It’s spring in the Northern Hemisphere and all around us is greenery. Outside our window, trees are full of leaves that are like little factories, collecting sunlight and converting it into food. We know this basic transaction takes place, but how does photosynthesis actually work?
Plants utilize quantum mechanical processes during photosynthesis. In an attempt to understand how plants do this, scientists at the University of Chicago recently modeled the workings of a leaf at a molecular level. They were astonished by what they saw: it turned out that the plant behaves like a strange fifth state of matter known as a Bose-Einstein condensate. What’s even stranger is that these condensates are usually found at temperatures close to absolute zero. The fact that they exist all around us on a normal, warm spring day is truly astonishing.
Low energy state
The three most common states of matter are solid, liquid, and gas. When pressure or heat is applied or removed, matter changes between these states. Plasma is often said to be the fourth state of matter. In plasma, atoms break down into a soup of positively charged ions and negatively charged electrons. This usually occurs when matter is superheated. For example, the Sun is mostly a big sphere of superhot plasma.
If matter can be superheated, it can also be supercooled, resulting in particles in a very low energy state. Understanding what happens next requires some knowledge of particle physics.
There are two main types of particles, bosons and fermions, and the difference between them is a property called spin. Spin is a strange quantum mechanical property related to the particle’s angular momentum. Bosons are particles with integer spin (0, 1, 2, etc.), while fermions have half-integer spin (1/2, 3/2, etc.). This property is described by the spin statistics theorem, which means that if you swap two bosons, the same wave function will be preserved. The same cannot be said for fermions.
In a Bose-Einstein condensate, the bosons in a substance have such low energy that they all occupy the same state and act as a single particle. This allows us to see quantum properties on a macroscale. A Bose-Einstein condensate was first created in a laboratory in 1995 at a temperature of just 170 nanokelvin.
Quantum Photosynthesis
Now let’s look at what happens in a typical leaf during photosynthesis.
Plants need three basic ingredients to make their own food: carbon dioxide, water, and light. A pigment called chlorophyll absorbs energy from red and blue wavelengths of light. Chlorophyll reflects other wavelengths of light, which is why plants appear green.
At the molecular level, something even more interesting happens: The absorbed light excites electrons in a chromophore, the part of a molecule that determines whether it reflects or absorbs light. This starts a series of chain reactions that ultimately produce sugars for the plant. Researchers at the University of Chicago used computer modeling to look at what happens in green sulfur bacteria, a type of photosynthetic microorganism.
Light excites electrons. Together, the electron and the void it leaves behind (a hole) act as a boson. This electron-hole pair is called an exciton. The exciton can then move on and deliver the energy to another location, where sugars are produced for living organisms.
“The chromophore passes energy in the form of excitons to the reaction center, where it can be used, similar to a group of people passing a ball towards a goal,” Anna Schouten, lead author of the study, explained to Big Think.
The scientists discovered that the orbitals of the excitons in the localized region are similar to those found in exciton condensates – Bose-Einstein condensates made of excitons. The problem with exciton condensates is that electrons and ions tend to recombine quickly. When this happens, the excitons disappear, often before the condensate can form.
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These condensates are extremely difficult to create in the lab, but they existed before scientists’ eyes in messy organic matter at room temperature. By forming a condensate, the excitons formed a single quantum state. In essence, the excitons behaved like a single particle. This created a superfluid (a fluid with zero viscosity and zero friction) that allowed energy to flow freely between the chromophores.
The results of their research PRX Energy.
A sticky situation
Excitons typically decay quickly and are no longer able to transfer energy once they have decayed. To extend their lifetime, excitons typically need to be kept very cold. In fact, exciton condensates have never been observed at temperatures above -173 degrees Celsius (100 Kelvin). It is therefore quite surprising to see such behavior at room temperature in a messy real-world system.
So what’s going on here? Just another way that nature continually surprises us.
“Photosynthesis works at room temperature because nature needs it to work at room temperature to survive, so the process evolved to make that happen,” Schouten says.
In the future, room-temperature Bose-Einstein condensates may have practical applications: because they act as single atoms, they may provide insight into quantum properties that are difficult to observe at the atomic level, and they could also be used in gyroscopes, atomic lasers, high-precision sensors of time, gravity, and magnetism, as well as higher levels of energy efficiency and transfer.
