We bring our paper, plastic, glass, and metal to be recycled in order to conserve the resources we have. Recycling of resources must also happen for the ecosystem to survive and not run out of what it needs. The matter (resources) recycled in ecosystems includes water, oxygen, carbon, nitrogen, and many other substances. Three of the most important cycles of matter are the water cycle, the
carbon-oxygen cycle, and the nitrogen cycle.
The hydrologic cycle, or water cycle, is the continuous process by which water moves from Earth’s surface to the atmosphere and back. The processes of evaporation, condensation, and precipitation make up the water cycle. Evaporation is the process by which molecules of liquid water absorb energy and change to the gas state. Some of this water evaporates from plants in the process of transpiration. Most of the water evaporates from the ocean, but it also comes from rivers, lakes, and streams on land. Once it evaporates from
Earth’s surface in the form of a water vapor, a gas, it goes up into the atmosphere. When water vapor in the atmosphere cools, it turns back into tiny droplets of liquid water. The process by which a gas changes to a liquid is called condensation. As more water vapor condenses, the drops grow larger and heavier. Eventually, the heavy drops fall back to Earth as a form of
precipitation—rain, snow, sleet, or hail. Then the water is recycled again and again.
The next important cycle in an ecosystem is the carbon cycle. Carbon is the building block for the matter that makes up the bodies of living things. Producers take in carbon dioxide from the atmosphere during photosynthesis. In this process, the producers use carbon from the carbon dioxide to produce other carboncontaining molecules. These molecules include sugars (C
6
H
12
O
6
) and starches.
Consumers obtain energy from these molecules by breaking them down into simpler molecules through the process of
cellular respiration.

The consumers release water and carbon dioxide as waste products of the process. At the same time, producers release oxygen during photosynthesis. Other organisms, like us, take in oxygen from the atmosphere and use it in their life processes. Carbon is also returned to the environment when burning fossil fuels. What problem may be caused by the burning of these fuels? ________________________________________________
Write the chemical equations for photosynthesis and cellular respiration.
Draw lines from the products of one equation that become reactants of the other equation.
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Like carbon, nitrogen is a necessary building block in the matter that makes up living things. Even though the air contains mostly nitrogen, (78%), most organisms cannot use nitrogen gas (N
2
) in the air. Nitrogen gas is called “free”
nitrogen because it is not combined with other kinds of atoms. Most organisms can use nitrogen only when it has been
“fixed”: or combined with other elements to form nitrogen- containing compounds.
The process of changing nitrogen gas into a usable form of nitrogen is called
nitrogen fixation. Lightning can break the bonds of N2 to make it useable. But most nitrogen fixation is performed by certain kinds of bacteria. Some of these bacteria live in lumps called nodules on the roots of certain plants. Once the producers have their nitrogen fixed, it can be used by consumers to build proteins and other complex substances. Consumers return the nitrogen in their wastes and when they die. Decomposers break down complex nitrogen compounds in these wastes and dead organisms to form ammonium ions (NH
3
). Nitrifying bacteria will change it to nitrites and nitrates that can be used again by plants (nitrification). Denitrifying bacteria, however, break down the nitrogen compounds completely into N
2
(denitrification). This becomes free nitrogen that is released back into the air, and the cycle starts again.

1. What are three important cycles of matter that an ecosystem must have to survive? the _________________________ , the ___________________________________, and the _______________________________________.
2. Another name for the water cycle is the ______________________ cycle.
3. Identify each process labeled in the diagram of the water cycle. a.______________________________________ b.______________________________________ c._______________________________________ d._______________________________________ e. ______________________________________
4. What is the source of energy for the process of evaporation? _________________________
5. What chemical can be found in the bodies of all living things? _________________________
6. How do producers use carbon dioxide? (Name the process)___________________________________________
7. How does the carbon cycle help give energy to the organisms in the ecosystem? DO NOT OMIT!!!
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8. How is oxygen returned to the atmosphere? _________________________________________________________
9. By what process do organisms return carbon dioxide to the ecosystem? ___________________________________
10. Another building block in the matter that makes you living things is _____________________.
11. How much of our air is nitrogen? _________________. What kind is it? ________________________
12. What is the problem with “free” nitrogen? __________________________________________________
13. True or False? Lightning can “fix” N
2
. (Circle your answer.)
13. What process makes nitrogen usable? _____________________________________
14.a. What organism is responsible for “fixed” nitrogen?__________________ b. Where can it be found? ______________________________ c. What happens to it next? ____________________
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More questions on back……..

15. Nitrogen is needed to build ______________________ in organisms.
16. How is nitrogen returned to the environment? ____________________________________________________
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17. What happens in nitrification? ______________________________________________________________
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18. What is denitrification? ____________________________________________________________________
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