How Plants Manage Light: New Insights Into Nature’s Oxygen-Making Machinery

A set of breakthroughs from scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) is offering a new understanding of how energy flows through one of nature’s most important molecular machines: the photosystem II supercomplex (PSII).

These massive protein complexes in plants, algae, and cyanobacteria are responsible for a critical reaction – splitting water using sunlight to produce breathable oxygen. Most PSII supercomplexes exist in a paired form, with two reaction centers and a network of light-absorbing chlorophyll proteins arranged around them. These paired assemblies work together to collect sunlight and drive photosynthesis. Through a series of four recent papers, the research team uncovered a surprisingly efficient and flexible energy transport system at work in these complexes.

“Photosystem II doesn’t just collect sunlight – it makes incredibly smart decisions about what to do with that energy. What we’ve uncovered is how nature balances two contradictory goals: getting the most from every photon while also protecting itself from too much light.”

— Graham Fleming

Rather than funneling energy directly to its reaction centers (as some bacterial systems do), PSII uses a flat, sprawling energy landscape that lets light energy explore multiple routes before locking into the photosynthesis process. The result is a dynamic system that can both harvest sunlight efficiently and protect itself from damage – qualities scientists would love to replicate in synthetic light-harvesting devices, such as photocatalysts and fuel-producing artificial photosynthesis systems. There’s potential for agriculture, too. By understanding and possibly mimicking PSII’s balancing act, scientists could engineer crops that recover faster, boosting yields.

“Photosystem II doesn’t just collect sunlight – it makes incredibly smart decisions about what to do with that energy,” said Graham Fleming, senior faculty scientist in Berkeley Lab’s Molecular Biophysics and Integrated Bioimaging Division and corresponding author on the four research papers. “What we’ve uncovered is how nature balances two contradictory goals: getting the most from every photon while also protecting itself from too much light.”

Nature’s Oxygen Engine

PSII supercomplexes are located in the chloroplasts of plant and algal cells and are absolutely essential to life on Earth. They’re the only system in nature that performs the tricky task of splitting water into oxygen and hydrogen using sunlight – a foundational step for both breathable oxygen and the food chain.

“It is the most essential piece of chemistry on the planet as far as anything that breathes is concerned,” Fleming said.

At the center of the PSII complex are two reaction hubs where this chemistry happens. Surrounding these is an array of hundreds of chlorophyll molecules that absorb light and pass the energy inward. In bacteria, this kind of setup tends to resemble a funnel – energy flows downhill straight to the center. But in PSII, the layout is unusually flat, not what one would expect from a system trying to be efficient.

Yet it works incredibly well. And that paradox is what Fleming’s team set out to understand.

New Tools, Deeper Insights

To explore how energy moves through this complex system, Fleming’s team at Berkeley Lab utilized recent innovations in spectroscopy and advanced modeling techniques. One of their key tools, described in an Aug. 2024 study in the Journal of Physical Chemistry B, is two-dimensional electronic-vibrational spectroscopy – a method developed by Fleming that provides high-resolution insights into where energy is going in a sea of nearly identical chlorophyll molecules.

“You’re looking at something with essentially 200 identical molecules – how on Earth are you going to tell where the excitation is?” Fleming said. “This new method gives us a lot more resolution.”

In an Oct. 2024 study in Nature Communications, the Berkeley Lab research team worked with colleagues at the University of Cambridge in England to build simulations of PSII’s energy landscape. These simulations helped explain why the system’s flat design actually supports fast energy spreading and offers built-in safeguards to prevent overheating.

The researchers reported in a March 2025 PNAS paper that energy in PSII doesn’t take a direct route to the reaction centers. In fact, it sometimes flows away from them before circling back. This strange behavior revealed a two-phase process: energy first roams randomly – driven by entropy – and only later moves in a more targeted direction.

This “wandering phase” turns out to be essential. It gives the system time to assess light intensity and avoid creating harmful byproducts. If energy builds up too quickly in one spot, it can turn into a kind of chemical stress that damages the cell. But by letting the energy roam, the system gains flexibility – it can let energy disperse when needed to prevent overheating in a particular location or funnel it to a reaction center when conditions are right.

“You put the energy close to where you think it wants to be used up, and it actually goes away,” Fleming said. “If it were wine, it would run back up the funnel.”

The final piece of the puzzle, reported in an April 2025 study in The Journal of Chemical Physics, was a first-ever measurement of the distance energy can travel within PSII – known as the exciton diffusion length. Using a technique called exciton–exciton annihilation, the team found that energy can travel across the entire complex and even between its two reaction centers, granting the system even more adaptability.

A Blueprint For Better Tech

All of this has major implications for building smarter light-harvesting systems.

Right now, most artificial photosynthetic systems are designed for raw efficiency. But they’re not great at protecting themselves or adapting to changing conditions. Fleming’s work shows that nature figured out how to do both – by giving energy room to explore before telling it where to go.

“It’s like having a really smart thermostat,” Fleming said. “You don’t just turn the heat on or off – you adjust based on what’s needed moment to moment.”

Insights gained from the research could ultimately lead to synthetic systems that use similar principles – spreading out energy, responding to intensity or damage, and then zeroing in to do useful work. Think molecular devices that re-route energy to avoid failure points.

There’s potential for agriculture too. Plants naturally shift into a protective mode when light levels spike, but they’re slow to switch back. By understanding and possibly mimicking PSII’s balancing act, scientists could engineer crops that recover faster – something Fleming’s collaborators have already shown can boost soybean yields by up to 25%.

What’s Next?

The researchers still have questions. One important protein, PsbS, is known to trigger PSII’s photoprotective response, but its precise role and location are still unclear. Fleming hopes future work – possibly using cryo-electron microscopy on larger assemblies called megacomplexes – will fill in the gaps.

“We’re not just trying to copy nature – we’re trying to understand the design principles that make it work so well,” Fleming said. “Once we do that, we can build systems that are not only efficient but smart – able to adapt, respond, and thrive under real-world conditions the way plants do.”

This research was supported by the U.S. Department of Energy’s Office of Science.

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Lawrence Berkeley National Laboratory (Berkeley Lab) is committed to groundbreaking research focused on discovery science and solutions for abundant and reliable energy supplies. The lab’s expertise spans materials, chemistry, physics, biology, earth and environmental science, mathematics, and computing. Researchers from around the world rely on the lab’s world-class scientific facilities for their own pioneering research. Founded in 1931 on the belief that the biggest problems are best addressed by teams, Berkeley Lab and its scientists have been recognized with 16 Nobel Prizes. Berkeley Lab is a multiprogram national laboratory managed by the University of California for the U.S. Department of Energy’s Office of Science.

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