teacher’s notes
student’s notes
Carbon cycle: where is
this crucial carbon?
(teacher’s notes)
Level
This material is designed for students aged 12 to 14
(years 8 and 9).
Topic
This activity is one of five aimed to teach students
about the nature of carbon, the different types of
compounds it exists in (eg charcoal, glucose, carbon
dioxide), the biochemical reactions it takes part in
(photosynthesis and respiration), the range of
processes that carbon and carbon compounds are
involved in on Earth, and how these link together form
the carbon cycle.
The other activities are
Carbon cycle in the lab: carbon products and the
processes that link them; which involves working in
small groups to place examples of carbon cycle
products in the correct places on a diagram of the
cycle.
Cycling carbon: seeing how plants use carbon dioxide
in the lab; a short pupil practical exploring the uptake
of carbon dioxide from the atmosphere by plants for
photosynthesis.
Carbon cycle: releasing dinosaur breath in the lab; a
short pupil practical exploring the storage of carbon in
the fossils that make up limestone and chalk.
Carbon cycle: exchanging carbon dioxide between the
atmosphere and ocean; a short pupil practical
comparing how well carbon dioxide dissolves in sea
water compared with fresh water.
Context
An understanding of the carbon cycle is essential to
the debate about global warming, an environmental
issue that most students will have heard about. Since
the Earth’s atmosphere formed, it seems to have
always contained carbon dioxide in varying amounts.
Carbon dioxide is a ‘greenhouse gas’ through which
light radiation can pass but which absorbs some of the
heat radiation produced by light irradiating the Earth’s
surface. This causes the Earth’s surface and
atmosphere to be warmer than it would otherwise be
and without the ‘greenhouse effect’ the Earth would
probably be completely frozen. Humans, as all life on
Earth, have always been part of the carbon cycle, but
now (since the industrial revolution) the large scale
burning of oil, coal and natural gas, along with
deforestation, is leading to increasing atmospheric
carbon dioxide levels. This in turn is related to an
enhanced greenhouse effect and consequent climatic
change. An understanding of the factors affecting
global warming leads to an understanding of the
measures required to reduce their impact. This can link
into economic and political debates on the subject.
Students should have already covered photosynthesis
and respiration in order to do this activity effectively.
Teaching points
Carbon is the fundamental building block of life (the
element that life is based on). It is contained by
everything from fossil fuels to DNA. Despite being very
important, carbon, is in fact not the most abundant
element on Earth. (The prize for abundance goes to
silicon that is, interestingly, similar to carbon in atomic
structure and some key chemical properties). Carbon is
cycled and re-cycled through the carbon cycle. Both
animals and plants need it although they obtain it in
different ways. It is worth revising respiration and
photosynthesis with students. Ensure that they
understand that the charcoal produced in the
demonstration is not the form in which it exists in the
body and that it is just a demonstration of the
presence of carbon in food and other materials.
Apparatus and materials
The teacher will need:
Eye protection
Bunsen burner
Safety screen
Tongs
Tray of sand
Whiteboard or A2 white paper
Lump of coal
Bacon, sausage
White bread, crisps, cereal
Dead twig (not green)
Artist’s charcoal
Safety
The teacher will burn objects over a Bunsen flame to
carbonise them and should wear eye protection. Pupils
should be a suitable distance away, perhaps behind a
safety screen, in case the objects spit as they burn.
Activity
Starting point: What is the connection between you
and a lump of coal?
Main activity: Teacher-led discussion. As it proceeds
and pupils provide answers to questions a record is
kept of their answers on the whiteboard. Suggested
sequence of questions and demonstrations:
Q 1. What do living things need in order to grow? =
Food (Revision of living processes)
Q 2. Where do plants get their food from? = They
make their own from carbon dioxide (CO2) (in the
air/atmosphere) and water (H2O), using energy from
the sun. The food they produce is in the form of sugars
and starch (charcoal). (Revision of photosynthesis).
Q 3. At this stage the teacher holds the twig over the
flames until it is suitably black. Do the same with the
cereal, potato crisp and bread (ensuring that the pupils
remember that these are made of plant matter!). For
each, the teacher asks what is left after burning. =
Carbon. Teacher (or pupil) writes this in on the record,
using the burned objects themselves or possibly, the
artist’s charcoal.
Q 4. Where do animals get their food from? = Eating
plants and/or eating animals that have eaten plants.
(Revision of respiration and energy production).
Q 5. At this stage, the teacher burns the bit of bacon
(or sausage) until it is suitably black. Teacher asks what
is left? = Carbon. Write this on the record.
Q 6. What happens to food in the body? = Digested,
absorbed, incorporated, used up. Revise respiration as
a process used in cells to release energy from food by
reacting with oxygen, then carbon dioxide is exhaled as
a by-product. This reaction is somewhat akin to the
burning just viewed but much more controlled. It is
worth discussing what burning actually is – where does
all the stuff that is not carbon go? What gases are
released? Why are you left with just carbon when you
hold a piece of plant or animal matter in a flame?
Compare this with the reactions that constitute
respiration.
Q 7. Review the record and ask: Where did the
carbon in all these objects come from originally? =
Atmosphere.
Q 8. What happens to carbon? = Incorporated into
living things, passes along food chain, some breathed
out by animals and returns to the atmosphere, some
“locked up” in animals and plants until they die and
decompose, some locked up for an very long time
when plans and animals are fossilised and forms rocks
like coal and also the widespread calcium carbonate-
rich rock, limestone.
Points to bring out
It could be mentioned that pupils themselves are
carbon recyclers! However, teachers beware that this
might involve discussion about burial versus
cremation!