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phenomena

By Susan Gaidos

May 9th, 2009; Vol.175 #10 (p. 26)

PHOTOSYNTHESIS GOES QUANTUMHi-Res (Click twice to zoom in) | Plants and
certain bacteria capture energy from light and use it to make food through
the process of photosynthesis. The initial stage of the process is remarkably
efficient — so efficient, in fact, that scientists are looking to quantum
phenomena to explain what’s happening.Illustration by Nicolle Rager Fuller

Until a century or so ago, nobody had any idea that there even was such a
thing as quantum physics. But while humans operated for millennia in quantum
darkness, it seems that plants, bacteria and birds may have been in the know
all along.

Quantum effects, human researchers have only recently discovered, may explain
how the first steps of photosynthesis convert light to chemical energy with
such high efficiency. Other studies suggest that quantum tricks may enable
migratory birds to navigate using Earth’s magnetic field lines.

Through studies like these, scientists are beginning to understand how
quantum mechanics — weirdness supposedly confined to the realm of subatomic
physics — affects everyday biology.

On one level, it seems perfectly natural that quantum mechanics would serve a
function at life’s foundation. After all, quantum principles define the
properties of atoms, from which living matter is made. And yet the quantum
rules, which allow particles like electrons to exist in two places at once
and sometimes behave like waves rather than particles, seem an unlikely
driver of life’s tightly regulated processes. Bizarre quantum properties are
supposed to govern objects such as individual atoms, not great clumps of
matter like redwoods or robins.

Now, with growing evidence that quantum weirdness indeed exists in biological
systems, scientists are looking for ways to tell how, or even if, nature
exploits these effects to confer an advantage.

“We can’t tell nature to ignore quantum mechanics, so we might need to
measure it and see what happens,” says Graham Fleming, a chemist at the
University of California, Berkeley, who coauthored a paper in the 2009 Annual
Review of Physical Chemistry outlining recent studies showing quantum effects
in photosynthesis.

Understanding how natural systems use quantum effects to their advantage
might help researchers find ways to control, and ultimately harness, such
processes. By copying the quantum tricks used by plants, for example,
researchers might be able to develop new technologies, such as more efficient
solar cells.

Making waves in the lab

Photosynthesis is carried out by molecular machinery embedded in membranes in
the interior of plant cells and some bacteria. Like all chemical reactions,
it relies on the action of electrons.

In green plants, light particles are absorbed by pigment molecules —
primarily chlorophyll — found in leaves. An incoming light particle, or
photon, boosts an electron in the chlorophyll into a mobile state. Once
excited, the electron is quickly shuttled from the chlorophyll to a nearby
“acceptor” molecule, setting off a series of electron transfers. Moving from
one molecule to another, the electron ultimately reaches the “reaction
center,” where the energy is converted into a form the cell can use to make
carbohydrates.

It’s these initial, near instantaneous energy transfers that are so
remarkably efficient — scientists estimate that more than 95 percent of the
energy in the light hitting a leaf reaches the photosynthesis reaction
center. Although each of the biochemical steps that follow adds a loss in
energy efficiency, the first steps in the process closely approach the ideal
of one photon leading to one electron transfer.

Previous models of photosynthesis assumed that the light energy stored in
excited electrons found its way to the reaction center via random hops,
particles moving in a step-by-step manner to successively lower energy
levels. But some scientists seeking to explain plants’ superefficient
energetics have considered the notion that plants may have a way to exploit
the quantum behavior of electrons.

In the odd quantum world, particles can behave like waves. Rather than simply
moving from one chlorophyll to another, electrons can exist as whirling
clouds of energy, jostling back and forth between the molecules. In this
wavelike state, the electrons become connected, or coupled, and act in a
concerted manner so the excitation is actually “sloshing around” between the
molecules, Fleming says.

Scientists theorized that this and other quantum effects could allow for more
efficient movement of energy but were faced with a problem in trying to
capture evidence of such effects in the lab. In the classical world, either
molecule A or B is excited, and scientists can track the transfer of
excitation by measuring changes in the molecules over time. But in the
quantum world, things appear to exist in a multitude of states, making
measurements more complicated. Besides measuring changes of excitation in A
and B over time, the scientists needed a way to measure simultaneous
excitations of A and B — a signature of a quantum effect called coherence.

In 2005, Fleming and his colleagues developed a way to capture these
simultaneous excitations, or oscillations, in a photosynthetic protein found
in green sulfur bacteria. Using ultrafast lasers, the scientists flashed the
sample with three pulses from different beams to stimulate energy absorption
and transfer. A fourth pulse was then delivered to amplify the signal.

The timing of the flashes allowed the scientists to follow energy flow in two
dimensions, watching it in time and space as it moved from one chlorophyll to
another.  access Enlargemagnify GOING FOR A SPINView Larger Version | Studies
suggest that migrating birds exploit quantum effects in their visual systems
to sense magnetic fields.Illustration by Nicolle Rager Fuller

The method provided a way to follow a system’s vibrational state, tracking
its many wavelengths to see when they are what scientists call “in phase.”
When numerous particles such as electrons move in phase, all atoms are
moving, spinning and tipping in synchronicity. Such a system is in a coherent
state.

