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EARTH SCIENCE AND PHYSICAL SCIENCE
Carbon Cycle Chemistry
The atmosphere is keeping you alive. Every time you breathe, you
take in the oxygen that you need to live. But that’s not the end of the
story. The food you eat would not exist without the carbon dioxide
in the air that you, and every other animal on Earth, breathe out.
A Closer Look at Oxygen and Carbon Dioxide
Gases in air are tiny molecules that are much too small to see,
even if you look through a microscope. Chemists use diagrams to
represent these molecules. Oxygen gas (O2) is made of two atoms
of oxygen, so a diagram of an oxygen gas molecule shows two red
balls stuck together. A diagram of a carbon dioxide molecule (CO2)
looks similar, but it has one black carbon atom in addition to two
red oxygen atoms.
oxygen
carbon dioxide
The Carbon Cycle
The Carbon Connection
1
The tree takes carbon
from the air.
1
The orange tree takes in carbon dioxide
from the air. Molecules of carbon dioxide
are broken apart, and some carbon atoms
become part of other more complex
molecules in the growing orange.
2
You take carbon-containing molecules
into your body when you eat the orange.
Later, your body uses the food to carry
out life processes. Some of the carbon
atoms become part of carbon dioxide
molecules, which you exhale into the air.
carbon
in food
oxygen
in air
carbon
dioxide
in air
2
You move carbon from
food back into the air.
The carbon dioxide you exhale may be
taken in again by the tree. This time, the
carbon may become part of the trunk of
the tree, and then return to the air when
the tree dies and decays. Carbon keeps
going around and around among living
things and the atmosphere.
EXPLORE
1. COMPARE AND CONTRAST What is the difference
between a carbon dioxide molecule and an oxygen
molecule?
2. CHALLENGE Draw a diagram showing how carbon can
move into and out of the air when a tree grows and
then later dies and decays.
Chapter 15: Earth’s Changing Atmosphere 511