Revision Notes

6.4 Investigating Gas Exchange

Extended Only

Effect of Light on Net 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, IGCSE & GCSE Biology revision notesPhotosynthesis 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

Investigating Gas Exchange table, IGCSE & GCSE Biology revision notes


  • 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:

Results from a typical gas exchange experiment table, IGCSE & GCSE Biology revision notes

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now