Investigating Gas Exchange (Cambridge (CIE) IGCSE Combined Science): Revision Note

Investigating Gas Exchange: Extended

• Plants are respiring all the time and so plant cells are taking in oxygen and releasing carbon dioxide as a result of aerobic respiration

• Plants also photosynthesise during daylight hours, for which they need to take in carbon dioxide and release the oxygen made in photosynthesis

• At night, plants do not photosynthesise but they continue to respire, meaning they take in oxygen and give out carbon dioxide

Photosynthesis and respiration in plants

• During the day, especially when the sun is bright, plants are photosynthesising at a faster rate than they are respiring, so there is a net intake of carbon dioxide and a net output of oxygen

• We can investigate the effect of light on the net gas exchange in an aquatic plant using a pH indicator such as hydrogencarbonate indicator 

• This is possible because carbon dioxide is an acidic gas when dissolved in water

• Hydrogencarbonate indicator shows the carbon dioxide concentration in solution

• The table below shows the colour that the indicator turns at different levels of carbon dioxide concentration

  

• Several leaves from the same plant are placed in stoppered boiling tubes containing some 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: