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. We have developed a way to film changes in chlorophyll fluorescence, by computer acquisition of fluorescence images. The coloured pea leaf image that is used in these pages as a link to the Plant Cell Biology home page, was obtained in this way. In the animation you now see, leaves of a young pea seedling were imaged by a camera that detects chlorophyll fluorescence. Fluorescence is initially high (seen as the red colour), and then falls as the light-harvesting protein becomes phosphorylated - this redistributes energy to photosystem I at the expense of the fluorescent photosystem, photosystem II. Photosystem I and photosystem II are connected in series, and must therefore work at the same rate. A far-red light, unseen by the camera, is switched on, and fluorescence falls further. This is because photosystem I absorbs the far-red light: the fluorescent photosystem 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 the light-harvesting protein, and its absorbed energy is redistributed back to photosystem II. When photosystem I light is switched off, fluorescence from photosystem II quickly rises, as photosystem I becomes rate-limiting. A second, slow, falling phase shows that the cycle of state transitions has begun again.
Fugue in D minor, BWV 948.
Imaging chlorophyll fluorescence - a page without animation.
A Quicktime animation (4.3 MB, like this page, suitable only for wide-band connections) of state 1-state 2 transitions in screening for mutants of Arabidopsis thaliana.
The Kautsky effect.