Alumni Relations
Faculty Highlights
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Professor Graham Fleming |
Fleming spends much of his time using lasers to observe
super-fast events, such as the main energy transfer steps in photosynthesis,
the process of using the energy from sunlight to convert carbon dioxide
and water into oxygen and carbohydrates. “Understanding how plants convert
sunlight into usable energy is a fascinating challenge with very practical
applications,” he said.
The primary chemical step in photosynthesis takes place
inside what is called a reaction center. Here an electronically excited
chlorophyll molecule transfers an electron to a donor, setting in motion
a series of steps culminating in the electron being transferred to carbon
dioxide. The entire cycle of events can run at 200-300 times per second,
yet even on a bright day, the reaction center electron donor only absorbs
a photon of sunlight per second. So the reaction center in plants is surrounded
by 200-300 absorber molecules that make up the light harvesting system.
If this works with perfect efficiency—and it does—then the photosynthetic
system can run optimally.
“In photosynthesis, nature has achieved an energy transfer
efficiency of approximately 97 percent, and we’d like to know how this
is done,” Fleming said.
Although scientists know the steps of the pathway, actually
following a single excitation in photosynthesis is a daunting task. “With
300 identical chlorophyll molecules absorbing the photon within each photosystem
(light harvesting unit), how do you follow the energy? It is like staring
at a maze in which every side is identical,” said Fleming.
“It turns out, however, that the chlorophyll molecules
are not exactly alike, differing slightly in the electronic energy,” he
explained. As the excitation jumps from molecule to molecule, it samples
these different energies. Thus the excitation progressively forgets what
the initial energy was. This memory loss can be observed by sophisticated
laser experiments called photon echo measurements.
Humankind’s energy consumption is expected to double by
2050. It is not clear where this exra energy will come from, but what
is clear is that a better understanding of photosynthesis is likely to
be of crucial importance in meeting our energy needs.
So, how to apply the study of photosynthesis to solar
energy? It turns out that knowing the principles that plants use to convert
sunlight to energy may change the materials of solar cells. In collaboration
with chemistry professor Martin Head-Gordon, Fleming and his colleagues
have studied the design of nature vs. manmade materials. “To our surprise,
we found that nature often breaks the rules developed by chemists. In
a chloroplast, chlorophyll and carotenoid molecules are packed so closely
together that transitions and energy levels that would be ineffective,
or even inoperative, in widely-spaced chromophores (which can occur in
the lab setting), play a crucial role in transferring the excitation energy
toward the reaction centers.
“We also know that leaves are responsive, constantly regulating
photosynthesis by reacting to changes in light, even to a slight cloud
passing over the sun. We would like to understand how light harvesting
is regulated in plants,” he added.
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Graham Fleming at one of his laser tables. (photo courtesy Tiff Dressen.) |
For example, no matter how efficient the process, an electron
in chlorophyll will eventually become unpaired and transfer its energy
to the oxygen, which slowly destroys the photo-system, which is the protein-pigment
complex present within higher plant and cyanobacterial thylakoid membranes.
“Nature has never found a way to fix this over billions of years of evolution,
although when Photosystem 2 is destroyed, it lowers the efficiency of
photosynthesis. Plants regulate the efficiency of light harvesting in
Photosystem 2 to minimize the damage caused by oxygen. This process is
very poorly understood, yet is important in determining worldwide crop
yields. We have an active collaboration with Kris Niyogi [in the
plant and microbial biology department] to determine the molecular actors
in this regulation process.”
Fleming started his career as a pure physical chemist. But he always had a biological bent and looked for crossover projects.
“If you want to use spectroscopy to study biology, photosynthesis
is a great process to focus on; it’s all about excited states,” he said.
“Thirty years ago, we didn’t have the ability to measure
in a useful manner something as complex as the photosynthetic systems—
measuring the absorption spectrum didn’t yield much information. It was
clear that the things we really wanted to see were too fast, even the
pico-second [10-12—a thousand times slower than the femtosecond] timescale
was too slow to work with,” Fleming noted. “This spurred me and others
to develop more sophisticated instruments.”
British by birth, Fleming obtained his Ph.D. in physical
chemistry in 1974 at the University of London, followed by postdoctoral
stints at Caltech and the University of Melbourne before landing at the
University of Chicago. In his 18 years on the faculty there, Fleming established
himself as an expert on the application of femto-second spectroscopy to
chemical and biological processes. He has been here in the chemistry department
since 1997.
And if research and teaching aren’t enough, Fleming is
also currently directing three entities—QB3 (the California Institute
for Quantitative Biomedical Research), the Physical Biosciences Division
at LBNL, and the new Stanley Building project. “I enjoy working with the
educational issues of QB3; the research
challenges at LBNL; and keeping everything on schedule and dealing with
occupancy issues with the Stanley Building,” he said.
Fleming’s research—breaking down the events that
occur after sunlight strikes a leaf—does put a different spin on contemplating
a flower-filled field, as he well knows. He sees plants for their color
and their beauty—as well as their trillions of super-efficient energy
converters. But don’t look for him in the garden. He gets hay fever.
