evident in the kidneys, through which 
1 80 liters of water are filtered daily. 
Although mitochondria were first 
observed in the 1 880's, it took many 
years for scientists to understand the 
organelles' function. The process by 
which mitochondria use oxygen to 
release the chemical energy stored 
in food is called cellular respiration. 
Early in this century, it was discov- 
ered that the biochemical reactions of 
respiration fall into two main groups: 
the carbon pathway, in which sugar 
is broken down into carbon dioxide 
and hydrogen; and the hydrogen 
pathway, which transfers hydrogen to 
oxygen in stages, forming water and 
releasing energy. 
In the hydrogen pathway, the 
hydrogen's electrons pass through an 
"electron transport chain" made up of 
enzymes. As they move from enzyme 
to enzyme, the electrons give up part 
of their energy. This energy is then 
stored in molecules of ATP. In the 
end, 38 molecules of ATP are formed 
for every molecule of sugar that is 
used up in respiration. 
Mitochondria are marvelously 
efficient at converting the chemical 
energy of sugar into ATP. Whereas a 
man-made engine would be consid- 
ered very efficient if it converted 25 
percent of the energy available in 
gasoline into mechanical work, mito- 
chondria routinely turn 54 percent of 
the available energy in sugar into 
ATP. This efficiency is achieved, in 
large part, because of the mitochon- 
dria's internal structure. In the early 
1 950's, Palade and a Swedish 
scientist, Fritiof Sjostrand, reported 
that mitochondria are bounded by 
a membrane and that they have a 
system of parallel, regularly spaced 
inner ridges that the scientists named 
cristae. It is now known that there 
are two membranes around a mito- 
chondrion: an outer membrane, 
separated from the rest of the organ- 
elle by a fluid-filled gap; and an 
inner membrane that is folded inward 
at various points to increase its 
surface, forming the cristae. This 
ridged surface allows the enzymes of 
the electron transport chain, which 
are attached to the cristae, to be 
packed more densely within each 
mitochondrion, thus increasing the 
organelle's efficiency. This general 
design seems to have existed un- 
changed from the time that mitochon- 
dria-like cells were free-living 
organisms. 
Mitochondria have also kept other 
vestiges of their existence as inde- 
pendent organisms. For example, 
mitochondria "reproduce" by splitting 
in half as many modern bacteria do; 
they are not formed by budding from 
existing cellular structures or built up 
from simple cellular constituents, as is 
the case for ribosomes. 
More significantly, after a billion 
or so years of residence within "host" 
cells, mitochondria (and chloroplasts| 
still retain some of their own DNA. 
The amount of this non-nuclear DNA 
varies significantly from organism to 
organism. The chloroplasts of plants, 
for example, have five times more 
DNA than do the mitochondria of 
38 
