I think of this practical activity as "Plants v Snails", but the Nuffield Foundation calls it "Investigate the rate of respiration in living organisms ".
Angie is holding three test tubes containing water and hydrogen carbonate indicator, which changes colour according to the amount of carbon dioxide dissolved in water. The lighter ones have had exhaled air bubbled through them, so their carbon dioxide concentration is higher as CO2 is partly soluble. The darker one had only room air bubbled through it. Hydrogen carbonate indicator is probably the most important indicator you need to know about for biology at this level, so it's a bonus that it gives pretty results.
We used an oxygenating aquarium plant, Cabomba, and some aquarium snails. We could only find tiny snails as the aquarium fish had been eating the indoors ones. The pond was searched but it was so silted up that no live snails were found, so we had to make do with the mini snails!
Two test tubes contained snails only, two had plants only, two both, and two controls had neither. We added some hydrogen carbonate indicator. This did not harm the snails.
Two test tubes contained snails only, two had plants only, two both, and two controls had neither. We added some hydrogen carbonate indicator. This did not harm the snails.
One of each type of test tube was put in a dark box, and one in the light. Each set was checked at intervals and the colours of the liquids compared to assess the carbon dioxide concentration.
The tubes in the light containing plants started to change colour around the base of the plants, showing decreased carbon dioxide concentration.
This effect intensified as the plants were exposed to light for longer.
When we took the yellow rack of tubes out of the dark box after a few hours, there was no noticeable change in the colour of the liquid. No pink was visible, only yellow. This suggested that the plants had not been photosynthesising.
After a few days, the experiment was ended and the following results were seen:
After a few days, the experiment was ended and the following results were seen:
The tubes have been arranged in order of colour, from the palest yellow on the left to the darkest pink on the right. This corresponds to carbon dioxide concentration - from highest to lowest. The plants which had been kept in the dark had respired - given off carbon dioxide - and had not photosynthesised. The plants which had been in the light had used up carbon dioxide in photosynthesis and the water had turned a rich pink as a result of the hydrogencarbonate indicator.
Our snails did not appear to make much difference. We think this is because they were too tiny to have a noticeable effect; we coudn't find any bigger ones. The tubes containing snails only may, perhaps, have been marginally paler yellow than the control tubes, but it's hard to say.
In this battle of Plants v Snails, plants won!
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