The ability of organisms to measure time is evidence that they are well adapted to the 24-hour day/night cycle of our planet. Circadian rhythms are the daily repeating patterns of many organisms, such as the closing of flowers at night or opening of stomata during the day. An interesting property of circadian rhythms is that many are endogenous, arising from an internal biological clock. For example, in 1729, a French scientist discovered that circadian movements were not dependent on the daily cycling of light and dark, even though they were synchronized with it.
Photosynthesis is an important circadian rhythm in plants. Photosynthesis is the process by which plants convert carbon dioxide and water to sugar and oxygen:
6CO2 + 6H2O + light energy --> C6H12O6 + 6O2
Tiny openings in the leaves, called stomata, open and close to regulate the amount of CO2 taken in, and the amount of water and O2 exiting. The two pathways of photosynthesis, photophosphorylation and the Calvin-Benson Cycle, proceed within the chloroplast. The products of photophosphorylation, ATP and NADPH + H+, are converted to sugars in the Calvin-Benson Cycle. Because photophosphorylation requires light energy to proceed, both pathways of photosynthesis stop in the dark. It is therefore beneficial for plants to coordinate their photosynthetic activity and stomata activity with daily changes in light availability. The internal clock coordinating photosynthesis produces a circadian rhythm.
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