Structural relationship of vitamin D 3 (cholecalciferol) and vitamin D 2 (ergocalciferol) with their respective provitamins, cholesterol, and a classic steroid hormone, cortisol (see inset box). The two structural representations presented at the bottom for both vitamin D 3 and vitamin D .2 are equivalent; these are simply different ways of drawing the same molecule. It is to be emphasized that vitamin D 3 is the naturally occurring form of the vitamin; it is produced from 7-dehydrocholesterol, which is present in the skin, by the action of sunlight (see Figure 2). Vitamin D 2 (which is equivalently potent to vitamin D 3 in humans and many mammals, but not birds) is produced commercially by the irradiation of the plant sterol ergosterol with ultraviolet light.
The use of oxygen as part of the process for generating metabolic energy produces reactive oxygen species.  In this process, the superoxide anion is produced as a by-product of several steps in the electron transport chain .  Particularly important is the reduction of coenzyme Q in complex III , since a highly reactive free radical is formed as an intermediate (Q · − ). This unstable intermediate can lead to electron "leakage", when electrons jump directly to oxygen and form the superoxide anion, instead of moving through the normal series of well-controlled reactions of the electron transport chain.  Peroxide is also produced from the oxidation of reduced flavoproteins , such as complex I .  However, although these enzymes can produce oxidants, the relative importance of the electron transfer chain to other processes that generate peroxide is unclear.   In plants, algae , and cyanobacteria , reactive oxygen species are also produced during photosynthesis ,  particularly under conditions of high light intensity .  This effect is partly offset by the involvement of carotenoids in photoinhibition , and in algae and cyanobacteria, by large amount of iodide and selenium ,  which involves these antioxidants reacting with over-reduced forms of the photosynthetic reaction centres to prevent the production of reactive oxygen species.  
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