6CO2 + 6H2O Carbon dioxide + water ‘Photosynthesis’
Photo means ‘light’ and synthesis means ‘to make with’. Together, photosynthesis means ‘to make with light’.
Confirming photosynthesis by testing for starch in leaves
The excess glucose produced during photosynthesis is stored as starch. Remember that iodine changes colour from orange-brown to blue-black if starch is present. As a result, iodine is a simple way to test if photosynthesis has taken place.
1. Before you start this investigation, consider the following:
a. If you use a plant that has been left on a windowsill, how can you be sure that the starch in this plant is actually the result of photosynthesis? (Hint: starch needs to be removed from the plant before this investigation to make sure the starch produced is the result of photosynthesis. How could starch be removed from the plant?)
b. What would be a suitable control to use in this investigation? (Hint: how might you stop a plant from photosynthesising?)
c. Before you test a leaf for starch, you will need to remove the chlorophyll. Why do you think it is necessary to remove the chlorophyll before testing for starch?
d. How long should you leave a plant to photosynthesise before testing for starch?
2. Consider changing this investigation to test for glucose. Is there any valid reason to test a leaf for glucose? Explain your answer.
3. Does this investigation confirm that sunlight is needed for photosynthesis? Explain your answer and mention the other controlled variables you would need for this to be a valid investigation.
