Why are plants green?

Date:
June 25, 2020
Source:
University of California - Riverside
Summary:
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 have developed
numerous tactics.

Scientists have been unable, however, to identify the underlying
design principle. A physicist has now constructed a model that
reproduces a general feature of photosynthetic light harvesting,
observed across many photosynthetic organisms.



FULL STORY ========================================================================== [Leaves in sunlight | Credit: (c) Olivier Le Moal / stock.adobe.com]
Leaves in sunlight (stock image).

Credit: (c) Olivier Le Moal / stock.adobe.com [Leaves in sunlight |
Credit: (c) Olivier Le Moal / stock.adobe.com] Leaves in sunlight
(stock image).

Credit: (c) Olivier Le Moal / stock.adobe.com Close 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 journalScience. "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.


========================================================================== Story Source: Materials provided by
University_of_California_-_Riverside. Original written by Iqbal
Pittalwala. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Trevor B. Arp et al. Quieting a noisy antenna reproduces
photosynthetic
light-harvesting spectra. Science, 2020 DOI: 10.1126/science.aba6630 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200625144900.htm

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