ZIMSEC O Level Combined Science: Carbon Dioxide and Photosynthesis

Aim: To test whether carbon dioxide is necessary for photosynthesis.

Materials: Potted green plants, potassium hydroxide solution (soda lime), two polythene bags/two bell jars, string, apparatus and reagents for the start test.

Image credit biologydiscussion.com
Image credit biologydiscussion.com


An experiment to determine whether Carbon dioxide is required in order for the photosynthesis process to occur in plants (leaves).

  1. Make sure the leaves have no residual starch

    De-starch the leaf to make sure that there is no residual starch in the plan’ts leaves left over from before the experiment began. You can achieve this by placing the plant in a dark room for 24 hours (at least)

  2. Verify that leaves have no residual starch

    Verify to make sure that both plants have no residual starch after the de-starching process. You can do this by taking one leaf from each plant and testing these leaves for the presence of starch.

  3. Place the plants outside in sunlight

    Set both plants into the light and cover them with polythene bags tied with strings at the bottom or just cover them with jar bells as shown in the diagram above.

  4. Remove carbon dioxide from one of the enclosures

    Inside one bag/jar place a container with soda lime/Potassium hydroxide solution. This absorbs carbon dioxide. Leave the other plant as the control.

  5. Leave the plants in sunlight

    Leave the plants in sunlight for a few hours in order to give them time to photosynthesise.

  6. Test each plant for starch

    Take two leaves from each of the plants and test them for starch. Take care to note the leaves from each plant.


Image credit rollitup.org
Leaf with carbon dioxide left, without carbon dioxide right .Image credit rollitup.org
  • The leaf with carbon dioxide tests positive for starch by turning blue-black in colour.
  • The one without carbon dioxide remains unchanged as shown to the right.


  • Carbon Dioxide is required in order for photosynthesis to take place.

To access more topics go to the Combined Science Notes page.