Engineering Chemistry S Datta

ROLE OF METALS IN BIOLOGY 565
Zinc, copper, vanadium, chromium, manganese and molybdenum are transported as simple
salts or loosely bound protein complexes. But for iron, high-affinity chelators or proteins are
required for transport.
Copper, cobalt, zinc, manganese and nickel, although essential in the diet, are also toxic
in excess and cause health hazards. Chromium in the form of chromate is mutagenic in bacterial
and mammalian systems.
Lastly silicon, ubiquitous as silica (SiO 2
) and as calcium aluminium silicate, is the
major component of clay and is required by rat, chick and other animals fed with synthetic
diet. The function of silicon is not known, but it occurs in greatest concentration in bone and
connective tissues of animals in an organic form.
Calcium and Magnesium
The adult human body contains more than a kilogram of calcium, nearly all of which is
concentrated in bones and teeth along with phosphate and as insoluble crystalline calcium
hydroxyapatite. Calcium also plays a very important role in all cells as an intracellular regulator
and messenger, mediating a wide range of biological processes like muscle contraction, secretion,
glycolysis and gluconeogenesis, iron transport, cell division; and growth.
Ca 2+ and the cytoplasmic Ca 2+ -binding proteins (e.g. Calmodulin) function as intracellular
messengers for different hormones. Free Ca 2+ present in the cytosol is very low about 10 –6 M.
Transitory increase in the Ca 2+ concentration which results from hormonal action on a
membrane receptor leads to the conformational changes of the Ca 2+ binding proteins. These
proteins in turn stimulate different protein kinases which are involved in different cellular
activities.
After these responses another important class of transport proteins (Ca 2+ ATPases)
transport the excess intracellular Ca 2+ (released from different sources) across the membrane
and maintain the original low concentration of Ca 2+ .
Ca 2+ ions are also known to play various roles outside cells. In the plants Ca 2+ forms link
between individual cells and is required for maintaining the rigidity of whole plants.
Calcium is abundant in foods like milk, cheese, cereal grains, legumes, nuts and
vegetables. Absorption of Ca from intestine is controlled by complex factors like pH, Ca:P ratio
in diet, presence of fatty acids and vitamin D in diet.
Pregnant and lactating women and children require more calcium (1200 mg/day)
compared to normal human (800 mg/day). The calcium present in the cereal grains is not
readily absorbed.
The body contains about 25 g of magnesium mostly present in bones. Concentration
of magnesium is rather high in all the cells, i.e., 5-10 mM, since it plays a vital role on the
action of a large number of enzymes involved in the glycolysis and ATP transport mechanisms.
Mg 2+ forms complexes with the phosphate groups of the glycolytic intermediates and ADP/
ATP and the substrate binding sites of many of these enzymes are specific for Mg 2+ complexes
of phosphorylated intermediates. Magnesium is abundant in chlorophyll of green leafy
vegetables. The recommended magnesium intake is 350 mg/day for adults. Alcoholics and
elderly people are found to be magnesium deficient.
Electron Transfer
Reactions in which electrons are transferred from one molecule to another are redox
reactions. Reducing and oxidising agents function as conjugate pairs (redox pairs).
electron donor e – + electron acceptor