1.2 photosynthesis

• Algae can also photosynthesise
• Photosynthesis is an endothermic process - meaning it requires heat to occur
• carbon dioxide + water -> (light and chlorophyll) glucose + oxygen
• 6CO2 + 6H2O -> C6H12O6 + 6O2
• Chlorophyll is present in chloroplasts and they absorb the light energy required for photosynthesis
• When photosynthesis occurs, the glucose produced usually gets converted to starch for storage
• Glucose's molecules are small and soluble while starch's molecules are large and insoluble.
• The advantage to a plant storing glucose as starch is :

  1. glucose is soluble, which means if stored in plant cells, it would affect the water balance.
  2. starch is insoluble, which means if stored in plant cells, it would not affect the water balance.
• To test a leaf for starch

  1. Place a leaf in boiling water for about 1 minute to soften it. (to kill the leaf)
  2. Place the leaf into a test-tube half filled with ethanol and put the test-tube in the hot water for 5 minutes (to remove chlorophyll and turn the leaf white)
  3. Rinse the LEAF in hot water to soften it (since ethanol dehydrates)
  4. Spread the leaf out onto a white tile and add iodine to the surface. If starch is present, the leaf would be blue-black.
• If light is required for photosynthesis:

  1. De-starch a germanium plant
  2. Cut out a bold shape of black card to make a stencil. Using paper clips, fasten the card onto the leaf. Do not remove the leaf from the plant.
  3. Leave the plant in a well-lit area for a few days
  4. After a few days, remove the black stencil and test the leaf for starch. ( POSITIVE RESULT - the area where the stencil was kept over must be orange-brown and the uncovered areas must be blue-black to signify that starch is present)
• If chlorophyll is required for photosynthesis:

  1. De-starch a germanium plant with variegated (meaning it has green and white) leaves