Hydrogen-carbonate indicator changes colour with different levels of CO2. In little CO2, it goes purple, in normal CO2, it's orange and in lots of CO2, it goes yellow. We need to know an experiment to show how light effects gas exchange, heres how its done...
Method
- Add the same volume pf hydrogen-carbonate indicator to four test tubes.
- Put a similar sized (healthy) leaf in 3 of the tubes and seal all four with a bung - trap the leaf stem in the bung to ensure it doesn't fall in the indicator. NOTE: the empty test tube is a control.
- Wrap one tube in aluminium foil (this ensures no light will get to that leaf - meaning photosynthesis can't occur)
- Wrap one tube in gauze (this means it has some light, but not full light - slowing the rate of photosynthesis)
- Place all four tubes in bright light
- Leave tubes for 1 hour
- Record the colour of the indicator
Conclusion
If all went well, there shouldn't be any colour change in the control tube, as no photosynthesis (so no gas exchange) occurred there. In the tube with aluminium foil around it, the hydrogen-carbonate indicator will be dark yellow. This is because it was not able to photosynthesise but respiration still took place, meaning the CO2 level in the tube will increase (as CO2 has been given out by respiration, but not taken in by photosynthesis). There will be no change as a little photosynthesis and respiration have taken place, so the CO2 level won't have changed much. The tube with access to full light will have gone purple. This is because the level of CO2 would have decreased as there will be some respiration, but lots of photosynthesis, meaning lots of CO2 was taken in by the leaf (but not much given out).
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