18.2 The Carbon Cycle and the Nitrogen Cycle
Lesson Objectives
- Describe the short term cycling of carbon through the processes of photosynthesis and
respiration. - Identify carbon sinks and carbon sources.
- Describe short term and long term storage of carbon.
- Describe how human actions interfere with the natural carbon cycle.
- Describe the nitrogen cycle.
Introduction
Carbon is a very important element. It is not the most abundant element in the universe or
even on the Earth, but it is the second most common element in the human body. You could
not live without carbon. If something you eat has protein orcarbohydratesor fats, then
it contains carbon. When your body breaks down that food to produce energy, you breathe
out carbon dioxide. Carbon is also a very important element on Earth. Carbon is provided
by the environment, moves through organisms and then returns to the environment again.
When all this happens in balance, the ecosystem remains in balance too. In this section,
let’s follow the path of a carbon atom over many years and see what happens.
Nitrogen is also a very important element. Nitrogen must be converted to a useful form so
that plants can grow. Without ”fixed” nitrogen, plants and therefore animals could not exist
as we know them.
Short Term Cycling of Carbon
The short term cycling of carbon begins with carbon dioxide and the process of photo-
synthesis. Our atmosphere is mostly made of nitrogen and oxygen, but there is a small
amount of carbon dioxide in the air too. Plants andalgaeuse this carbon dioxide, along
with water and energy from sunlight to produce their own food. This is a little miracle that
is happening everywhere around you each and every day. Plants and algae have the ability
to take the inorganic carbon in carbon dioxide and make it into organic carbon, which is
food. That is something that we cannot do at all! Imagine the difference between what
would happen if you tried to eat a piece of coral or a shell and what happens when we eat
sugar. We can’t get energy from the bits of rock at all, but you know how quickly sugar can
be used for energy in our bodies.
Through photosynthesis, carbon dioxide plus water and energy from sunlight is transformed
into food with oxygen given off as a waste product. Chemists write shorthand equations for
different types of chemical reactions. The equation for photosynthesis looks like this (Figure