Uncertain he would find such wavelike behavior in a photosynthetic bacterium,
Fleming nonetheless considered it possible. “What changed is that we could
stop considering [the quantum effect] as a possibility and actually measure
it,” Fleming says.

In 2007, a sharp-eyed postdoc using the two-dimensional laser technique
spotted the telltale signature in a sample of green sulfur bacteria after
blasting it with the laser.

When the scientists repeated the experiment, their data showed the
oscillations meeting and interfering constructively, forming wavelike motions
of energy flowing through the system.

Fleming’s team, publishing in Nature, noted that quantum coherence could
explain the extreme efficiency of photosynthesis by enabling electrons to
simultaneously sample all the various potential pathways to the reaction
center and choose the most efficient one (SN: 4/14/07, p. 229). Rather than
hopping from one molecule to another in a step-by-step manner, the electrons
could try various routes to find the path of least resistance.

Intelligent design

Photosynthetic organisms are designed for efficiency. The light-absorbing
chlorophyll molecules found in leaves, for example, aren’t just arbitrarily
scattered throughout the cell, but are tightly packed into tiny organelles,
crammed into spaces where they touch each other frequently. So when excited
by a photon, the chlorophylls no longer act as individuals, but band together
to create a system that works in concert, says Thorsten Ritz, a theoretical
physicist at the University of California, Irvine.

And acting in concert has advantages. For one, it allows plants to absorb
energy in different ranges of light. Such a system also permits other
light-absorbing pigment molecules, such as carotenoids, to transfer energy
into the system in an efficient manner.

Early this year, scientists in Ireland and England used an ultrafast laser
with multiple color wavelengths to get an even closer view of energy moving
through a photosynthetic system. Ian Mercer of University College Dublin,
along with researchers at Imperial y inside the protein. The resulting map
showed how individual electrons coordinated their movements as they jostled
energy back and forth: Shifts to the left or right showed electrons
connecting, while vertical shifts indicated energy was being passed or
received.

The methods allowed the scientists to distinguish random hopping of energy,
or particle behavior, from the wavelike movements of electrons behaving
collectively. The study, published in the Feb. 6 Physical Review Letters,
will help scientists better model how quantum effects such as coherence
influence energy transfer in photosynthesis, Mercer says.

“We’ve been needing a better pair of eyes to see how molecules are doing the
tricks that they do,” he says.

Going for a spin

Birds may give scientists another pair of eyes in which to view quantum
effects in living cells. Studies suggest that migratory birds about to embark
on their seasonal journeys may tap into a quantum property called spin to
help them “see” Earth’s magnetic field using photosensitive proteins in their
eyes.

The idea that birds rely on some sort of biochemical reaction to orient
themselves during migration was first proposed more than 30 years ago. Eleven
years ago, Ritz and his colleagues identified cryptochrome, a protein
containing a light-sensitive pigment, as a candidate molecule capable of
creating such a reaction.

Cryptochrome is found in the nerve layers of birds’ eyes. Research shows that
when cryptochrome interacts with a specific wavelength of blue-green light it
can trigger a cascade of electron transfers similar to those that occur in
photosynthesis.

Normally, the electrons in cryptochrome exist in pairs. The energy from
light, however, can rip the electrons apart, leaving one electron on the
original molecule and sending the other off to another molecule. The result
is two charged molecules, or ions.

Initially, the electrons in these molecules spin in opposite directions. In
the presence of an external magnetic field, however, the dynamics of the
spins willence of quantum effects in living systems, researchers have yet to
demonstrate that those effects can actually influence the efficiency of
photosynthesis or migrating birds’ ability to navigate.

“Would plants not work so well if this didn’t happen?” Fleming asks. “I think
we need to be a bit cautious about answering that at this point. It’s a
complicated question. You have to be very sophisticated in how you model
things to show that the quantum effect is really making the system work
better. You can’t just turn it on and off.”

Not yet, anyway. Fleming, who says he is looking for “a higher standard of
proof,” has worked out two new theoretical models that will allow scientists
to perform experiments and better simulate bioquantum effects in the lab. The
new models will appear in an upcoming issue of the Journal of Chemical
Physics.

“Once you have a really good theory, you can turn things off to see what
happens,” he says.

Discovering how quantum effects play out in photosynthesis and bird
navigation may point scientists to other examples of the quantum in
biological systems.

“Photosynthesis, after all, is one of the oldest processes around,” Ritz
says. “If we see that nature learned at the very beginning, when they were
still bacteria, to control quantum processes, there’s no reason why nature
should have forgotten that in the future for more complex things.”

Susan Gaidos is a science writer in Maine.