I T 
OCHONDRIA, ENERGY CONVERTERS IN THE CELL 
M 
III 
36 
ne and a half 
billion years 
ago, scientists 
believe, eukaryotic 
cells derived the 
energy they needed through a 
variety of relatively inefficient 
processes, none of which 
required oxygen. Oxygen, a 
waste product of some of these 
processes, gradually began to 
accumulate in the atmosphere. 
It was at this time, scientists 
hypothesize, that a primitive 
eukaryotic cell engulfed a 
primitive bacterium that had 
acquired the abdity to use 
oxygen to produce large 
quantities of energy. Over the 
eons, a symbiotic relationship 
evolved between the cells, and 
today almost all plant and 
animal cells have organelles 
that are the descendants of the 
primordial energy producers. 
In animal cells, these 
organelles are called mito- 
chondria. Plant cells have 
both mitochondria and a sec- 
ond kind of energy-producing 
organelle, the chloroplast. 
Chloroplasts use the energy in 
sunlight to convert molecules 
of carbon dioxide and water 
into molecules of sugar, a form 
of energy that can be stored in 
the plant cell. (Oxygen is 
given off as a byproduct of 
this process, which is called 
photosynthesis.) When an 
animal eats a plant (or 
another animal that has itself 
eaten plants), the plant’s 
sugars are broken back down 
into carbon dioxide and 
water, with the help of oxygen 
and an arsenal of enzymes, 
releasing large amounts of 
stored energy. This energy is 
immediately converted to yet 
another form — molecules of 
adenosine triphosphate (ATP). 
ATP is often called the 
universal currency of cellular 
energy. It is a convenient way 
for cells to store the energy 
they need for such processes 
as protein manufacture, DNA 
replication, and the construc- 
tion of new organelles. ATP is 
also required for such 
mechanical work as muscle 
contraction, pumping water 
through membranes, and cell 
movement. Following the first 
stages of sugar breakdown, 
the complicated process of 
energy transfer from sugar to 
ATP takes place within the 
animal cell’s mitochondria. 
Besides supplying energy, 
mitochondria help to control 
the concentration of calcium 
and other electrically charged 
particles in the cytoplasm. 
They also break down and 
recycle the energy contained 
in fatty acids and amino acids. 
Mitochondria are the largest 
organelles in an animal cell, 
after the nucleus, yet some 
cells have more than a 
thousand of them. They vary 
in diameter from 0.5 to 1 
micrometer and in length up 
to 7 micrometers, and can 
be seen with a good light 
microscope. Mitochondria are 
usually represented as 
oval-shaped, but in life they 
can change shape quite readily. 
They swell or contract in 
response to various hormones 
and drugs and during ATP 
