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*Elements in boldface are known to be essential to humans.

Table 7.6 Relative Abundance of Some Essential Elements in Earth’s Crust and Oceans Element* Not surprisingly, both molybdenum and iodine are used by many organisms.

In contrast, molybdenum and iodine, though not particularly abundant, are highly soluble-molybdenum as molybdate (MoO 4 2−) and iodine as iodide (I −) and iodate (IO 3 −)-and thus are more abundant in seawater than iron. Many organisms have therefore developed elaborate strategies to obtain iron from their environment. This is also the case for iron and aluminum, which form insoluble hydroxides. Although silicon is the second most abundant element in Earth’s crust, SiO 2 and many silicate minerals are insoluble, so they are not easily absorbed by living tissues. Instead, they tend to form insoluble oxides, hydroxides, or carbonate salts.

Adequate amounts of the element must be available in the environment in an easily accessible form.Īs you can see in Table 7.6, many of the elements that are abundant in Earth’s crust are nevertheless not found in an easily accessible form (e.g., as ions dissolved in seawater).

The element must have some unique chemical property that an organism can use to its advantage and without which it cannot survive.

What makes some elements essential to an organism and the rest nonessential? There are at least two reasons: Of the more than 100 known elements, approximately 28 are known to be essential for the growth of at least one biological species, and only 19 are essential to humans.

To describe some of the roles of trace elements in biological systems.