How Nature Harvests Sunlight: The Physics of Photosynthesis

Photosynthesis is one of the great-impact inventions of biological evolution.  Indeed, life on Earth is fueled energy-wise mainly by sun light.  Many, so-called photosynthetic, life forms harvest sun light directly, for example, plants, algae and bacteria; other life forms use sun light indirectly, like herbivorous animals.  This lecture tells the story a particular simple, yet amazing photosynthetic apparatus, the chromatophore, found in purple bacteria.

The photosynthetic chromatophore is a spherical shell of 50 nm diameter that exists in hundreds of copies in the bacterial cell and converts sun light into chemical synthesis of an energy-rich molecule, adenosin triphosphate (ATP). Each chromatophore is made of over hundred protein complexes with thousands of light absorbing and electron conducting molecules embedded in them; the complexes are held together by a membrane made of 20,000 lipid molecules. Despite its complexity and heterogeneity the chromatophore can be viewed today through advances in experimental and computational biology at atomic- and electronic-level detail in its entire structure and function. One sees a clockwork of linked, mostly rather elementary processes: light absorption, coherent and incoherent exciton formation, intermolecular electron and proton transfer, charge carrier diffusion, electrostatic steering of protein- mediated electron conduction, molecular motor action driven by proton conduction, and lastly mechanically driven ATP synthesis.

For the first time a major part of a biological cell has been resolved in its entirety at the level of truly basic physics, showcasing how Angstrom- scale processes lead to 100-nm-scale intelligent overall function. In viewing the chromatophore through a beautifully detailed movie one can recognize in an exemplary fashion how evolution engineered an apparatus crucial for solar energy-driven life on Earth, utilizing amazing processes on the small scale by linking them together in a clock-work fashion such that an efficient, robust and adaptive cell-scale function emerges.