124 GROUPS I AND IIBeryllium is added to copper to produce an alloy with greatly
increased wear resistance; it is used for current-carrying springs and
non-sparking safety tools. It is also used as a neutron moderator
and reflector in nuclear reactors. Much magnesium is used to
prepare light metal alloys; other uses include the extraction of
titanium (p. 370) and in the removal of oxygen and sulphur from
steels; calcium finds a similar use.BIOLOGICAL IMPORTANCE
Sodium and potassium ions are found in all animal cells and,
usually, the concentration of potassium ions inside the cell is greater
than that of sodium. In many cells, this concentration difference is
maintained by a 'sodium pump', a process for which the energy is
supplied by the hydrolysis of adenosine triphosphate (ATP).
Diffusion of excess potassium ions outwards through the cell wall
gives the inside of the cell a net negative charge (due to the anions
present) and a potential difference is established across the cell wall.
In a nerve cell, a momentary change in the permeability of the cell
wall to sodium ions can reverse the sign of this potential difference,
and this produces the electrical impulse associated with the action
of the nerve.
The ability of living organisms to differentiate between the chemic-
ally similar sodium and potassium ions must depend upon some
difference between these two ions in aqueous solution. Essentially,
this difference is one of size of the hydrated ions, which in turn
means a difference in the force of electrostatic (coulombic) attraction
between the hydrated cation and a negatively-charged site in the
cell membrane; thus a site may be able to accept the smaller ion
Na+(aq) and reject the larger K+(aq). This same mechanism of
selectivity operates in other 'ion-selection' processes, notably in ion-
exchange resins.
All organisms seem to have an absolute need for magnesium. In
plants, the magnesium complex chlorophyll is the prime agent in
photosynthesis. In animals, magnesium functions as an enzyme
activator; the enzyme which catalyses the ATP hydrolysis mentioned
above is an important example.
Calcium plays an important part in structure-building in living
organisms, perhaps mainly because of its ability to link together
phosphate-containing materials. Calcium ions in the cell play a vital
part in muscle contraction.