When sunlight shining on a leaf changes
rapidly, plants must protect themselves from the ensuing sudden surges of solar
energy. To cope with these changes, photosynthetic organisms — from plants to
bacteria — have developed numerous tactics. Scientists have been unable,
however, to identify the underlying design principle.
An
international team of scientists, led by physicist Nathaniel M. Gabor at the
University of California, Riverside, has now constructed a model that
reproduces a general feature of photosynthetic light harvesting, observed
across many photosynthetic organisms.
Light
harvesting is the collection of solar energy by protein-bound chlorophyll
molecules. In photosynthesis — the process by which green plants and some other
organisms use sunlight to synthesize foods from carbon dioxide and water —
light energy harvesting begins with sunlight absorption.
The
researchers’ model borrows ideas from the science of complex networks, a field
of study that explores efficient operation in cellphone networks, brains, and
the power grid. The model describes a simple network that is able to input
light of two different colors, yet output a steady rate of solar power. This
unusual choice of only two inputs has remarkable consequences.
“Our model shows that by absorbing only very specific
colors of light, photosynthetic organisms may automatically protect themselves
against sudden changes — or ‘noise’ — in solar energy, resulting in remarkably
efficient power conversion,” said Gabor, an associate professor of physics and
astronomy, who led the study appearing today in the journal Science.
“Green plants appear green and purple bacteria appear purple because only
specific regions of the spectrum from which they absorb are suited for
protection against rapidly changing solar energy.”
Gabor
first began thinking about photosynthesis research more than a decade ago, when
he was a doctoral student at Cornell University. He wondered why plants
rejected green light, the most intense solar light. Over the years, he worked
with physicists and biologists worldwide to learn more about statistical
methods and the quantum biology of photosynthesis.
Richard
Cogdell, a botanist at the University of Glasgow in the United Kingdom and a
coauthor on the research paper, encouraged Gabor to extend the model to include
a wider range of photosynthetic organisms that grow in environments where the
incident solar spectrum is very different.
“Excitingly,
we were then able to show that the model worked in other photosynthetic
organisms besides green plants, and that the model identified a general and
fundamental property of photosynthetic light harvesting,” he said. “Our study
shows how, by choosing where you absorb solar energy in relation to the
incident solar spectrum, you can minimize the noise on the output — information
that can be used to enhance the performance of solar cells.”
Coauthor
Rienk van Grondelle, an influential experimental physicist at Vrije
Universiteit Amsterdam in the Netherlands who works on the primary physical
processes of photosynthesis, said the team found the absorption spectra of
certain photosynthetic systems select certain spectral excitation regions that
cancel the noise and maximize the energy stored.
“This
very simple design principle could also be applied in the design of human-made
solar cells,” said van Grondelle, who has vast experience with photosynthetic
light harvesting.
Gabor
explained that plants and other photosynthetic organisms have a wide variety of
tactics to prevent damage due to overexposure to the sun, ranging from
molecular mechanisms of energy release to physical movement of the leaf to
track the sun. Plants have even developed effective protection against UV
light, just as in sunscreen.
“In
the complex process of photosynthesis, it is clear that protecting the organism
from overexposure is the driving factor in successful energy production, and
this is the inspiration we used to develop our model,” he said. “Our model
incorporates relatively simple physics, yet it is consistent with a vast set of
observations in biology. This is remarkably rare. If our model holds up to
continued experiments, we may find even more agreement between theory and
observations, giving rich insight into the inner workings of nature.”
To
construct the model, Gabor and his colleagues applied straightforward physics
of networks to the complex details of biology, and were able to make clear,
quantitative, and generic statements about highly diverse photosynthetic
organisms.
“Our
model is the first hypothesis-driven explanation for why plants are green, and
we give a roadmap to test the model through more detailed experiments,” Gabor
said.
Photosynthesis
may be thought of as a kitchen sink, Gabor added, where a faucet flows water in
and a drain allows the water to flow out. If the flow into the sink is much
bigger than the outward flow, the sink overflows and the water spills all over
the floor.
“In
photosynthesis, if the flow of solar power into the light harvesting network is
significantly larger than the flow out, the photosynthetic network must adapt
to reduce the sudden over-flow of energy,” he said. “When the network fails to
manage these fluctuations, the organism attempts to expel the extra energy. In
doing so, the organism undergoes oxidative stress, which damages cells.”
The
researchers were surprised by how general and simple their model is.
“Nature
will always surprise you,” Gabor said. “Something that seems so complicated and
complex might operate based on a few basic rules. We applied the model to
organisms in different photosynthetic niches and continue to reproduce accurate
absorption spectra. In biology, there are exceptions to every rule, so much so
that finding a rule is usually very difficult. Surprisingly, we seem to have
found one of the rules of photosynthetic life.”
Gabor
noted that over the last several decades, photosynthesis research has focused
mainly on the structure and function of the microscopic components of the
photosynthetic process.
“Biologists
know well that biological systems are not generally finely tuned given the fact
that organisms have little control over their external conditions,” he said.
“This contradiction has so far been unaddressed because no model exists that
connects microscopic processes with macroscopic properties. Our work represents
the first quantitative physical model that tackles this contradiction.”
Next,
supported by several recent grants, the researchers will design a novel
microscopy technique to test their ideas and advance the technology of
photo-biology experiments using quantum optics tools.
“There’s
a lot out there to understand about nature, and it only looks more beautiful as
we unravel its mysteries,” Gabor said.
Journal article: https://science.sciencemag.org/content/368/6498/1490
Source: https://myfusimotors.com/2020/06/27/why-are-plants-green/
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