ENVIRNONMENTAL SCIENCE A

3.2  THE CYCLING OF MATERIALS

• Materials in ecosystems are endlessly recycled by natural processes.
• Carbon, water, and nitrogen are three materials essential for life, and each follows a recognizable cycle.

Carbon Cycle

• Plants take in carbon from the atmosphere during photosynthesis; organisms then eat the plants and release the carbon to the atmospere by cellular respiration.
• Producers take in carbon dioxide from the air and use it to make simple sugars during a process called photosynthesis.  Many sugar molecules can then be linked to fom starches, which the plant uses to store energy.  The energy and carbon in the starches are used by the animals that eat the plants.
• Humans add carbon to the atmosphere by burning large amounts fo fossil fuels.  The increasing population also increases carbon dioxide through the process of cellular respiration.  Also by cutting down large amounts of forests are decreasing plants that take out CO₂ out of the atmosphere.  All these activities may contribute to global warming.
• Animals exhale CO₂ into the atmospehre.

Nitrogen Cycle

• The atmosphere is 78% nitrogen
• Lighning fixes some nitrogen with then falls to the Earth
• Nitrogen-fixing bacteria in soil and root nodules (legume plants) produce ammonia
• bacteria convert ammonia into nitrates, which plants can use
• When animals die or deposit urine or dung, the nirogen they obtained from eating plants or other animals is converted by bacteria into ammonia.  
• Some ammonia is abosrbed by the soil, and some is released into the atmosphere as nitrogen gas.

Water Cycle

• The sun drives the water cycle.
•  In the water cycle, evaportation releases water vapor into the air.  The vapor condenses into liquid vapor at cooler temperatures in the atmospehere, the liquid then falls to earth as percipitaiton.