Unit 3: Matter & Energy Flow on
Earth
Objectives:
E2.3c - Explain how the nitrogen cycle is part of the Earth system.
E2.3d - Explain how carbon moves through the Earth system and
how it may benefit or harm society.
Nitrogen Cycling
Nitrogen is an important element that is part of the DNA
molecule, which is responsible for the replication of living
cells. Nitrogen is also an important component of protein
molecules.
Approximately 5% of the human body is composed of
nitrogen. The source of nitrogen for the world’s
ecosystems is the atmosphere. Earth’s atmosphere is
composed of 78% nitrogen. The nitrogen gas that fills
the atmosphere is in a diatomic form (N 2), which means it
is composed of two atoms of nitrogen bonded together.
Nitrogen Cycle*
The Nitrogen Cycle begins
when microscopic bacteria
located in the soil and in the
root systems of specialized
plants combine atmospheric
nitrogen with other atoms to
form nitrogen compounds.
This process is called
nitrification, and it produces
nitrogen-containing molecules
such as nitrites, nitrates and
ammonia.
Nitrogen Cycle
Nitrogen compounds can then
be taken up by the root system
of plants to be used to make
proteins. The plants that
harbor these specialized
bacteria in their root systems
are called legumes. Common
legumes include clover, beans,
peanuts, and alfalfa. Legumes
are often used in agriculture to
add nitrogen to the soil, and
they act as natural fertilizers.
Carbon Cycling
Carbon molecules form the base for
all life (at least here on Earth),
making the availability of carbon to
living things essential. Most carbon
that finds its way into living
organisms comes from the
atmosphere.
Approximately 19% of the human
body is composed of the element
carbon. All of this carbon was
derived from eating food.
Carbon Cycle*
Carbon dioxide gas (CO2) present
in the Earth’s atmosphere is used
by green plants and algae for
photosynthesis. Photosynthesis
uses solar energy to take carbon
dioxide from the atmosphere and
combines it with water to form
sugars.
A common form of sugar that is a
product of photosynthesis is
glucose (C6H12O6).
Carbon dioxide + water + sunlight = sugar + oxygen
Carbon Cycle*
Respiration breaks down
sugars and starches in the
presence of oxygen and
produces carbon dioxide as a
by-product. Every time
humans and animals exhale air
out of their lungs, they are
removing carbon dioxide from
their bodies and returning it to
the atmosphere.
Carbon Cycle*
Another path carbon can take
in the food chain is to be
eliminated from an organism in
its waste. The waste becomes
part of the nonliving soil
environment.
Once there, the organic
molecules that are in waste
can be used by decomposers
such as fungi or bacteria.
Eventually this carbon is also
returned to the atmosphere as
carbon dioxide.
Carbon Cycle
Sometimes the bodies of plants
and animals are buried deep in
the Earth for millions of years.
These carbon-containing
organisms are then exposed to
the great heat and pressure
below the Earth’s surface. This
causes chemical changes that
transform the long-dead
organisms into what we call
fossil fuels.
Common fossil fuels include oil
and coal.
Carbon Cycle
When fossil fuels are burned, the
carbon that was locked in the
Earth for millions of years is
released back into the
atmosphere as carbon dioxide
gas.
In the past 100 years, the
burning of fossil fuels as an
energy source is causing an
imbalance of carbon dioxide in
the Earth’s atmosphere.
Carbon Cycle
Part of the carbon cycle also
involves the creation of
methane gas (CH4). Methane
is a molecule produced by
anaerobic bacteria. Anaerobic
means without oxygen.
Anaerobic bacteria survive only
without the presence of
oxygen.
Examples include methane gas
that is created as a by-product
as cows and sheep digest tough
grasses, from the mud of rice
paddies found in Asia, and from
human sewage and garbage
waste.