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Quantum effects are necessary for the energy collection process in photosynthesis (Source: iStockphoto)

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Quantum life

Tuesday 2nd June 2015 1:03 pm

Last time, I talked about that strange counter-intuitive bit of physics called quantum mechanics.

We normally see quantum mechanics acting only on truly tiny things — smaller than an atom. Incredibly, it does have flow-on effects in the world of big things — such as USB memory sticks and the like.

But what about the big, big one — life itself?

In the late 1940s and into the 1950s, scientists began to speculate that quantum mechanics might provide a link or a bridge between non-life and life.

Erwin Schrodinger, one of the gods of quantum mechanics, published What is Life? in 1944. Other quantum heavies such as Neils Bohr, Werner Heisenberg and Eugene Wigner also pondered on this.

But to quote the physicist and astrobiologist, Paul Davies, writing in 2009:

“Life seems little short of miraculous — all those stupid atoms getting together to perform such clever tricks. … Today, we know that no special ‘life force’ is at work in biology; there is just ordinary matter doing extraordinary things, all the while obeying the familiar laws of physics.”

At this stage, we can’t prove (or disprove) that quantum mechanics was part of that big jump, from non-life to life.

But we are pretty definite that quantum mechanics is involved with a few biological processes.

Consider photosynthesis.

The word literally means using light (that’s ‘photo’) to make something (that’s ‘synthesis’). So photosynthesis usually exploits the energy in light to grab carbon dioxide from the atmosphere, and then causes a chemical reaction to make carbohydrates and oxygen. Carbohydrates and oxygen are essential if you want to eat and breathe.

The critters that do photosynthesis are plants (both in the oceans and on land), as well as algae and cyanobacteria. These guys work hard.

Worldwide, they catch about 130 terawatts of power from the Sun, which is roughly three times as much power as we humans generate. And each year, they use this power to grab about 100 billion tonnes of carbon and turn it into biomass.

But there’s still one big mystery about photosynthesis. Why is it so fiendishly efficient? Here, I’m using the word ‘efficient’ in its engineering usage, as in how much you get back, for what you put in.

Most biological and human processes run at somewhat less than 50 per cent efficiency. But in the early stage of capturing the light energy, photosynthesis runs at around 95 per cent efficiency — amazingly high.

The whole process of photosynthesis is very complex, and we still don’t fully understand it. But the process begins with collections of atoms, which are formed into tiny antennas. These antenna complexes of pigment-protein absorb the sunlight, and then very efficiently, funnel this excitation energy to a reaction centre — where the next set of chemical reactions happen. Ultimately, the process finishes with the production of carbohydrates and oxygen.

But at the very beginning, the molecules that gather up the incoming light are arranged like antennas, and actually look like rings located next to each other. When a photon of light energy lands on one of these antennas, it seems to stop being a single particle and instead turns into a smeared and spread-out wave. This wave somehow manages to ‘see’ or sample all the possible different energy pathways, all at the same time — and pick the most efficient one.

Now because this is all happening at warm temperatures, the atoms and molecules will jiggle around a lot. So over time, the most efficient pathway to carry the light energy will change — sometimes this pathway, and then several tenths of a billionth of a second later, a different pathway. Somehow, the light energy nearly always picks the most efficient pathway.

In 2013, in photosynthesis, scientists managed to measure the fleeting existence (that’s ‘fleeting’ in our terms) of unexpectedly long-lived quantum coherence states — about four tenths of a billionth of a second.

But it now seems that quantum effects are necessary, not just for the energy collection process in photosynthesis, but also in enzymes. There are many thousands of different enzymes in your body. They’re essential for life. Enzymes are chemicals that accelerate chemical reactions by factors of trillions of times faster, or even more. It’s long been a mystery as to how they run so quickly.

But now it seems as though enzymes use quantum tunneling, which previously has been seen in radioactive decay, and it’s also seen in how our Sun burns hydrogen to make helium and keep us warm. In some biochemical reactions, it seems that an enzyme can encourage protons and electrons to vanish from one location on the enzyme’s surface, and instantly, with zero delay, to rematerialise in a different location. It’s a kind of quantum teleportation.

If we could capture and control such quantum effects, we may well one day be able to say: “Beam me up, Scotty”.


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This blog first appeared on Dr Karl's Great Moments in Science

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  1. joseph kelly says:

    Dr Karl,
    love your efforts. I am an chemical engineer; can you put some links to the works you are talking about? It is all very interesting, and (joke) as Monty Burns said in The Simpsons, I ask you to “continue the research”.
    Joe K

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