Technology & Science

Quantum physics at work in photosynthesis

University of Toronto researchers have observed quantum mechanics, an endeavour usually reserved for the high-energy physics lab, working in the biological molecules that algae use to make food out of light.

University of Toronto researchers have observed quantum mechanics, an endeavour usually reserved for the high-energy physics lab, working in the biological molecules that algae use to make food out of light.

The University of Toronto chemists used light-harvesting molecule complexes from cryptophyte algae like these. The red areas indicate where photosynthesis is taking place. Each single-celled organism is nine millionths of a metre long. ((Tihana Mirkovic/University of Toronto))
Chemists led by Greg Scholes isolated light-harvesting protein complexes from algae that can photosynthesize, just as plants do.

In a study published this week in Nature, Scholes and his colleagues found that these proteins display quantum effects, properties of energy and matter that become important at the scale of atoms.

Scholes said it's possible that there's a lot for physicists and engineers to learn about getting energy from light as efficiently as algae and plants do.

"Perhaps there are optimizations that these organisms have worked out over three billion years that help them gather light better than maybe what we can do in a plastic solar cell," he said.

The green pigment of photosynthesis, chlorophyll, is housed inside the light-harvesting protein complexes. The complexes act as antennas to gather light and channel the energy to reaction centres.

"The reaction centre is the first and most simple chemical reaction that stores the energy from the light in moving an electron of charge … exactly like the initial steps we would use in making any solar cell," said Scholes.

Each reaction centre has several light-harvesting complexes directing light energy to the centre. What's remarkable is that the antennas move the energy with almost perfect efficiency.

"To understand how the process works, really you need the quantum mechanics," said Scholes.

A computer model shows the light-harvesting protein complex, the antennas that carry light energy. ((Greg Scholes))
When the energy from the light is transferred to the electrons in the light-harvesting complex, they start to oscillate.

"You can think of that as a wave. If that wave extends to more than one molecule, then they can actually work together to capture the light, they can work together to move the light," said Scholes.

That wave can be disrupted very easily, a process called decoherence, because of the water and other molecules in the cell bumping around,

"That removes all the quantum effects and we expect that to happen very quickly," said Scholes.

Previous research found quantum effects in the molecules of photosynthesis at very low temperature, as cold as liquid nitrogen, but it wasn't clear whether this was relevant to how the physics of photosynthesis works in living plants.

"What we were able to find here is, yes, [quantum effects] really are used in biology," he said.

Laser spectroscopy

They were able to determine that quantum mechanics is at work in the molecules of photosynthesis through a process called femtosecond laser spectroscopy, which uses very short pulses of light to probe molecules.

"While three laser beams go into the sample, four come out, essentially, and that fourth one carries with it a lot of information. It tells us what the quantum system looked like as a function of time after we put light in there," said Scholes.

The light from the different molecules also creates interference patterns, like ripples of water from two stones thrown into a still pond, only instead of water waves, they're quantum waves.

"You only get that interference when there's quantum mechanical effects linking the molecules that we're hitting with the laser pulse," said Scholes. "That's essentially the smoking gun for these quantum mechanical effects."

Scholes said the next step is to plot the evolution of these quantum effects in algae to see when they originated.

"We have a whole group of these algae where, and by analyzing the genes, you can sort of go back in time and see that one of these species was the first one," he said.