The Water Cycle

Title of Paper : Cycles
Grade Received on Report : 100

The movement of water from the atmosphere to the Earth and back to the atmosphere is called the
water cycle. The water cycle consists of an alternation of evaporation and condensation. Water molecules
enter the air by evaporation from the ocean and other bodies of water. In the air, the water molecules
condense (in clouds) and then return to the Earth in the form of precipitation (rain). On land, most of the
rainwater runs along the surface of the ground until it enters a river or stream that carries it to a larger body
of water. Some water sinks into the ground and is called ground water. The upper surface of ground water
is known as the water table.

The Nitrogen Cycle

All organisms require nitrogen to build proteins. Nitrogen is available to organisms in several
ways. Free nitrogen gas makes up 78 percent of the atmosphere. Nitrogen is also found in the wastes
produced by many organisms and in dead and decaying organisms. The movement of nitrogen through the
biosphere is called the nitrogen cycle. However, most of this nitrogen cannot be directly used by living
things. It must be converted into other forms.
Certain bacteria that live on roots of plants such as legumes (beans, peas, and peanuts) change free
nitrogen atmosphere into nitrogen compounds (nitrates and nitrites) that can be used by living things. this
process is known as nitrogen fixation, and the bacteria are called nitrifying bacteria. Once the nitrogen
compounds are available, plants use them to make plant proteins. Animals then eat the plants and use the
proteins to make animal proteins. When the plants and animals die, the nitrogen compounds return to the
Eventually other bacteria in the soil break down these nitrogen compounds into free nitrogen in a
process called denitrification. These bacteria are called denitrifying bacteria. Through the process of
denitrification, free nitrogen is returned to the atmosphere.

Carbon and Oxygen Cycles

The process by which carbon is moved through the environment is called the carbon cycle.
During photosynthesis, green plants and algae use carbon dioxide from the atmosphere to form glucose.
Consumers and decomposers use glucose in respiration, during which they produce carbon dioxide.
Carbon dioxide is then released into the atmosphere, completing the carbon cycle.
The movement of oxygen through the environment is called the oxygen cycle. During
photosynthesis, water molecules are split, releasing oxygen into the atmosphere. The oxygen is used by
most organisms for respiration. During respiration, water is released. The water is absorbed by plants, and
the cycle begins again.