Investigating Gas Exchange (Cambridge International IGCSE Biology)

Spec point — 6.1.7 · Investigate and describe the effect of light and dark conditions on gas exchange in an aquatic plant using hydrogencarbonate indicator solution

Investigating gas exchange

• 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
• The effect of light on net gas exchange can be investigated using an aquatic plant and 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: