Investigating the Need for Chlorophyll, Light & Carbon Dioxide (CIE IGCSE Biology: Co-ordinated Sciences (Double Award))

Testing leaves for starch

• Although plants make glucose in photosynthesis, leaves cannot be tested for its presence as the glucose is quickly used, converted into other substances and transported or stored as starch
• Starch is stored in chloroplasts so testing a leaf for starch is a reliable indicator of photosynthesis
• Leaves can be tested for starch using the following procedure:
  • A leaf is dropped in boiling water to kill the leaf
  • The leaf is left for 5-10 minutes in hot ethanol in a boiling tube
    • This removes the chlorophyll so colour change due to iodine can be seen more clearly
  • The leaf is dipped in boiling water again to soften it
  • The leaf is spread out on a white tile and covered with iodine solution
  • Photosynthesising regions of the leaf will turn blue-black due to the presence of stored starch

Demonstrating the need for chlorophyll

• This method can be used to demonstrate that chlorophyll is needed for photosynthesis by using variegated leaves
  • When the leaf is tested only the areas that contain chlorophyll stain blue-black
  • The areas that have no chlorophyll remain orange-brown as no photosynthesis is occurring here and so no starch is stored

Leaves can be tested for starch as an indicator of photosynthesis

• Care must be taken when carrying out this practical as ethanol is extremely flammable, so at that stage of the experiment the Bunsen burner should be turned off.
• The safest way to heat the ethanol is in an electric water bath rather than using a beaker over a Bunsen burner with an open flame

• The starch test can be used to demonstrate that light is needed for photosynthesis as follows:
  • The plant should be destarched by placing in a dark cupboard for 24 hours
    • This ensures that any starch already present in the leaves will be used up and will not affect the results of the experiment
  • Following destarching, a leaf of the plant can be partially covered with aluminium foil and the plant placed in sunlight for a day
  • The leaf can then be removed and tested for starch using iodine
• The area of the leaf that was covered with aluminium foil will remain orange-brown as it did not receive any sunlight and could not photosynthesise, while the area exposed to sunlight will turn blue-black
• This indicates that light is necessary for photosynthesis

• The starch test can be used to demonstrate the need for carbon dioxide as follows:
  • Destarch two plants by placing in the dark for a prolonged period of time
  • Place one plant in a bell jar which contains a beaker of sodium hydroxide
    • Sodium hydroxide absorbs carbon dioxide from the surrounding air
  • Place the other plant in a bell jar containing a beaker of water, which will not absorb carbon dioxide from the surrounding air
    • This is an experimental control
  • Place both plants in bright light for several hours
  • Test both plants for starch using iodine
• The leaf from the plant placed near sodium hydroxide will remain orange-brown as it could not photosynthesise due to lack of carbon dioxide
• The leaf from the plant placed near water should turn blue-black as it had all necessary requirements for photosynthesis

The need for carbon dioxide in photosynthesis can be demonstrated using sodium hydroxide