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D8. Plant Protection

Plants have evolved a great ability to absorb light over the entire visible range of the spectra.  Can they absorb to much energy?  The answer is yes, so plants have developed many ways to protect themselves. IF too much light is absorbed, the pH gradient developed across the thylacoid membranes becomes greater.  This is sensed by a protein, PsbS, and through subsequent conformational changes transmitted through the light-harvesting antennae, the excess light energy is dissipated as thermal energy.  Mutants lacking PsbS showed decreased seed yield, a sign that it became less adaptable under conditions of stress (such as exposure to rapidly fluctuating light levels).  Molecules called xanthophylls (synthesized from carotenes - vit A precursors) such as zeaxanthin are also important in excess energy dissipation.  These molecules appear to cause excited state chlorophyll (a singlet like excited state dioxygen) to become deexcited.  Without the xanthophylls, the excited state chlorophyll could deexcite by transfer of energy to ground state triplet dioxygen, promoting it to the singlet, reactive state, which through electron acquisition, could also be converted to superoxide.  These reactive oxygen species (ROS) can lead to oxidative damage to proteins, lipids and nucleic acids, alteration in gene transcription, and even programmed cell death.  Carotenoids can also acts as ROS scavengers.  Hence both heat dissipation and inhibition of formation of ROS (by such molecules as vitamin E) are both mechanism of defense of excessive solar energy

Given that both plants and animals must be protected from ROS, antioxidant molecules made by plants may prove to protect humans from diseases such as cancer, cardiovascular disease, and general inflammatory diseases, all of which have been shown to involve oxidative damage to biological molecules.  Humans, who can't synthesize the variety and amounts of antioxidants that are found in plants, appear to be healther when they consume large amounts of plant products.  These phytomolecule also have other properties, including regulation of gene transcription which can also have a significant effect on disease propensity.

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