Investigating Gas Exchange: Extended (CIE IGCSE Biology: Co-ordinated Sciences (Double Award))
Revision Note
Investigating Gas Exchange: Extended
Extended Tier Only
- In order to carry out respiration and photosynthesis plants must carry out gas exchange with their environment
- Plants respire all the time; for this they need to take in oxygen and release carbon dioxide
- Plants photosynthesise during daylight hours, for which they need to take in carbon dioxide and release oxygen
Plants carry out respiration and photosynthesis
- During the day plants photosynthesise at a faster rate than they respire so the overall direction of gas exchange is:
- Carbon dioxide diffuses out of leaves
- Oxygen diffuses in
- At night plants respire at a faster rate than they photosynthesise so the overall direction of gas exchange is different:
- Oxygen diffuses into leaves
- Carbon dioxide diffuses out
- The overall direction of movement of a gas is described as its net movement
Investigating gas exchange in plants
- We can investigate the effect of light on the net gas exchange in an aquatic plant using a pH indicator such as hydrogencarbonate
- This is possible because carbon dioxide is an acidic gas when dissolved in water
- Hydrogencarbonate indicator shows the carbon dioxide concentration in solution:
- Several leaves from the same plant are placed in stoppered boiling tubes containing hydrogencarbonate indicator
- The effect of light can then be investigated over a period of a few hours
- Results from a typical experiment are shown in the table below:
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