Living Physics: From Green Leaves to Bird Brains, Biological Systems May Exploit Quantum Phenomena

By Gaidos, Susan | Science News, May 9, 2009 | Go to article overview

Living Physics: From Green Leaves to Bird Brains, Biological Systems May Exploit Quantum Phenomena


Gaidos, Susan, Science News


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Until a century or so ago, nobody had any idea that the re 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. …

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