Photons provide a potential source of energy for all living organisms. However, only photosynthetic organisms can use this energy. To use it they require a photosynthetic apparatus (e.g., scaffolds of membrane-bound pigments and reaction centers that include chlorophyll-like molecules) to absorb the light and a system to convert this energy into a useful form. Most light-absorbing systems can dissipate absorbed energy as heat. While this heat is occasionally directly beneficial to organisms, it is critical for photosynthetic organisms to convert the absorbed energy to biochemically useful forms. Because absorption of light involves exciting electrons in the photosystem apparatus to a higher energy state, energy storage is achieved by using these excited electrons to create reduced, energy-rich intracellular compounds. The reduced compounds formed during photosynthesis then yield their energy in subsequent oxidation-reduction reactions that drive cellular biosynthesis.
A key aspect of this energy-trapping process lies in its timescale. Excited bacteriochlorophyll, for example, has a life time of about 10~10 s and would be difficult to use to drive the majority of biochemical processes. In contrast, reduced organic molecules like fats, lipids, and carbohydrates can serve as intracellular reserve materials for days, months, or years and then be drawn upon as necessary to create the ATP pool that are directly fuels for anabolic (biosynthetic) cellular reactions.
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