upstream nerves release neurotransmitter molecules into these junctions much as neurotransmitters are released at the nerve–muscle junction; this triggers the opening of ion channels in the membrane surrounding the nerve cell body, thereby allowing positively charged ions to rush inside, causing its voltage to drop sharply.

Most voltage drops caused by the opening of a handful of ion channels in a synapse will have little or no effect. But if lots of neurotransmitter arrives, then lots of ion channels will flip open. The ensuing rush of positive ions into the cell causes its membrane voltage to dip below a critical threshold of about −0.04 volts. When this happens, another set of nerve ion channels come into play. These are voltage-gated ion channels, which means they are sensitive not to neurotransmitters but to the voltage difference across the membrane.

The ensuing voltage drop caused more voltage-gated ion channels to pop open, allowing more ions to rush inside the cell, causing more of the membrane to short-circuit. The long cable of the nerve, the axon, is lined with these voltage-gated channels, so once the short-circuiting was kicked off at the cell body, it triggered a kind of domino effect of membrane short-circuiting—the action potential—that quickly traveled down the nerve until it reached the nerve ending (figure 8.3).

For a start, the current, the movement of charges, is not down the length of nerve cables in the direction of the nerve signal, but perpendicular to the direction of the action potential: from outside in, through those ion channels in the cell membrane.

Also, immediately after the action potential is initiated by the opening of the first ion channels, they are slammed shut again and the ion pumps get to work on reestablishing the original battery voltage across the membrane.

So another way of viewing the nerve signal is as a wave of opening and closing of membrane ion doors that travels from the cell body to the ...

The nerve–nerve junctions for most motor nerve cells are located in the spinal cord, where they receive neurotransmitter signals from hundreds or even thousands of upstream nerves (figure 8.1). Some upstream nerves release neurotransmitters into the junction box (synapse) that open ion channels in the cell body to increase the likelihood of firing up the motor nerve, whereas others tend to close them.

In this way the cell body of each nerve cell seems to be acting like the logic gate of a computer, generating an output—whether or not it fires—based on its inputs. So, if the neuron is like a logic gate, then the brain, made up of billions of neurons, might be thought of as some kind of computer; or at least, this is the assumption of most cognitive neuroscientists who subscribe to what is called the computational theory of mind.

Between sensory inputs and motor output is the brain’s neural network that performed the computations dictating the decision to generate, or not to generate, the precise motor output needed to draw the outline of a bison.

From the perspective of quantum computing, entanglement can be visualized as each qubit sphere being connected by elastic strings*4 to every other qubit (figure 8.4c). Now, let us imagine that we rotate just one of the spheres. Without entanglement, the rotation will not affect neighboring qubits. But if our qubit is entangled with other qubits, then the rotation changes the tensions in all the connecting strings between these connected qubits. The computational resource of all those entanglement strings increases exponentially with the number of qubits, which means that it increases very rapidly indeed.

But, and this is a very big but, for the quantum computer to work, the qubits must interact only with one another to perform calculations (via their invisible entangled “strings”). This means they must be completely isolated from their environment. The problem is that any interaction with the outside world causes the qubits to become entangled with their environment, which we can envisage as the formation of many more strings, all pulling on the qubits from different directions, competing with the strings between the qubits and therefore interfering with the calculation they are performing. This, essentially, is the process of decoherence (figure 8.5).

A quite different approach has been pioneered by the Canadian company D-Wave, which encodes qubits in the motion of electrons in electrical circuits.

In 2007, the company claimed to have developed the first commercial 16-qubit quantum computer, able to solve a Sudoku puzzle and other pattern-matching and optimization problems.

Consider a familiar logical system, such as language, which is capable of reasoning through statements such as “All men are mortal. Socrates is a man” to conclude that “Socrates is mortal.” It’s easy to see, and easy to formally prove, that the last statement follows logically from the first two, given a simple set of algebraic rules (if A = B and B = C then A = C). But Gödel showed that any logical system complex enough to prove mathematical theorems has a fundamental limitation: application of their rules can generate statements that are true, but these statements cannot be proved with the same tools that were used to generate them in the first place.

Consciousness, he argues, requires a quantum computer.

structures called microtubules that are found in neurons are the qubits of quantum brains.10

There is much more to the Penrose–Hameroff consciousness theory, including, possibly even more controversially, a proposed involvement of gravity.*6

The channels are only about one-billionth of a meter long (1.2 nanometers) and less than half that wide, so the ions have to pass through them in single file. Yet they do so at an extraordinarily high rate of about a hundred million per second.

perform a quantum mechanical simulation of an ion passing through a voltage-gated ion channel and discovered that the ion is delocalized (spread out) when it travels through the channel: more of a coherent wave than a particle. Also, this ion wave oscillates at very high frequencies and transfers energy to the surrounding protein by a kind of resonance process so that the channel effectively acts as an ion refrigerator that reduces the kinetic energy of the ion by about half. This effective cooling of the ion helps to maintain its delocalized quantum state by keeping decoherence at bay and thereby promotes rapid quantum transport through the channel.

It also contributes to selectivity, since the degree of refrigeration will be very different if potassium is replaced with sodium: constructive interference can promote potassium ion transport while destructive interference can inhibit sodium ion transport. The team concluded that quantum coherence plays an “indispensable” role in the conduction of ions through nerve ion channels, and is thereby an essential part of our thinking process.13

have recently seized upon the idea that shifting consciousness from the discrete particles of matter in the brain to the joined-up EM field could potentially solve the binding problem and provide a seat for consciousness.

In physics, the term “field” has the same essential meaning, but usually refers to energy fields that are able to move objects.

Gravitational fields move anything that has mass, and electric or magnetic fields move electrically charged or magnetic particles such as the ions in nerve ion channels.

In fact, what the field seems to do is to coordinate nerve firing: that is, bring lots of neurons into synchrony so that they all fire together.

The findings suggest that the brain’s own EM field, generated by nerve firing, also influences nerve firing, providing a kind of self-referencing loop that many theorists argue is an essential component of consciousness.17

The scheme outlined above, involving quantum coherent ion channels and EM fields, is certainly speculative, but it does at least provide a plausible link between the quantum and classical realms in the brain.

Haldane and Oparin proposed that the emergence of this primordial replicator was the key event that led to the origin of life as we know it.

Even the simplest life is, much like this chemical gunk, extraordinarily complex. Unlike gunk, however, it is also highly organized. The problem with using gunk as the starting material for generating organized life is that the random thermodynamic forces that were available in the primordial earth—the billiard-ball-like molecular motions that we discussed in chapter 2—tend to destroy order rather than create it. You throw a chicken into the pot, heat it up and stir it, and make chicken soup. No one has ever poured a can of soup into a pot and made a chicken.

The probability of random chemical processes coming together to generate life, he said, was as likely as a tornado blowing through a junkyard and assembling a jumbo jet by chance.