Gas exchange experiments

Hydrogencarbonate indicator is used to show carbon dioxide concentration in solution. The table shows the colour that the indicator turns at different levels of carbon dioxide concentration.

ConcentrationIndicator turns
Atmospheric levelRed

A leaf is placed in a stoppered boiling tube containing some hydrogen carbonate indicator solution. The effect of light intensity can then be investigated.

The table shows some typical results.

Tube 1Tube 2Tube 3Tube 4
Light turned on
Paper on tubeBlack paperTissue paperNoneNone
LeafLivingLivingLivingDead (boiled)
Indicator colour at the endYellowMagentaPurpleRed
Carbon dioxide concentrationHighestLowLowestAtmospheric level
  • Tube 4 was a control. The results in tubes 3 and 4 show that the leaf has to be alive for the carbon dioxide concentration to change.
  • Tubes 1, 2 and 3 show the effect of increasing the light intensity. The black paper stopped light reaching the leaf in tube 1, so only respiration could happen.
  • The tissue paper stopped some of the light reaching the leaf in tube 2, and the leaf in tube 3 received the most light.
  • Photosynthesis happened as well as respiration in tubes 2 and 3, so there was a net absorption of carbon dioxide.
  • The rate of photosynthesis was greatest in the leaf in tube 3, and it had the greatest net absorption of carbon dioxide.
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