Chemistry Reference
In-Depth Information
hats' more rigid. Prolonged exposure to the mercury vapours caused mercury poisoning. Victims developed severe
and uncontrollable muscular tremors and twitching limbs, called “hatter's shakes”; other symptoms included
distorted vision and confused speech. Advanced cases developed hallucinations and other psychotic symptoms.
This may well explain the expression 'mad as a hatter', illustrated classically in Lewis Carrol's alluring
FIGURE 1.10
The Mad Hatter's Tea Party.
(From Lewis Carroll's Alice in Wonderland.)
nonmetals. The important biological role of some of the nonmetals, oxygen, nitrogen, phosphorus, sulfur, and
selenium together with the halogens, chlorine and iodine, will be discussed in Chapter 18.
The metalloids boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), and tellurium (Te) are
located along a diagonal line separating the metals from the nonmetals
11
(
Fig. 1.11
). Whereas some metalloids
have beneficial and essential roles in biological systems (B, Si), others are highly toxic (As, Sb). All organisms,
therefore, face the challenge of dealing with the presence of metalloids either in terms of the need to acquire
sufficient amounts for effective metabolism, or conversely the need to extrude them to prevent toxicity. Major
intrinsic proteins (MIPs) are a family of selective membrane channels which facilitate diffusion of water and small
uncharged solutes. Recently, in bacteria, fungi, protozoa, mammals, and plants, specific members of the MIP
family of solute channels have been shown to facilitate the diffusion of reduced and noncharged polyhydroxylated
forms of various metalloids (
Fig. 1.11
).
The essential role of boron in plants was first described more than 80 years ago, and there is increasing
evidence that boron is also an essential element in yeast and mammals, including man. During the past decade, it
has been shown that the principal function of boron in plant cell walls is to crosslink rhamnogalacturonan
molecules, an abundant and crucial structural component in plants, which means there is a high demand for B in
plants. B is widely distributed in the earth's crust (5
The elements of groups 13
e
100 mg kg
1
) and mainly occurs as boric acid [B(OH)
3
]. At
increasing pH, this weak Lewis acid forms the charged borate anion [B(OH
4
)
], which has physico-chemical
similarity to bicarbonate. Recently, molecular genetics experiments using Arabidopsis thaliana have identified
two types of boron transporters, a boric acid channel, NIP5;1, and a boric acid/borate exporter, BOR1. Both
e
11. Polonium, with chemical resemblance to bismuth and tellurium, is often considered as a metal.
