The key to life on Earth, photosynthesis, begins on the quantum level with a single photon

Shedding light on the minutiae of life’s processes, a recent study published in Nature on June 14th has brought us one step closer to fully understanding photosynthesis. It is the process that converts sunlight into the chemical energy needed to support life.

The research confirms that photosynthesis can be triggered by the absorption of a single photon, the smallest possible unit of light. This groundbreaking discovery adds to our comprehension of the intersect between quantum physics and biology, which is where life happens at its smallest scale.

“Worldwide, an enormous amount of work has been conducted, both theoretically and experimentally, to understand what follows after a photon is absorbed. However, we noted that no one was addressing the first step, which still needed detailed answering,” Graham Fleming, the study’s co-lead author, a senior faculty scientist at Lawrence Berkeley National Laboratory’s Biosciences Area, and a professor of chemistry at UC Berkeley, stated.

Fleming, alongside co-lead author Birgitta Whaley, a senior faculty scientist in the Energy Sciences Area at Berkeley Lab, and their research groups made the pioneering discovery. They demonstrated that in photosynthetic purple bacteria, photosynthesis’ initial step can be set off by a single photon.

How life uses light

“Nature invented a very clever trick,” Fleming commented, referring to the universality of this phenomenon. Their confidence in the findings is based on the fact that all photosynthetic organisms share an evolutionary ancestor and use similar processes. This suggests that the mechanism of photosynthesis in plants and algae is likely the same.

Scientific consensus has long assumed that only a single photon is required to initiate photosynthesis, due to its impressive efficiency at transforming sunlight into energy-rich molecules. In this process, absorbed photons pass energy onto electrons.

These electrons then swap places with electrons in different molecules, ultimately creating the precursor ingredients required for sugar production. This happens even though the sun only provides a limited number of photons – merely a thousand photons reach a single chlorophyll molecule each second on a sunny day.

No studies had been done to prove this single photon theory

However, Quanwei Li, the study’s first author and a joint postdoctoral researcher in the Fleming and Whaley groups, pointed out that, “no one had ever backed up that assumption with a demonstration.”

To further complicate matters, many studies that had elucidated specific details about the later stages of photosynthesis were performed using ultra-fast laser pulses. Li noted, “There’s a huge difference in intensity between a laser and sunlight – a typical focused laser beam is a million times brighter than sunlight.”

The differences are not just in intensity, but also in the quantum properties of light, known as photon statistics. As Li explained, the nature of the photon being absorbed is yet unknown.

“But just like you need to understand each particle to build a quantum computer, we need to study the quantum properties of living systems to truly understand them, and to make efficient artificial systems that generate renewable fuels.”

Scientific understanding of chemical reactions, including photosynthesis, started with a bulk view, focusing on the overall inputs and outputs. From there, possible interactions between individual molecules were inferred.

In the 1970s and 80s, technological advancements allowed the direct study of individual chemicals during reactions. Currently, scientists are starting to explore reactions on an even smaller scale, looking at individual atoms and subatomic particles.

Designing an experiment to create facts from assumptions

The authors of the new study designed an experiment that allowed the observation of individual photons, bringing together a team of theorists and experimentalists from the fields of quantum optics and biology.

Whaley, also a professor of chemical physics at UC Berkeley, said, “It was new for people who study photosynthesis, because they don’t normally use these tools, and it was new for people in quantum optics because we don’t normally think about applying these techniques to complex biological systems.”

How the single photon study was conducted

The team set up a photon source that generates a single pair of photons through a process known as spontaneous parametric down-conversion. A highly sensitive detector observed the first photon—dubbed “the herald”—confirming the second photon was en route to the assembled sample of light-absorbing molecular structures. These were derived from photosynthetic bacteria.

Another photon detector was set up near the sample to measure the lower-energy photon emitted by the photosynthetic structure after absorbing the second photon.

The light-absorbing structure used in the experiment, referred to as LH2, has been widely studied before. In LH2, photons with a wavelength of 800 nanometers (nm) get absorbed by a ring of nine bacteriochlorophyll molecules. This causes energy to be passed to a second ring of 18 bacteriochlorophyll molecules, which can then emit fluorescent photons at 850 nm.

