Section 22.5 - Nonessential elements PDF

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Nonessential Elements - Professor Kelly ...

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Section 22.5 - Nonessential elements Tuesday, March 12, 2019

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11:50 PM

The ten elements listed in Table 22.2 are found in the human body but are classi- fied as nonessential. In this section we discuss how some of these elements may end up in our bodies, working our way from left to right across the periodic table Rubidium is generally regarded as nonessential in humans, yet it is the 15th most abundant element in the body. It is believed that Rb1 is retained by the body because of the similarity of its size and chemistry to that of K1. Like the other cations of group 1, cesium ions (Cs1) are also readily absorbed by the body. Cesium cations have no known function, although they can substitute for K1 and interfere with potassium-dependent functions. In most cases, the concentration of cesium in the environment is low, so exposure to Cs1 is not a health concern. The nuclear accident at Chernobyl in 1986, however, released significant quantities of radio- active 137Cs into the environment. The ability of Cs1 to substitute for K1 led to the incorporation of 137Cs1 into plants, which rendered crops grown in the imme- diate area unfit for human consumption because of the radiation hazard posed by this long-lived (t1/2 < 30 yr) β emitter. Some single-celled organisms build exoskeletons made with SrSO4 and BaSO4, but the human body appears to have no use for Sr21 and Ba21 ions. These ions do find their way into human bones, where they replace Ca21 ions. At the low concentrations of Sr21 and Ba21 that are typically present in the human body, these elements appear to be benign. However, as in the case of radioactive 137Cs, incor- poration of 90Sr (t1/2 5 29 yr) in bones can lead to leukemia. Atmospheric testing of nuclear weapons over the Pacific Ocean and in sparsely populated regions of the American West in the 1950s released 90Sr into the environment. The full extent of the toxic effects of the fallout from these tests did not become apparent for several decades. It is generally agreed that germanium is a nonessential element and is barely detectable in the human body. Bis(carboxyethyl)germanium sesquioxide (Figure 22.16) has been touted as a nutritional supplement, but its efficacy remains controversial. The role of antimony is also poorly understood. Most antimony compounds are toxic because they cause liver damage. However, ultratrace amounts of

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antimony may have a stimulatory effect, and selected antimony compounds have been used medically as antiparasitic agents, as discussed in the next section. Bromine has no known function in the human body but is consumed in foods such as grains, nuts, and fish in amounts ranging from 2–8 mg per day, leading to average concentrations of Br2 in blood of about 6 mg/L. Br2 has sedative and anticonvulsive properties but becomes toxic at concentrations around 100 mg/L, limiting its use to veterinary medicine. Bromide ion concentrations in seawater typically range from 65–80 mg/L. A select group of aquatic species can metabolize Br2 into bromomethane, CH3Br, and other brominated organic compounds....