Investigating Gas Exchange (Cambridge International IGCSE Science)

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 into the leaves
  • Oxygen diffuses out

• 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: