Monday, 15 October 2018
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Photosynthesis - the light reactions (under construction)

Photosynthesis is the process that uses sunlight energy to synthesise organic compounds, which are used by biological organisms to live, grow and multiply. The process has primary importance because it provides energy, organic matter and oxygen for nearly all life on earth. Oxygenic photosynthesis in green plants and algae occurs in chloroplasts. A composite system of membranes inside the chloroplast (the thylakoids) harbours all the complexes which participate in the light reactions of photosynthesis. In these reactions, sunlight is absorbed by a large array of chlorophylls (Chls) and excitation energy is transferred to the reaction centre where it is used to perform charge separation. The electrons move along an electron-transport chain and are finally used for the reduction of NADP+ to NADPH. The primary donor obtains its electrons from water, producing O2. During the electron transport an H+ gradient is formed across the membbrane, which is used by the ATPase for the production of ATP.

The goal of our research is to understand at the molecular level the mechanisms of the light reactions of photosynthesis, with particular emphasis on light absorption, excitation energy transfer and photo-protection. For these studies we use an integrated approach from genetics, molecular biology, biochemistry to spectroscopy, steady-state and time-resolved.

Light harvesting and the conversion of light energy into chemical energy are catalysed by two multiprotein complexes, called Photosystems (I and II). Both these complexes can be divided in two moieties, an antenna system, which harvests the light energy and transfers it to the reaction centre and a core complex in which charge separation and electron transfer take place. In higher plants the antenna complexes of Photosystem I and II are named Lhca (Lhca1-4) and Lhcb (Lhcb1-6), respectively, and belong to the Lhc multigenic family. These proteins are characterised by three transmembrane helices and they coordinate Chl a, Chl b and carotenoids (lutein, neoxanthin, violaxanthin and zeaxanthin in stress conditions). In addition to the light-harvesting function these complexes are involved in photoprotection mechanisms which regulate the flow of energy toward the reaction centre under varying environmental conditions. Within a short period of time a plant can experience a wide range of illumination conditions and the system should be able to respond to these changes in a very fast and efficient way. When the light-harvesting efficiency exceeds the electron transport capacity, the plant is in extreme need to get rid of the excess energy to avoid major damage which can result from the production of radical oxygen species in the membrane. In the antenna complex of PSII a process called NPQ (non-photochemical quenching), regulates the level of single excited states by converting excess energy into heat. The understanding of the dissipative mechanisms is of primary importance for the future attempt of direct production of clean fuels from sunlight by using the operating principles of solar energy conversion of photosynthesis.