8.6Absorption of Light Energy
Antenna pigments that absorb light energy include chlorophyll a, chlorophyll b and carotenoid*9. Blue and red pigments, such as phycobilin, are also known in red algae and cyanobacteria. When visible light excites these pigments, nearly 100% of the excitation energy moves between the pigments and is conveyed to the photosynthetic reaction center. For this purpose, the antenna pigments need to be located close to each other, and are incorporated into proteins and accumulated within the thylakoid membrane. Although plant leaves may appear evenly green, only the thylakoid membrane of chloroplasts, which contain chlorophyll a and b, is in fact green. Other organelles, such as those found in animal cells, have no color. While most terrestrial plants have green leaves, pigments found in algae vary in color (e.g., phycocyanin is blue, phycoerythrin is red and fucoxanthin is brown) because the spectrum of sunlight deviates in water depending on the conditions of absorption and scattering. On the other hand, anthocyan - a water-soluble colorful pigment found in plant tissues such as flowers - is located in vacuoles, and cannot use absorbed light energy for photosynthesis.
Carotenoid: A fat-soluble material with a long-chain polyene. The material functions as an antenna pigment, and also known as a vitamin A precursor.
Demonstration of ATP Synthase Rotation
A Japanese research group demonstrated the rotation of ATP synthase through an elegantly designed experiment. In the setup, F1 of ATP synthase was fixed to a glass slide, and an actin filament treated with a fluorescent dye was bound to the stalk, thus making one complex visible under a fluorescence microscope. When ATP was applied to the glass, the complex started rotating as ATP hydrolysis proceeded, and this action was video-observed. Interestingly, the rotation took place in an anticlockwise direction in three 120-degree steps, indicating that ATP is degraded via three stable intermediates and the energy is transferred to the stalk. It can be considered that ATP synthesis proceeds in the opposite direction. It is believed that, through H+ transport, the energy of H+ causes ATP synthesis through the rotation of the stalk. Crystal structure analysis of F1 had already shown that an asymmetric stalk-like γ-subunit is located on the bottom surface of the threefold symmetrical sphere containing three α-subunits with ATP synthetic activity, and that these three subunits are in different states. This indicated that the structural change of the α-subunits, which are involved in ATP synthesis, is linked to the positional relationship with the stalk, thus rotating the complex in three 120-degree steps.