Section 2- Structures and functions in living organisms
a) Levels of organisation - b) Cell structure - c) Biological molecules - d) Movement of substances - e) Human Nutrition - e) Plant Nutrition - f) Respiration - g) Gas exchange (Humans) - g) Gas exchange (Plants) - h) Transport (Humans) - h) Transport (Plants) - i) Excretion - j) Coordination and response

This practical is in two parts:
  • Setup a variegated plant to photosynthesize
  • Test the leaves for starch and analyse your findings


Setup a variegated (leaves coloured both green and white) plant
  1. De-starch the plant by keeping the plant in darkness for 48 hours prior to the lab (this should have been done for you)
  2. Cover both part of the green region and part of the white region of one leaf
  3. Put the plant in a well lit location for 1-2 days


Testing the leaf for starch
  1. Cut off the leaf you partially covered.
  2. Use the whole leaf if it is small if not make sure the section of leaf you use includes: a green covered section, a green uncovered section, a white covered section and a white uncovered section.
  3. Pour some boiling water from a kettle into a large beaker. (alternatively water can be boiled using a bunsen burner)
  4. Place the leaf in the hot water for about two minutes
    PB_testing-leaves-for-starch-1.jpg
  5. Using forceps, remove the leaf from the boiling water. Note how it has changed.
  6. Drop the leaf into a test tube and push it to the bottom
  7. Add enough ethanol to cover the leaf, and stand the test tube in your beaker of hot water. (BUNSEN MUST BE TURNED OFF)
    PB_testing-leaves-for-starch-2.jpg
  8. Wait a few minutes, watch as the ethanol boils and the green colouring (chlorophyll) is removed from the leaf.
  9. Using forceps, remove the leaf from the boiling tube and rinse the leaf in cold water. Be careful when removing the leaf as it will be very brittle until you rinse it.
    PB_testing-leaves-for-starch-3.jpg
  10. Put the leaf on a white tile and cover with iodine solution.
    PB_testing-leaves-for-starch-4.jpg
  11. Wait a few minutes and watch for colour changes. Does any part of the leaf become blue-black?


Safety considerations
  • Use eye protection
  • Be careful of the hot liquids
  • Wash your hands to remove any traces of plant sap, or the chemicals that you have used.


Image of a variegated leaf tested for starch
Results.jpeg


Analysis of the results
  1. What was the role of the boiling water?
  2. What was the reason for soaking the leaf in ethanol?
  3. Which regions of the leaf did not produce starch, give reasons why?
  4. Which regions of the leaf did produce starch?
  5. The lab proves that two things are needed for photosynthesis to occur, what are they?
  6. Photosynthesis produces glucose, why do we test for starch not glucose?

Images courtesy of Practical Biology