How can we show that plants use carbon dioxide in photosynthesis?

1. How can we show that plants use carbon dioxide in photosynthesis?

2. Hydrogencarbonate Indicator Decreasing CO2in indicator Increasing CO2 in indicator Atmospheric CO2 level Hydrogencarbonate indicator is used to measure carbon dioxide levels in aquatic systems. It is red in equilibrium with atmospheric air. It becomes more orange/yellow with increased carbon dioxide levels. It changes from red through magenta to deep purple as carbon dioxide is removed.

3. Method • Take four transparent containers that can be easily sealed. • Rinse the containers with a small amount of indicator. • Add a standard volume of indicator to each container and in three of the containers, place equal length sprigs of pondweed e.g. Cabomba • Seal all the containers. • Place one with pondweed into a dark cupboard. • Take another containing pondweed and cover it with shading. • Place the three remaining containers (including the one with no plant in), the same distance from a bright white light source and leave until there is a noticeable change in colour. • Make sure the lamp will not cause your pond water to overheat. You might want to place a transparent screen between the light and the plant, or a flat sided container of cold water to absorb the heat but none of the light. • When there is a noticeable change in colour in the indicator, remove the shading and compare the colours in the four containers to those in the picture.

4. Table of results

5. Questions • Describe your observations • Link these observations with what you know about photosynthesis? • Do they confirm what you set out to show in this experiment? • What happened to the indicator when the pondweed was placed in the dark? • What do you deduce from this? • Can you think of any improvements to the method which you would carry out if you had the time to repeat this experiment?

6. Important Information: Green plants and algae use up carbon dioxide – removing it from the indicator as they carry out photosynthesis. BUT.. they also produce carbon dioxide as they respire – all living things respire all the time • What colour would the indicator go if you breathed out into it? • What colour would the indicator go if the plant is respiring and carrying out photosynthesis at the same rate? Why? • Select the two times of the day that this is most likely to happen? NIGHT, DAWN, MID MORNING, LUNCHTIME, AFTERNOON, DUSK.

7. Result in bright light

8. Result with thin shading

9. Result in dark