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Cycles in Nature
Chapter 25, Section 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.
Water Cycle

A model that describes how water moves from
the surface of the earth to the atmosphere and
back to the surface again.
Water Cycle

Processes that “add” water to the air:

Evaporation


Transpiration


Occurs when plants release water into the air.
Respiration


Occurs when liquid water turns into water vapor (gas).
Releases water vapor to the air.
Animal Wastes

Returns water to the environment.
Water Cycle

Processes that “subtract” water from the air.

Condensation


Clouds


Occurs when water vapor changes to a liquid.
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.
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.
Nitrogen Cycle

Nitrogen Cycle

A model that describes how nitrogen
moves from the atmosphere to the
soil to living organisms and back
again.
Nitrogen Cycle

Importance of
nitrogen


Needed to build proteins (building
blocks of cells).
Essential part of DNA.
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.
Nitrogen Cycle

Processes that
return nitrogen
to the soil:


Animal wastes.
Dead and decaying organisms.
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.
Carbon Cycle

Carbon Cycle

A model that describes how carbon
moves from living to non-living
things.
Carbon Cycle

Carbon Cycle

Carbon is located in humus in the
soil and as part of carbon dioxide in
the atmosphere.
Carbon Cycle

Processes that
remove carbon
from the
atmosphere:

Photosynthesis

Uses carbon dioxide, water, and
sunlight to produce sugars.
Carbon Cycle

Processes that
add carbon to
the atmosphere:

Respiration

Uses oxygen and releases carbon
dioxide.
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.