A clear sign emerges

Under normal conditions, in the native bacteria, the energy from the photons would continue to be transferred to subsequent molecules until it is used to initiate the chemistry of photosynthesis. However, in the experiment, when the LH2s were separated from the other cellular machinery, the detection of the 850 nm photon served as a clear sign that the process had been activated.

Dealing with individual photons proved challenging, as Fleming noted, “If you’ve only got one photon, it’s awfully easy to lose it. So that was the fundamental difficulty in this experiment and that’s why we use the herald photon.”

The team rigorously analyzed over 17.7 billion herald photon detection events and 1.6 million heralded fluorescent photon detection events to ensure that the observations could only be attributed to single-photon absorption, and that no other factors were influencing the results.

“This experiment has shown that you can actually do things with individual photons. So that’s a very, very important point,” said Whaley. Looking towards the future, she added, “The next thing is, what else can we do? Our goal is to study the energy transfer from individual photons through the photosynthetic complex at the shortest possible temporal and spatial scales.”

This study offers a significant leap forward in our understanding of photosynthesis and points towards the fascinating potential of studying life at the quantum level. As research continues to push the boundaries of our knowledge, the future promises exciting revelations in the realm where quantum physics and biology converge.

More about photosynthesis

Photosynthesis is a fundamental biochemical process in which plants, algae, and certain bacteria convert light energy, usually from the sun, into chemical energy stored in the form of glucose (a simple sugar) and other energy-rich carbohydrates. This process is vital for life on Earth as it is the primary source of all organic matter that organisms need for nutrition, either directly or indirectly.

Here’s a basic overview of how photosynthesis works:

Light-Dependent Reactions

The first stage of photosynthesis, also known as the “photo” part of photosynthesis, takes place in the thylakoid membranes of the chloroplasts in plant cells. Here, sunlight (consisting of packets of energy called photons) is absorbed by a complex of proteins and pigments (including chlorophyll) called the photosystem. Two main photosystems are involved: Photosystem II and Photosystem I.

When photons strike the chlorophyll molecules in Photosystem II, it excites electrons, raising them to a higher energy state. These “excited” electrons are then passed along an electron transport chain, and in the process, they lose some of their energy. This energy is used to pump hydrogen ions (H+) into the thylakoid space, building up a concentration gradient.

The high-energy electrons are then passed to Photosystem I, where they are re-energized by light before being transferred to a carrier molecule called NADP+, forming NADPH. Meanwhile, water molecules are split (photolysis) in the process to replace the lost electrons in Photosystem II, releasing oxygen as a by-product.

The concentration gradient of H+ across the thylakoid membrane leads to a flow of H+ ions back across the membrane, through a protein called ATP synthase. This flow drives the conversion of ADP into ATP, storing energy in the form of this “energy currency” molecule.

Light-Independent Reactions (Calvin Cycle)

The second stage, often referred to as the “synthesis” part, occurs in the stroma of the chloroplasts, and does not require direct sunlight. This stage uses the ATP and NADPH produced in the light-dependent reactions to convert carbon dioxide (CO2) into glucose.

The process involves a cycle of reactions known as the Calvin Cycle. CO2 is captured from the environment and attached to a 5-carbon sugar called ribulose-1,5-bisphosphate (RuBP) by an enzyme called RuBisCO. This results in an unstable 6-carbon compound, which quickly breaks down into two 3-carbon molecules.

ATP and NADPH power the reactions

These 3-carbon molecules then go through a series of reactions powered by ATP and NADPH, resulting in the production of glucose and other carbohydrates. The cycle also regenerates RuBP to continue the process.

Photosynthesis plays a key role in the carbon cycle, helping to regulate levels of carbon dioxide in the Earth’s atmosphere. It is also the process that produces the oxygen in the air that animals and humans breathe. Furthermore, the principle of photosynthesis has inspired a range of scientific innovations, from solar power technologies to attempts to create artificial photosynthesis for energy production.