has been s tripped down to a form in which it is ready to combine with a new molecule coming
in and to s tart the cycle anew.
This proces s by which the cell functions as a chemical factory is one of the wonders of the living
worl d. The fact that all the functioning parts are of infinitesimal size adds to the miracle. With
few exceptions cells themselves are minute, s een only with the aid of a micros cope. Yet the
greater part of the work of oxidation is performed in a theater far smaller, in tiny granules
within the cell called mitochondria. Although known for more than 60 years , thes e were
formerly dis mis sed as cellular elements of unk nown and probably uni mporta nt function. Only
in the 1950s did their s tudy become an exciting and fruitful field of res earch; s uddenly they
began to engage s o much attention that 1000 papers on this s ubject alone appeared within a
five-year period. Again one s tands in awe at the marvelous ingenuity and patience by which the
mys tery of the mitochondria has been solved. Imagine a particle so small that you can barely
see it even though a micros cope has enlarged it for you 300 times. Then imagine the s kill
required to isolate this particle, to take it apart and analyze its components and determine their
highly complex functioning. Yet this has been done with the aid of the electron micros cope and
the techniques of the biochemis t.
It is now known that the mitochondria are tiny packets of enzymes , a varied ass ortment
including all the enzymes neces sary for the oxidative cycle, arranged in precis e and orderly
array on walls and partitions. The mitochondria are the ‘powerhous es ’ in which mos t of the
energy-producing reactions occur. After the first, preliminary steps of oxidation have been
performed in the cytoplas m the fuel molecule is taken into the mitochondria. It is here that
oxidation is completed; it is here that enormous amounts of energy a re released. The endles s ly
turning wheels of oxidation within the mitochondria would turn to little purpos e if it were not
for this all-important res ult. The energy produced at each s tage of the oxidative cycle is in a
form familiarly spoken of by the bioche mis ts as ATP (adenos ine triphos phate), a molecule
containing three phos phate groups. The role of ATP in furnis hing energy comes from the fact
that it can trans fer one of its phos phate groups to other s ubs tances , along with the energy of
its bonds of electrons s huttling back and forth at high s peed. Thus , in a mus cle cell, energy to
contract is gained when a terminal phos phate group is trans ferred to the contracting mus cle. So
anothe r cycle takes place—a cycle within a cycle: a molecule of ATP gives up one of its
phos phate groups and retains only two, becoming a diphos phate molecule, ADP. But as the
wheel turns further anothe r phos phate group is coupled on and the pote nt ATP is res tored. The
analogy of the s torage battery has been us ed: ATP repres ents the charged, ADP the dis charged
battery.
ATP is the unive rs al currency of e nergy—found in all organisms from microbes to man. It
furnis hes mechanical energy to muscle cells; electrical energy to nerve cells. The sperm cell, the
fertilized egg ready for the enormous burs t of activity that will transform it into a frog or a bird
or a human infant, the cell that mus t create a hormone, all are s upplied with ATP. Some of the
energy of ATP is us ed in the mitochondrion but mos t of it is immediately dis patched into the
cell to provide power for other activities. The location of the mitochondria within certain cells is
eloquent of their function, s ince they are placed s o that energy can be delivered precisely
where it is needed. In mus cle cells they clus ter around contracting fibers; in nerve cells they are
found at the junction with anothe r cell, s upplying energy for the trans fer of impuls es ; in s perm
cells they are concentrate d at the point where the propellant tail is joined to the head.
backadmin
(backadmin)
#1