The Three Cycles
Purpose: Organic and inorganic substances must constantly be recycled in ecosystems so
that chemicals will always be available for other organisms to grow and rebuild. In this
activity, you will become experts in the carbon, nitrogen and water cycles.
Instructions:
Work in small groups to answer questions related to the three cycles. How you organize the
work within your group is up to you. By the end of this task you should be able to describe
each of the following cycles and explain human activities that might have an effect on them
(positive or negative)



The Carbon Cycle
The Nitrogen Cycle
The Water Cycle
NOTE: There are many cycles that exist within an ecosystem, but these are the three that
we will focus on.
PART A: THE CARBON CYCLE
Atmospheric Carbon Dioxide
Photosynthesis
Plants (Producers)
Cellular
Respiration
e
Animals
(Consumers)
Oceans
air
soil
Fossil Fuels
Decomposers Break Down
Organic Matter
Carbonates in
shells, skeletons
and rocks
Figure 1.The carbon cycle
1. Why are photosynthesis and cellular respiration considered to be complimentary
processes? *HINT* Look at the chemical equations
2. Why are decomposers important in the carbon cycle?
3. Explain the ways that humans impact the carbon cycle. If it is a negative impact, come
up with a solution to reduce or eliminate it.
4. Explain why carbon is cycled more slowly in cold northern ecosystems than in the hot
tropics. *HINT* Particle theory can help you answer this question.
PART B: THE NITROGEN CYCLE
http://www.uwsp.edu/geO/faculty/ritter/geog101/textbook/earth_system/nitrogen_cycle_EPA.jpg
Figure 2.The nitrogen cycle
1. Since Nitrogen makes up 79 % of the atmosphere, why is it so difficult for living
organisms to obtain the nitrogen they need?
2. How do animals obtain usable nitrogen?
3. What would happen to animals if all of the nitrogen fixing bacteria in soil and on plant
roots was killed by pesticides?
PART C: THE WATER CYCLE
http://www.dask.org.tr/english/want_to_know/meteorology/moisture/water-cycle.gif
1. Describe the ways in which liquid water becomes atmospheric water vapour. Use terms
from the diagram above.
2. A bottled water company gets permission from the City of Oakville to take ground water
from Oakville and sell it in China. How would this impact the Oakville water cycle? List
ways that this could impact your daily life.
3. Use your knowledge of the water cycle to explain why cities and towns make sure to
leave parks and green space when developing and building on new land. What would
happen if all exposed soil was covered in concrete, buildings and asphalt?