1. Cycles in an Ecosystem Chapter 25.3

2. Cycles All organisms need certain substances to live, grow, and reproduce.

3. Many substances, such as water, pass through natural cycles in which they are used and reused.

4. Some cycles include nonliving things. Many of the chemicals move in and out of the living realm.

5. The Water Cycle Water moves continually throughout the earth by means of the water cycle.

6. evaporation Energy from the sun changes water from a liquid to a gas through the process of evaporation.

7. condensation water vapor changes back to a liquid by the process of condensation.

8. precipitation The condensed water forms clouds, steam, or fog, and eventually falls back to the earth as precipitation.

9. The Nitrogen Cycle Nearly 80 percent of the earth's atmosphere is made up of nitrogen. Changing atmospheric nitrogen into usable compounds is called nitrogen fixation.

10. The Oxygen-Carbon Dioxide Cycle When you inhale, you take in air that contains oxygen. When you exhale, you release carbon dioxide back into the atmosphere.

11. Another way in which carbon dioxide enters the atmosphere and becomes part of the cycle is through the decay of dead organisms.

12. Sometimes the bodies of dead organisms do not decompose.