Carbon and Oxygen Cycle
** Carbon dioxide and oxygen are recycled during respiration and
photosynthesis. Carbon dioxide is released by living organisms during respiration
and taken in by plants for photosynthesis. During photosynthesis, plants release
oxygen as a waste product. Oxygen enters the air to be used again by living
organisms during respiration.
Water Cycle
** The sun heats the water on the Earth’s surface causing it to evaporate. Plants
release water vapor during transpiration and animals release water when they
breathe and release waste. The water vapor rises into the atmosphere where it
cools and condenses, forming water droplets. The water droplets condense,
forming clouds. The water falls back to the Earth’s surface as precipitation. Much
of the precipitation falls back into the bodies of water, but some falls onto the land
and runs off into the bodies of water or soaks into the ground becoming ground
water.
Nitrogen Cycle
** All living things need nitrogen to build proteins and certain other body
chemicals. Most organisms cannot get the nitrogen they need from the free
nitrogen in the air. They can only use nitrogen that is combined with other
elements to make compounds (fixed nitrogen).
** Free nitrogen in the atmosphere is combined with other elements when
lightning occurs. The nitrogen compounds are brought down to the soil by
precipitation. Free nitrogen in the soil or water is combined with other elements
to make nitrogen compounds by nitrogen-fixing bacteria. The nitrogen
compounds are taken up by plant roots and changed into proteins. Animals eat
the plants, taking in proteins. Animals release waste or die. Decomposers break
down the animal waste or decay in nitrogen compounds that are released into the
soil. Denitrifying bacteria in the soil breaks down the nitrogen compounds and
release free nitrogen back into the atmosphere.