manufacture. This swelling 
and contracting appears 
related to the movement of 
water through cells, and is 
particularly evident in the 
kidneys, through which 180 
liters of blood are filtered 
daily. 
Although mitochondria were 
first observed in the 1880’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. In the early 
1900’s, it was discovered that 
the biochemical reactions of 
this type 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. 
Electron micrograph 
showing one of the cells 
many mitochondria , the 
organelles that convert 
energy from food into a 
form that can be stored. 
