• 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: