Investigating the Need for Chlorophyll, Light & Carbon Dioxide (Cambridge O Level Biology)

• The occurrence of photosynthesis can be demonstrated by observing the presence of its products
• Although plants make glucose in photosynthesis, leaves cannot be tested for its presence as the glucose is quickly used or converted into other substances
• Starch is stored in chloroplasts, where photosynthesis occurs, so testing a leaf for starch is a reliable indicatorthat photosynthesis is taking place
• Leaves can be tested for starch using the following procedure:
  1. A leaf is dropped in boiling water to kill the cells and break down cell membranes
  2. The leaf is left for 5-10 minutes in hot ethanol in a boiling tube; this removes the chlorophyll so that colour changes from iodine can be seen more clearly
     • Care must be taken at this stage as ethanol is extremely flammable; the Bunsen burner should be turned off before any ethanol is poured into the boiling tube
     • A water bath could be used to avoid the need for naked flames
  3. The leaf is dipped in boiling water to soften it
  4. The leaf is spread out on a white tile and covered with iodine solution
  5. Any parts of the leaf where photosynthesis has recently occurred will turn blue-black, indicating the presence of starch
     • In a green leaf, the entire leaf will turn blue-black as photosynthesis is occurring in all areas of the leaf
• This method can be used to test whether chlorophyll is needed for photosynthesis by using a variegated leaf that is partially green and partially white
  • The white areas of the leaf contain no chlorophyll, so 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

Testing a leaf for starch diagram

Iodine can be used to test for the presence of starch in different parts of a leaf

• The procedure above can also be used to demonstrate that light is needed for photosynthesis:
  1. Remove any starch stored in the leaves by placing a plant 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
  2. Partially cover a leaf of the plant with aluminium foil and place the plant in sunlight for a further 24 hours
  3. Remove the leaf and test for starch as shown above
• The area of the leaf 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 demonstrates that light is necessary for photosynthesis and the production of starch

• The need for carbon dioxide in photosynthesis can be demonstrated using the following procedure:
  1. Remove starch from two plants by placing them in the dark for 24 hours
  2. Place one plant in a bell jar which contains a beaker of sodium hydroxide
     • Sodium hydroxide will absorb carbon dioxide from the surrounding air
  3. Place the other plant in a bell jar which contains a beaker of water, which will not absorb carbon dioxide from the surrounding air
     • Water here acts as an experimental control, demonstrating that it is the presence of the sodium hydroxide, and not any other factor, that is affecting the plant
  4. Place both plants in bright light for 24 hours
  5. Test both plants for starch using iodine, as shown above
• The leaf from the plant placed near sodium hydroxide will remain orange-brown, as a lack of carbon dioxide will prevent it from photosynthesising
• The leaf from the plant placed near water should turn blue-black as it had all necessary materials for photosynthesis

Experiment that demonstrates the need for carbon dioxide in photosynthesis diagram

Bell jars and sodium hydroxide can be used to demonstrate that plants need carbon dioxide for photosynthesis