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Chemical Cycles
What is a cycle?
 cy·cle
n. A
periodically
repeated
sequence of
events.
Why are chemical cycles
necessary for life to exist?
1.
2.
Nutrients such as carbon, oxygen, and
nitrogen are needed for organisms to live
and grow.
These elements, along with all others,
are not constantly replenished from an
outside source.

Remember that the energy cycle has a constant
input of energy from the sun.
The three cycles
1.
2.
3.
The carbon cycle—the movement of
carbon through the environment.
The nitrogen cycle—the pathway by
which nitrogen moves through the
environment.
The water cycle—the continual
movement of water from the Earth’s
surface to the atmosphere and back to
the surface again.
The Carbon Cycle



Carbon is the building block
atom of all organic compounds.
Carbon-based compounds
move through air or water,
through living things, and back
to air or water again.
The driving forces behind the
carbon cycle are:
1.
2.
photosynthesis
and cellular respiration
Photosynthesis
CO2 + H20 + energy → C6H1206 + O2


The process by which
autotrophs convert
sunlight into a
useable form of
energy (glucose).
Occurs in the
chloroplasts of plant
cells (an organelle).
Cellular Respiration
C6H12O6 + O2 → CO2 + H20 + energy



res·pi·ra·tion n. The act
or process of inhaling and
exhaling; breathing.
Cellular respiration is the
process by which glucose
is broken down to release
energy.
Occurs in in the
mitochondria of ALL cells
(another organelle).

Photosynthesis—the construction of
glucose to store energy.
CO2 + H20 + energy → C6H1206 + O2

Cellular Respiration—the breakage of
glucose to release energy
C6H12O6 + O2 → CO2 + H20 + energy
Humans and the carbon cycle

Humans have begun to interfere with the
carbon cycle by:
1.
2.
Cutting down forests
Burning fossil fuels (energy-rich organic
materials that are formed from geological
processes: coal, gasoline, oil, etc.)
The Nitrogen Cycle


Nitrogen is needed to make amino acids,
proteins, and other necessary
substances.
It moves through atmosphere and soil,
through living things, and back to the
atmosphere and soil again.
Composition of the atmosphere

Nitrogen makes up
almost 80% of the air
you breathe every
day!

The rest is made up
of O2, Ar, CO2, and
other traces of gases.
Two important N2-cycle processes
1. Nitrogen fixation—
bacteria in the soil
or the root nodules
of legumes convert
N2 that is present in
the atmosphere to
ammonium ions that
plants can take up
and use.
Two important N2-cycle processes
2. Denitrification—
another group of
bacteria convert some
of the ammonium and
nitrates in the soil into
free nitrogen gas.
Humans and the N2 cycle


Nitrogen compounds dissolve
easily in water, so are lost with
erosion (the result of
deforestation).
Crop plants like corn and cotton
absorb nitrogen excessively—
causing farmers to use nitrogen
fertilizers, which are very
expensive to make (and pollute
the environment during the
process).
The Water Cycle



Water is necessary for all life processes.
Water cycles from the atmosphere, to the
Earth, through organisms, and back to
the atmosphere.
The two driving forces of the water cycle
are:
1.
2.
Solar energy
Gravity
Definitions (write down what you
don’t know)
 Evaporation—liquid
changing to a
gas.
 Transpiration—evaporation from the
leaves of trees.
 Condensation—gas changing to a
liquid.
 Precipitation—rain, snow, sleet, and
hail.
The steps of the water cycle
1.
2.
3.
4.
Water evaporates or transpires and rises in the
atmosphere (this requires energy from the
sun).
As it rises, it cools and condenses to form
clouds.
The droplets of the clouds eventually get big
enough and fall back to the surface of Earth by
gravity.
The water is absorbed and collected in lakes,
streams, ponds, and oceans—the cycle begins
all over again.
Humans and the water cycle



Chemicals, sewage, trash,
and toxic wastes find their
way into rivers, ponds, and
lakes all over the world.
These eventually travel
underground and
contaminate our
groundwater—the source of
all our fresh drinking water.
Deforestation also prevents
absorption of rainfall making
land susceptible to flooding
and erosion.
To Review


Nutrients such as carbon, oxygen, and
nitrogen are needed for organisms to live and
grow.
These elements, along with all others, are not
constantly replenished from an outside source.
1.
2.
3.
The carbon cycle—the movement of carbon through
the environment.
The nitrogen cycle—the pathway by which nitrogen
moves through the environment.
The water cycle—the continual movement of water
from the Earth’s surface to the atmosphere and
back to the surface again.