The Unexplained Efficiency of Photosynthesis

Plants convert sunlight into energy with 99% efficiency, but biologists are puzzled because classical mechanics suggests this should be far less. A quantum phenomenon known as superposition might be the solution to the riddle.

The Happy Neuron
Dec 17, 2020 · 4 min read
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The common explanation of photosynthesis is that plants take sunlight, carbon dioxide, and water and turn them into sugar, oxygen, and energy. This is the formula: 6CO 2 + 6H 2O + sunlight → C 6H 12O 6 (glucose) + 6O 2 + energy. All of this happens inside a plant cell’s chloroplasts, which contain layers of thylakoids, which in turn contain pigment called chlorophyll.

Chlorophyll is where the whole process begins. These green pigments are mostly made up of long chains of carbon and oxygen, with some areas composed of a magnesium atom surrounded by nitrogen and carbon. This configuration results in the magnesium atom having a single valence electron, and when a photon from the Sun enters the chlorophyll, it knocks it loose. When this happens, the outer shell of the magnesium atom develops a positively charged gap due to the loss of a negative election. This positively charged gap and the negatively charged electron can form a pair called an exciton. Excitons store energy much like a battery, as they have both a positive and negative pole. However, to release this energy, the excitons need to be transferred to a reaction center that can split the pair. Once this energy is released, it powers the rest of the photosynthesis.

Here’s the problem: the exciton needs to randomly travel from one chlorophyll to the next to reach the reaction center, like a blind frog hopping on lily pads. The chances of the exciton reaching the reaction center is a small fraction of the 99% that actually happens.

The quantum world is counter-intuitive. It operates under a different set of rules than everyday objects, as quantum particles behave as both a particle and a wave. In our world, the pencil on the desk or my car driving on the highway is in one place and one place only. In the quantum world, particles do not exist in a particular location until they are forced to choose one. Before being forced to choose, they exist as a wave of probability.

This is demonstrated by the double slit experiment. Imagine quantum particles being fired at a barrier with two vertical slits. Behind this barrier is a wall that serves as a detector. What pattern do you think will be produced on the back wall? If the quantum particles are indeed particles, then the pattern will be two vertical bands, as some will collide with the barrier, while others will pass through one of the slits and hit the back wall. However, this is not what happens. Instead, an interference pattern is produced, which can only mean that they behaved as waves.

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The crests of one wave cancel out the trough of the other, while two crests add together. This produces an alternating pattern of light and dark areas on the back wall. (Creative Commons License)

It gets even weirder. When the quantum particles were detected as they went through a slit, they were forced to stop existing as a wave of probability and pick a location. Therefore, the pattern on the back wall changed to two bands, as would be expected from particles.

Furthermore, when only one quantum particle was fired the outcome was the same. When it wasn’t detected going through the slits and it was allowed to stay as a wave of probability, it went through both slits, interfered with itself, and produced an interference pattern on the back wall. When it was detected going through the slits, it was forced to choose a location, passed through only one slit, and struck the back wall at only one point.

Returning to the problem of an exciton getting to the reaction center, the only way for it to do so at a rate of 99% is if it behaves as a wave. If this is true, then the exciton exists as a wave of probability that spreads across all chloroplasts at once.

You’re right if you think this sounds far-fetched. This idea was originally suggested in the 1930s and was not taken seriously by physicists until the 2000s. In the first experiment, researchers from University of California at Berkeley created excitons in chlorophyll with laser bursts. After crunching the data, they found what were looking for: the tell-tale inference pattern produced by interacting waves. This suggested that the exciton was indeed traveling as a wave.

After numerous other experiments, not all physicists are convinced, though. At the top of their list of problems is quantum decoherence. The wave nature of a quantum particle is difficult to maintain, as interactions with pretty much anything forces it to collapse into an individual particle. When physicists measure such phenomena they need to work in carefully controlled environments to keep the wave in a state of coherence and to avoid it collapsing into a state of decoherence. A plant is likely not able to create these conditions.

Other physicists believe the inference pattern is produced by vibrating molecules or even lab equipment.

Whether or not photosynthesis relies on superposition is still an open question. However, the more we learn, the more it seems processes we take for granted are not possible without quantum phenomena. For example, the Sun isn’t hot enough to create light and only does so with quantum tunneling, which also helps explain how DNA mutates and enzymes function. Even birds can’t navigate without quantum entanglement. All of these are just a handful of other examples emerging from the rising field of quantum biology, so it may not be as crazy as it seems.

Originally published at http://thehappyneuron.com on December 17, 2020.

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