SOLAR BREAKTHROUGH: HOW PLANTS DO IT

Efficiently converting sunlight into energy is the dream. Plants do it at nearly 100% efficiency. Humans have figured out how to do it at anywhere from 10% to 40% (or so) efficiency (depending on how much you want to spend).

Maybe it’s because plants don’t waste their efforts on American Idol or Britney Spears.

No, no – plants use a network of pigment-protein complexes. Scientists want to know more about how those networks work.

Maybe the most amazing part of plant photosynthesis is that it consumes carbon dioxide, the biggest if not the baddest of the greenhouse gas (GhG) emissions, and produces oxygen, the stuff humans need to breathe.

For obvious reasons, most of the brilliant scientists doing deep research in the area of solar energy are trying to replicate the process. It has been slow going. A new breakthrough will allow them to see the process a little more clearly.

For those seeking a technical understanding of the breakthrough, quotes from the researchers are below.

Even more informative is Inorganic Mimics of Photosynthesis, Professor Harry Gray’s November 9, 2007, CalTech lecture, from which some of the illustrations below are borrowed.

Schematic of sunlight photon havesting/energy transfer via pigment-protein complexes. (From Nat'l Energy Research Scientific Computing Center - click to enlarge)

On The Energy Trail: Researchers Find New Details Following the Path of Solar Energy During Photosynthesis
April 25, 2008 (Lawrence Berkeley National Labs via PhysOrg)

WHO
Graham Fleming, physical chemist, Department of Energy’s Lawrence Berkeley National Laboratory and University of California at Berkeley; Lawrence Berkeley researchers/co-authors: Elizabeth Read, Gabriela Schlau-Cohen, Gregory Engel, Jianzhong Wen, Robert Blankenship

The earliest breakthru in mimicking photosynthesis: Ru-bpy. (from Prof. Gray - click to enlarge)

WHAT
“Visualization of Excitonic Structure in the Fenna-Matthews-Olson Photosynthetic Complex by Polarization-Dependent Two-Dimensional Electronic Spectroscopy” moves researchers a step closer to understanding why plants can do what human technology has so far failed to do – turn light into energy efficiently.

WHEN
2.5 to 3.5 billion years ago: Converting light into energy allowed plants to evolve. The byproduct, oxygen, allowed animal life to arise.
1839: French Physicist Alexandre Edmond Becquerel first observed the photoelectric effect.
1883: Charles Fritts made the first solar cell.
1888: First solar cell patent. (Not Fritts)
1901: Nikola Tesla on the case.
1904: Einstein’s paper on the photoelectric effect.
1916: Robert Millikan proves the photoelectric effect.
1950s: Bell Labs make solar cells for space satellites.
1954: Bell Labs makes first silicon solar cell. Early work was with selenium, germanium and dyes.
1955: Western Electric licenses the first commercial solar cell. 2% efficiency. $1,785/watt.
The race is on.
Lot’s more detail at the full-length version of Wikipedia’s Timeline.

Current inorganic photosynthesis replication is focusing on the properties of titanium oxides. (from Prof. Gray - click to enlarge)

WHERE
Published in Biophysical Journal

WHY
- Fleming and his group have used a laser-based technique (two-dimensional electronic spectroscopy) to track the flow of energy. For the first time, they’ve connected that flow to energy-transferring processes in pigment-protein complexes.
- One of the researchers describes the breakthrough as improving “reception” of the images already being made.
- Three laser beams in femtosecond bursts are amplified and phase-matched by a fourth (oscillator) beam.
- In photosynthesis: Frenkel excitons (named after Russian physicist Yakov Frenkel) are released when light excites plant pigments to release chemical energy. The excitons carry energy down specific pathways in the plant to produce its activity and leave it in a new state (nourished, releasing oxygen, etc.). The new imaging breakthrough allows better visualization of this process.

Cal Tech and MIT are working to make solar energy by mimicking photosynthesis. (from Prof. Gray - click to enlarge)

QUOTES
- Fleming, lead author: “To fully understand how the energy-transfer system in photosynthesis works, you can’t just study the spatial landscape of these pigment-protein complexes, you also need to study the electronic energy landscape. This has been a challenge because the electronic energy landscape is not confined to a single molecule but is spread out over an entire system of molecules…Our new 2D electronic spectroscopy technique has enabled us to move beyond the imaging of structures and to start imaging functions. This makes it possible for us to examine the crucial aspects of the energy-transfer system that enable it to work the way it does.”
- Read, co-author: ” The optical properties of bacteriochlorophyll pigments are well understood, and the spatial arrangement of the pigments in FMO is known, but this has not been enough to understand how the protein as a whole responds to light excitation…By polarizing the laser pulses in our 2D electronic spectroscopy set-up in specific ways, we were able to visualize the direction of electronic excitation states in the FMO complex and probe the way individual states contribute to the collective behavior of the pigment-protein complex after broadband excitation.”
- Fleming: “By providing femtosecond temporal resolution and nanometer spatial resolution, 2D electronic spectroscopy allows us to simultaneously follow the dynamics of multiple electronic states, which makes it an especially useful tool for studying photosynthetic complexes…Because the pigment molecules within protein complexes have a fixed orientation relative to each other and each absorbs light polarized along a particular molecular axis, the use of 2D electronic spectroscopy with polarized laser pulses allows us to follow the electronic couplings and interactions (between pigments and the surrounding protein) that dictate the mechanism of energy flow. This suggests the possibility of designing future experiments that use combinations of tailored polarization sequences to separate and monitor individual energy relaxation pathways.”
- Read: “Using specialized polarization sequences that select for a particular cross-peak in a spectrum allows us to probe any one particular electronic coupling even in a system containing many interacting chromophores…The ability to probe specific interactions between electronic states more incisively should help us better understand the design principles of natural light-harvesting systems, which in turn should help in the design of artificial light-conversion devices.”