Sci. STKE, 22 October 2002
Absorbed light energy is redistributed between the two energy-converting systems of photosynthesis. Chlorophyll is the green pigment that harvests and converts light in photosynthesis. Chlorophyll also emits light, as fluorescence, and variations in fluorescence emission report on changes in the efficiency of photosynthesis. At room temperature, changes in the yield of chlorophyll fluorescence come from photosystem II (PS II). In Movie 1, leaves of a young pea seedling were imaged by a camera screened with a 660-nm optical interference filter, detecting chlorophyll fluorescence. Fluorescence is initially high (seen as the red color), and then falls as the light-harvesting protein LHC II becomes phosphorylated--this redistributes energy to photosystem I (PS I) at the expense of the fluorescent photosystem, PS II. This is the state 2 transition. PS I and PS II are connected in series and must, therefore, function at the same rate. A far-red light is then switched on, and fluorescence falls further. This is because PS I absorbs the far-red light: the fluorescent PS II can then use more energy, and less of its energy is wasted as fluorescence. Fluorescence then rises slightly, as the phosphate group is removed from phospho-LHC II, and absorbed energy is redistributed back to PS II. This is the state 1 transition. When PS I light is switched off, PS I becomes rate-limiting, and PS II has nowhere to pass electrons, so fluorescence from PS II quickly rises. A second, slow, falling phase is a second transition to state 2: The cycle of state transitions has begun again.
Citation: J. F. Allen, H. L. Race, Will the real LHC II kinase please step forward? Science's STKE (2002), http://www.stke.org/cgi/content/full/sigtrans;2002/155/pe43.
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