1. Cycles in Nature
Chapter 25, Section 2

2. Cycles of Matter
The biosphere contains a fixed amount of water, carbon, nitrogen, oxygen, and other materials required for life.
These materials are cycled through the biosphere and used by many different organisms.

3. Water Cycle
A model that describes how water moves from the surface of the earth to the atmosphere and back to the surface again.

4. Water Cycle Processes that “add” water to the air: Evaporation
Occurs when liquid water turns into water vapor (gas).
Transpiration
Occurs when plants release water into the air.
Respiration
Releases water vapor to the air.
Animal Wastes
Returns water to the environment.

5. Water Cycle Processes that “subtract” water from the air. Condensation
Occurs when water vapor changes to a liquid.
Clouds
When water vapor cools, it condenses on tiny particles of matter (like dust) in the air and forms tiny droplets.
Precipitation
Eventually the “cloud” droplets become so heavy that they fall to the earth as precipitation.

6. Water Cycle Affects on the cycle by humans:
Water use by humans (households, factories, agriculture, etc) can reduce the amount of water that evaporates into the atmosphere and limits the amount of water available to plants and animals.

7. Nitrogen Cycle Nitrogen Cycle
A model that describes how nitrogen moves from the atmosphere to the soil to living organisms and back again.

8. Nitrogen Cycle Importance of nitrogen
Needed to build proteins (building blocks of cells).
Essential part of DNA.

9. Nitrogen Cycle Processes that remove nitrogen from the atmosphere:
Nitrogen fixation:
Plants cannot use nitrogen directly from the air.
So, bacteria in soil convert the nitrogen from the air into compounds that plants can use.
Animals get the nitrogen they need by eating plants.

10. Nitrogen Cycle Processes that return nitrogen to the soil:
Animal wastes.
Dead and decaying organisms.

11. Nitrogen Cycle Affects on the cycle by humans:
Removing crops from the land removes any nitrogen that would be added to the soil through the decay of the plants.
Adding fertilizer, compost, or animal manure returns nitrogen to the soil.
Planting nitrogen-fixing crops (such as peas, clover, and beans) adds nitrogen to the soil.
Nitrogen-fixing crops have roots with nodules that contain nitrogen-fixing bacteria.

12. Carbon Cycle Carbon Cycle
A model that describes how carbon moves from living to non-living things.

13. Carbon Cycle Carbon Cycle
Carbon is located in humus in the soil and as part of carbon dioxide in the atmosphere.

14. Carbon Cycle Processes that remove carbon from the atmosphere:
Photosynthesis
Uses carbon dioxide, water, and sunlight to produce sugars.

15. Carbon Cycle Processes that add carbon to the atmosphere: Respiration
Uses oxygen and releases carbon dioxide.

16. Carbon Cycle Affects on the cycle by humans:
Carbon dioxide is released into the air when humans burn fossil fuels (gas, coal, oil, etc), which are the remains of once-living organisms.
Use of trees for fuel or for building reduces the number of plants that remove carbon dioxide from the air.