MITOCHONDRIA, 
ENERGY CONVERTERS 
IN THE CELL 
One and a half billion years ago, 
scientists believe, 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 hypothe- 
size, that a pre-eukaryotic cell 
engulfed another primitive cell that 
had somehow acquired the ability to 
use oxygen to produce large quanti- 
ties of energy. Over the eons, a 
symbiotic relationship evolved 
between the cells, and today all 
plant and animal cells have organ- 
elles that are the descendants of the 
primordial energy producers. In 
animal cells, these organelles are 
called mitochondria, while the 
energy-producing organelles in green 
plants are called chloroplasts. 
Chloroplasts use the energy in 
sunlight to convert molecules of 
carbon dioxide and water into mole- 
cules of sugar, a form of energy that 
can be stored in the plant cell. 
(Molecular oxygen is given off as a 
byproduct of this process, whi ch 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 replica- 
tion, and the construction of new 
organelles. ATP is also required for 
such mechanical work as muscle 
contraction, pumping water through 
membranes, and cell movement. 
Except for the first stage of sugar 
breakdown, the entire, complicated 
process of energy transfer from sugar 
to ATP takes place within the animal 
cell's mitochondria. (Plants also have 
mitochondria, although they make 
most of the ATP they need within their 
chloroplasts.) 
Besides supplying energy, mito- 
chondria apparently help to control 
the conceniration of water, calcium, 
and other charged particles (ions) in 
the cytoplasm. They also break down 
and recycle the energy contained in 
fatty acids and amino acids. 
Mitochondria are the largest organ- 
elles 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 manufacture. 
This swelling and contracting appears 
related to the movement of water 
through cells, and is particularly 
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