As was first mentioned in Chapter 10, the free
energy of hydrolysis of ATP into ADP and inorganic phosphate is
unusually high for organic phosphate compounds, around 7.3 kcal
mole-'. This is the energy that must be supplied to produce
ATP and water from ADP and phosphate, and this is the free energy
that is released again when ATP is hydrolyzed. The further hydrolysis
of ADP to AMP and phosphate releases a similar amount of energy,
but the free energy of hydrolysis of AMP to adenosine and phosphate
is only 3.4 kcal mole-', which is similar to that of
other organic phosphate compounds. The unusually large hydrolysis
energies, which arise partly from delocalization of electrons and
partly from repulsions between negative charges on the polyphosphate
groups, make ATP a useful means of storing chemical energy in living
No matter how a particular organism obtains its
chemical energy, or what compounds it employs for long-term energy
storage, every living organism first converts chemical energy into
ATP molecules, and then uses this ATP for its subsequent purposes.
It is tempting to think that life began as a scavenger of ATP from
the primordial seas, and that all the other energy-gathering processes
developed only as alternative ways of making artificial ATP when
the natural supply ran out.
Right: The two pyrimidines in DNA, cytosine
and thymine, have single rings. In RNA, thymine is replaced by uracil,
which does not have the thymine methyl group.