Friday, April 13, 2007

Quantum thermodynamics in photosynthesis

A study led by researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) at Berkeley reports that the answer lies in quantum mechanical effects. Results of the study are presented in the April 12, 2007 issue of the journal Nature.

Through photosynthesis, green plants and cyanobacteria are able to transfer sunlight energy to molecular reaction centers for conversion into chemical energy with nearly 100-percent efficiency. Speed is the key – the transfer of the solar energy takes place almost instantaneously so little energy is wasted as heat. How photosynthesis achieves this near instantaneous energy transfer is a long-standing mystery that may have finally been solved.

“We have obtained the first direct evidence that remarkably long-lived wavelike electronic quantum coherence plays an important part in energy transfer processes during photosynthesis,” said Graham Fleming, the principal investigator for the study. “This wavelike characteristic can explain the extreme efficiency of the energy transfer because it enables the system to simultaneously sample all the potential energy pathways and choose the most efficient one.”

One of the most sensitive signatures of phytoplankton optical properties is chlorophyll fluorescence. The coloration of phytoplankton is a consequence of evolutionary selection of pigments that can absorb and transfer excitation energy to photosynthetic reaction centers, where the excitation energy (in the form of an “exciton,” an excited state within a lattice structural matrix) can be used to induce physical charge separation.Phytoplankton comprises at least eight taxonomic divisions (the equivalent of animal phyla), each of which has one or more distinctive light-harvesting pigments. Excitation energy is transferred from pigment to pigment within a protein scaffold via either Forster resonance or excitation coupling (Falkowski and Raven, 1997). The excitation transfer almost always proceeds “downhill” (i.e. to longer-wavelength pigments).

The terminus of excitation transfer is chlorophyll a, from whence a fraction of the excitation energy is emitted to the environment in the red portion of the spectrum (centered at 683 nm). In vivo, chlorophyll fluorescence competes for excitons with two other energy-dissipating processes, photochemistry and nonradiative energy dissipation (heat). As such, it is possible to relate changes in chlorophyll fluorescence quantitatively to the quantum yield of photochemistry.

There are several interesting connections between bio-optics, biogeochemistry,upper ocean physics, and the biological pump. As described earlier, primary productivity and phytoplankton biomass are dependent on photosynthetic processes, which implicitly involve the availability of light (e.g., PAR) and nutrients. Macronutrients (e.g., nitrate, silicate, and phosphate) and micronutrients (e.g., iron) are important. The spectral quality of light varies with depth and is important for specific phytoplankton species with special pigmentation or photoadaptive characteristics, as described earlier (e.g., see also Bidigare et al., 1990; Bissett et al., 1999). Light exposure for individuals is affected by variation in physical conditions, including mixed layer depth, turbulent mixing, and currents as well as incident solar radiation, which varies in time and space (e.g., astronomical forcing, cloud variability). An important feedback concerns the modulation of the spectral light field at depth as phytoplankton concentrations and communities wax and wane. The determination of primary production in the upper ocean is a vital step in quantifying the carbon flux associated with the biological pump (Laws et al., 2000). Fundamental measurements and models have been developed to estimate primary production . Such models often use measurements of chlorophyll a concentration and PAR. The choices of values for spectral absorption and quantum efficiency are often critical, as both of these parameters can vary in time and geographically.

Thus in summary we can see the importance of the spectral wavelengths and species for photosynthesis ,the transformation of energy from radiative to chemical energy, the limitations for absorbtion and modulation of thermal radiation in the oceans by the biosphere.A system of inter and overlying sets,which when studied in isolation seem trivial but as a dynamic system in total, show us a complex non linear "engine"that undercorrects and overcorrects due to self organisation and autocatalytic mechanisms.


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