How Plants Use CO2


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.

Materials:(Each group of pupils will need)

• 3 test tubes

• a drinking straw for each student

• distilled water

• Phenol red indicator (goes from red to yellow in presence of CO2)

• sprig of Elodea


• Pour about 2 cm depth of water into each test tube

• Add a few drops of indicator to each.

• Breathe out gently through the straw into two of the tubes until the indicator color changes to yellow.

• Put the sprig of Elodea into one tube.

• Place all three test tubes in bright light and leave them for about 40 minutes.


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?

A   Photosynthesizing Elodea took in C

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.


Pupils should blow through the straw and not suck. Dispose of the straws at the end of the activity.

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