At one level, photosynthesis looks very simple. Plants, green algae and some bacteria take in sunlight and carbon dioxide, and turn them into energy. What niggles in the back of biologists minds, though, is that photosynthetic organisms make the process look just a little bit too easy.
It’s one part of photosynthesis in particular that puzzles scientists. A photon – a particle of light – after a journey of billions of kilometres hurtling through space, collides with an electron in a leaf outside your window. The electron, given a serious kick by this energy boost, starts to bounce around, a little like a pinball. It makes its way through a tiny part of the leaf’s cell, and passes on its extra energy to a molecule that can act as an energy currency to fuel the plant.
The trouble is, this tiny pinball machine works suspiciously well. Classical physics suggests the excited electron should take a certain amount of time to career around inside the photosynthetic machinery in the cell before emerging on the other side. In reality, the electron makes the journey far more quickly.
What’s more, the excited electron barely loses any energy at all in the process. Classical physics would predict some wastage of energy in the noisy business of being batted around the molecular pinball machine. The process is too fast, too smooth and too efficient. It just seems too good to be true.
Then, in 2007, photosynthesis researchers began to see the light. Scientists spotted signs of quantum effects in the molecular centres for photosynthesis. Tell-tale signs in the way the electrons were behaving opened the door to the idea that quantum effects could even be playing an important biological role.
This could be part of the answer to how the excited electrons pass through the photosynthetic pinball machine so quickly and efficiently. One quantum effect is the ability to exist in many places at the same time – a property known as quantum superposition. Using this property, the electron could potentially explore many routes around the biological pinball machine at once. In this way it could almost instantly select the shortest, most efficient route, involving the least amount of bouncing about.
