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THE CARBON CYCLE
Cycles of Matter
Matter is recycled in ecosystems. Matter includes water, oxygen, carbon, nitrogen and
many other substances. Three of the most important cycles of matter are the water
cycle, carbon cycle and nitrogen cycle.
1. The Water Cycle
Water on Earth is never created or destroyed – it is only changed from one form to
another over time in a the water cycle. The water cycle is a continuous process by which
water moves from Earth’s surface to the atmosphere and back again. The energy of the Sun
drives the water cycle. The four processes of evaporation, condensation, precipitation,
and runoff make up the water cycle.
Evaporation is the process by which molecules of liquid water absorb heat energy and
change into gas. Water evaporates from the Earth’s surface and forms water vapor in the
atmosphere.
The process by which a gas changes to a liquid is called condensation. When water
vapor in the atmosphere cools, it turns back into tiny droplets of liquid water. As more and
more water vapor condenses, the drops grow larger and heavier. Eventually, they heavy drops
fall back Earth as a form of precipitation – rain, snow, sleet, or hail.
The precipitation that falls to Earth ultimately flows over the ground and into a body of
water. This is called runoff. Runoff flows into streams and rivers and eventually back to the
ocean. The runoff can also soak into the ground where it might be stored as groundwater
between particles of soil, sand, or rock. Groundwater moves slowly, but it eventually flows
back to the ocean. When water returns to the ocean, one turn of the water cycle is complete.
2. The Carbon Cycle
Carbon is the building block of life. It is present in all organic (living) things and in other
inorganic (nonliving) things such as coal and oil that came from once-living things. You may
think of carbon as a solid, but it can also occur in the form gases like methane (CH4) and
carbon dioxide (CO2). The Earth’s carbon is constantly changing from one form to another.
The carbon cycle is a process where carbon atoms circulate in the environment. The carbon
cycle involves carbon sources and carbon sinks.
Carbon Sources
Carbon sources release carbon into the atmosphere. Carbon can be released in
several ways. Living things, such as animals, breathe out CO2 when they perform aerobic
respiration. Decomposers, organisms that break down dead organic material, give off CO2 in
the process of decomposition. When carbon-based materials burn, like when trees burn in a
forest fire, they release it CO2 by the process of combustion. Carbon also enters the
atmosphere from nonliving things: volcanic eruptions release CO2 from inside Earth and
burning fossil fuels releases CO2 and CH4 through combustion.
Carbon Sinks
Carbon sinks take in and store carbon. Carbon can be removed from the
atmosphere in several ways. Plants remove CO2 during photosynthesis. The carbon the
plants take in is stored in their tissues as carbohydrates, such as sugars and starches. The
carbohydrates are then passed on to animals that consume the plants. Algae and
phytoplankton are tiny oceanic plants that also undergo photosynthesis in the sea.
This means that carbon is stored inside living organisms as long as they are alive. When
organisms die, their bodies are decomposed by bacteria. As mentioned earlier, decomposition
releases CO2 back into the atmosphere. In addition, the motion of oceanic waves dissolves
CO2 in seawater. There, the dissolved CO2 is turned into carbonate compounds that form a
major part of sea shells.
The death of living things can also store carbon. If a plant or animal dies and falls into
a low-oxygen environment, such as marshland, the carbon in its tissues can turn into fossil
fuels over millions of years. The fossil fuels remain in the ground for long periods of time, until
humans remove and burn them for energy, releasing the carbon back into the atmosphere.
When the tiny oceanic plants die, their bodies sink to the bottom of the sea, taking with
them the carbon stored in their cells. Sediments cover the dead organisms and the carbon in
them is stored for long periods of time. When shelled organisms sink to the bottom of the sea,
their bodies sometimes turned into rocks such as chalk and limestone. This also stores
carbon for long periods of time.
REVIEW QUESTIONS – THE CARBON CYCLE
1. The continuous process by which water moves from Earth’s surface to the atmosphere and
back again is called ____________________________.
2. Match the term with the definition.
a. _____ evaporation
X. Process by which liquid water changes to water vapor
b. _____ condensation
Y. Forms of water that fall from clouds
c. _____ precipitation
Z. Process by which water vapor turns to liquid water.
3. List four forms of precipitation: ________________________________________________.
4. Circle the letter that is true about the carbon cycle.
a.
b.
c.
d.
Producers take in oxygen during photosynthesis.
Producers release carbon dioxide as a result of photosynthesis.
Consumers release carbon dioxide as a waste product of respiration.
Consumers take in oxygen for respiration.
5. Use the word bank to fill in the blanks to summarize the biological parts of the carbon cycle.
Word Bank (some terms may be used more than once)
photosynthesis
respiration
decomposition
consuming
fossilization
fossil fuels
combustion
death
1.
6.
2.
3.
5.
4.
dissolving
6. Use the word bank to fill in the blanks of the non-biological parts of the carbon cycle.
Word Bank (some terms may be used more than once)
photosynthesis
respiration
decomposition
consuming
fossilization
fossil fuels
combustion
death
4.
dissolving
3.
2.
6.
7.
1.
11.
5.
8.
9.
10.