Deciphering Nature’s Quantum Blueprint: The Mysteries of Photosynthesis Explained

Photosynthetic beings, via elaborate biochemical mechanisms, transform light energy into chemical energy that is essential for life. A current study has substantiated that a solitary photon can initiate this reaction, creating a link between quantum physics and biology. Photo acknowledgment: Jenny Nuss/Berkeley Lab

An avant-garde experiment has exposed the quantum mechanics that govern one of the most fundamental processes in nature.

Using a sophisticated ensemble of pigments enhanced with metals, proteins, enzymes, and co-enzymes, organisms capable of photosynthesis transform light energy into the chemical energy required for existence. Nature recently published a study revealing that this organic chemical conversion is receptive to the minutest quantity of light conceivable – just one photon.

This discovery reinforces our prevailing comprehension of photosynthesis and will assist in solving inquiries regarding life at the most microscopic levels, where quantum physics intersects with biology.

“A massive volume of work, both theoretical and empirical, has been pursued globally to comprehend what transpires after a photon’s absorption. But we perceived that no one was addressing the initial step. That remained a question demanding an exhaustive explanation,” stated co-lead researcher Graham Fleming, a top faculty scientist in the Biosciences Area at Lawrence Berkeley National Laboratory (Berkeley Lab), and a professor of chemistry at UC Berkeley.

In the research conducted by Fleming and co-lead author Birgitta Whaley, who is also a senior faculty scientist in the Energy Sciences Area at Berkeley Lab, along with their teams, they demonstrated that a lone photon can indeed trigger the initial stage of photosynthesis in photosynthetic purple bacteria. The similarity in the processes among all photosynthetic organisms, sharing a common evolutionary forebear, gives them confidence that the process in plants and algae operates identically. “Nature has devised a quite intelligent maneuver,” remarked Fleming.

How Living Systems Utilize Light

Based on the efficacy of photosynthesis in translating sunlight into molecules rich in energy, it has been a long-standing assumption that a single photon suffices to spark the reaction. This reaction involves photons transferring energy to electrons, which then swap places with electrons in diverse molecules, ultimately leading to the formation of sugar precursors. Though sunlight provides limited photons – with only a thousand photons arriving at a solitary chlorophyll molecule every second on a bright day – this process takes place consistently worldwide.

Nevertheless, “that presumption had never been substantiated through practical demonstration,” mentioned the study’s primary author, Quanwei Li, a joint postdoctoral researcher involved in developing novel experimental techniques with quantum light in the Fleming and Whaley teams.

Adding to the complexity, much of the research that has elucidated detailed aspects of photosynthesis’s later stages was conducted by activating photosynthetic molecules with intense, ultra-rapid laser pulses.

Co-senior author Graham Fleming, to the left, and the main author Quanwei Li near some equipment employed in their groundbreaking experiment. Photo acknowledgment: Henry Lam/Fleming Lab

“The disparity in intensity between a laser and sunlight is colossal – a focused laser beam is typically a million times more luminous than sunlight,” Li noted. Even if you can generate a weak beam with sunlight’s intensity, significant differences remain due to the light’s quantum attributes known as photon statistics. Since the absorption of the photon hasn’t been observed, the differences remain unclear, as well as the exact nature of the photon. He elaborated, “Just as understanding each particle is crucial for constructing a quantum computer, the study of living systems’ quantum attributes is vital for comprehensive comprehension and the creation of effective artificial systems producing renewable fuels.”

Originally, photosynthesis, like other chemical reactions, was understood in bulk – meaning scientists knew the aggregate inputs and outputs, and from that deduced possible interactions among individual molecules. The technology of the 1970s and 80s enabled the direct study of individual chemicals during reactions. Today, scientists are venturing into new territory, examining individual atoms and subatomic particles, thanks to even more refined technologies.

From Hypothesis to Reality

Designing a study that would allow individual photon observation required a distinct group of theorists and experimentalists, who merged cutting-edge instruments from quantum optics and biology. “This was novel for those studying photosynthesis, as they typically do not utilize these tools, and it was unprecedented for those in quantum optics because we seldom contemplate applying these techniques to intricate biological systems,” said Whaley, also a professor of chemical physics at UC Berkeley.

The researchers assembled a photon source that produced a singular pair of photons through a method called spontaneous parametric down-conversion. During each pulse, the initial photon – “the harbinger” – was detected by a highly sensitive device, which confirmed the second photon’s journey to the sample containing molecular structures extracted from photosynthetic bacteria. Another photon detector near the sample was positioned to measure the lower-energy photon emitted by the photosynthetic structure following absorption of the original pair’s second “announced” photon.

The light-absorbing structure, referred to as the LH2, has been extensively investigated. It is known that photons at an 800 nanometers (nm) wavelength are absorbed by a ring of 9 bacteriochlorophyll molecules in LH2, causing energy to be transferred to a second ring of 18 bacteriochlorophyll molecules, emitting fluorescent photons at 850 nm. Within native bacteria, the photon energy would continue to transfer to subsequent molecules until utilized to begin photosynthesis. However, in the experiment, when the LH2s were isolated from other cellular components, the detection of the 850 nm photon served as a conclusive indicator that the process had been set in motion.

“If you have only one photon, it’s extraordinarily simple to misplace it. Thus, the primary challenge in this experiment was the fundamental difficulty, which is why we utilized the harbinger photon,” Fleming explained. The scientists meticulously analyzed over 17.7 billion harbinger photon detection occurrences and 1.6 million heralded fluorescent photon detection occurrences to ensure that the observations were solely attributable to single-photon absorption, devoid of other influencing factors.

“I believe the foremost aspect is that this experiment has demonstrated the feasibility of operations with individual photons. So that’s a highly significant point,” remarked Whaley. “The subsequent query is, what more can we achieve? Our objective is to explore the energy transfer from individual photons through the photosynthetic complex at the briefest conceivable temporal and spatial scales.”

Reference: “Single-photon absorption and emission from a natural photosynthetic complex” by Quanwei Li, Kaydren Orcutt, Robert L. Cook, Javier Sabines-Chesterking, Ashley L. Tong, Gabriela S. Schlau-Cohen, Xiang Zhang, Graham R. Fleming, and K. Birgitta Whaley, 14 June 2023, Nature.
DOI: 10.1038/s41586-023-06121-5

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