teacher’s notes
student’s notes
Cycling carbon: seeing
how plants use carbon
dioxide in the lab
(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 to form the carbon cycle.
The other activities are
Carbon cycle: where is this crucial carbon?; a
teacher-led discussion interspersed with
demonstrations in which the teacher burns a
range of materials over a Bunsen flame, forming
charcoal, to illustrate that they contain carbon.
Carbon cycle in the lab: carbon products and the
processes that link them; which involves working
in small groups to place examples of the carbon
cycle products in the correct places on a diagram
of the cycle.
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
This activity demonstrates the uptake of carbon
dioxide by plants, using Elodea as the example.
Elodea is a pond plant that lives below the water
surface and thus extracts dissolved carbon
dioxide from the water rather than directly from
the atmosphere as terrestrial plants do.
Carbon exchange between living things and the
atmosphere mostly happens through
photosynthesis and respiration. During the
growing season leaves take up carbon dioxide.
Carbon is stored in the living biomass.
Students should work in pairs for this activity.
Apparatus and
materials
Each group of pupils will need:
3 boiling tubes
A drinking straw
Boiled water
Phenol red indicator (which is red and goes
yellow in the presence of carbon dioxide)
A sprig of Elodea
Bright light
Carbon cycle diagram (Figure 1)
Safety
Pupils should blow through the straw and not
suck. Only one person in the group should use
the straw. Dispose of the straws at the end of the
activity.
Activity
Pour about 2- 3 cm depth of water into each
boiling tube (the same depth in each).
Add a few drops of indicator to each.
Breathe out gently through the straw into two
of the tubes until the indicator colour changes to
yellow.
Put the sprig of Elodea into one tube.
Place all three in bright light and leave them
for about 40 minutes.
After this time (during which the students could
be doing another related activity), they should
observe the boiling tubes’ contents and record
their observations by answering the following
questions:
Q 1. What happened to the indicator in the
tube containing Elodea? = Went back to red.
Q 2. What does this mean? = CO2
gone/decreased.
Q 3. How do you know it was due to the
Elodea? = Because there was no change in other
tube.
Q 4. What caused this to happen? =
Photosynthesising Elodea took in CO2.
Q 5. Shade in the stage of the carbon cycle on
the diagram that this relates to.
Q 6. State two ways in which this carbon can
be returned to the atmosphere. = Decay of plant
when dead, respiration by plant. Animal eating
plant and respiring/ decaying.