Mitochondria are marvelously 
efficient at converting the 
chemical energy of sugar into 
ATP. Whereas an engine 
would be considered very 
efficient if it converted 25 
percent of the energy available 
in gasoline into mechanical 
work, mitochondria routinely 
turn 54 percent of the available 
energy in sugar into ATP. 
This efficiency is achieved, in 
large part, because of the 
mitochondria’s internal 
structure. In the early 1950’s, 
Palade and a Swedish scientist, 
Fritiof Sjostrand, reported 
that mitochondria are 
surrounded 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 
mitochondrion: an outer 
membrane, separated from 
the rest of the organelle by a 
fluid-filled gap; and an inner 
membrane that is folded 
inward in many places to 
increase its surface area, form- 
ing 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 mitochon- 
drion, thus increasing the 
organelle’s efficiency. This 
general design seems to have 
existed unchanged from the 
time that mitochondria-like 
cells were free-living organisms. 
Mitochondria have also kept 
other vestiges of their existence 
as independent organisms. 
For example, mitochondria 
“reproduce” by splitting in 
half, as many modern bacteria 
Inner membrane 
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, mitochon- 
dria (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 mammalian 
cells. Human mitochondrial 
DNA is a circular molecule 
Outer membrane 
