Page 2 - 2. Lesson notes- Bio- geo chemical cycles-1

The stages involved in a complete water cycle are:

Stage I: Evaporation and Transpiration-The sun’s energy heats up the lakes, rivers, oceans,
swamps and other water bodies to evaporate as vapor into the air. The rising air currents take

the vapor up in the sky. Simultaneously, plants and trees also lose water to the atmosphere in

the form of vapor (through transpiration) which rises up in the sky.
Stage II: Condensation-As the vapors rise high, the cooler temperatures make them cool down

and turn back into liquid –condensation. Wind and air currents move the moisture around,
leading to the formation of clouds.

Stage III: Precipitation- Wind movements cause the clouds particles to collide. As they become
water laden, they develop into rain bearing clouds and fall back onto the earth’s surface by the

process known as precipitation. This may occur in the form of rain, hail, snow or sleet depending

upon the temperature conditions.
Stage IV: Runoff and Infiltration -The precipitation either runs off into oceans, rivers and ground

surface or is absorbed into the soil (infiltration).
Nitrogen cycle :-

“Nitrogen Cycle is a biogeochemical process which transforms the inert nitrogen present
in the atmosphere to a more usable form for living organisms.”

Important note: However, the abundant nitrogen in the atmosphere cannot be used directly by

plants or animals.

Importance of nitrogen to animals.

1.Nitrogen is an important component of all Amino acids which forms proteins required for
growth and metabolism in living organisms.

2. Nitrogen is an important component of all types of nucleic acids found in the

living organisms i.e. DNA and RNA.
Types of Nitrogen Fixation :

a) Atmospheric fixation: A natural phenomenon where the energy of lightning breaks the
nitrogen into nitrogen oxides and is then used by plants.

b) Industrial nitrogen fixation: Is a man-made alternative that aids in nitrogen fixation by the use

of ammonia. Ammonia is produced by the direct combination of nitrogen and hydrogen, and
later, it is converted into various fertilizers such as urea.

c) Biological nitrogen fixation: We already know that nitrogen is not usable directly from the air
for plants and animals. Bacteria like Rhizobium, Azotobacter and blue-green algae (Nostoc)