The short answer is that when we breathe in
air, there's more than oxygen in it. About
20% is oxygen, about 80% is nitrogen, and less
than one percent is carbon dioxide.
But we breathe out more carbon dioxide
than we breathe in. Our cells all need oxygen
to stay alive. They use it in the process of
breaking down sugar to release the energy we need.
Our lungs bring in air, and the oxygen flows from
the air in our lungs into our blood. Then the
oxygen flows from the blood into the cells that
need it. The cells basically "use up" the
oxygen and produce carbon dioxide
(CO2). This CO2 flows into
the blood, is carried back to the lungs, and
The molecules that make up the CO2
and oxygen don't disappear, they just get
(C6H12O6) and 6
oxygen (O2) go into the process and 6
carbon dioxide (CO2) and 6 water
(H2O) come out.
Plants use the sun's energy to make sugar out
of water and CO2. What do they
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