Investigating Gas Exchange: Extended (CIE IGCSE Biology: Co-ordinated Sciences (Double Award))

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: