Skip to content

The Effect of light exposure on Photosynthesis

AIM

To test the relationship between the presence of light and photosynthesis.

HOW DOES IT WORK?                                                    

Preparation Time: 24 hours
Time allocation: 35 Min

 The hot water will kill the leaf, where photosynthesis has taken place, and the alcohol will break down and remove the chlorophyll from the leaf sample. Chlorophyll is the green pigment that is responsible for the photosynthesis in plants. Iodine reacts with starch to form a blue-black colour. When Iodine solution is applied to the leaf sample it will turn blue-black if starch is present. The part of the leaf that is occluded from sunlight will not contain chlorophyll and the iodine will not change colour there (Iodine Solution remains brown)

A plant can be ‘de-starched’ by leaving it in the dark for a few hours. Parts of its leaves are covered with dark paper, and the plant is left in the light for a few hours. Only the uncovered parts become blue-black with iodine solution, showing the importance of light in photosynthesis.

APPARATUS

The box includes:

  • 1 x 250ml Glass beaker
  • 1 x 100ml Glass beaker
  • 100ml Ethanol
  • 10ml Iodine solution
  • 1 x forceps
  • 1 x spotting tile
  • strip of foil
  • 1 x Paperclip
  • 1 gas stove and fuel cartridge
  • 1 Gauze wire

To be supplied by Educator

  • 1 Seedling or plant exposed to sun

Refillable items:

  • Ethyl Alcohol (Ethanol)
  • Iodine solution

METHOD:

  1. Fold the foil so that it covers half of a leave, place the plant in sun for a few days.
  2. Place 100ml of water into a 250ml beaker and bring to a boil on the gas burner.
  3. Remove the leaf from the plant and dip the leaf in boiling water for 60 seconds, using the forceps.
  4. Remove the leave from the boiling water.
  5. Put about 25-50ml of ethyl alcohol in a 100ml beaker. Place the beaker in the boiling water, with the heat turned off. Place the leaf in the ethyl alcohol for 2 minutes or until they turn almost white in colour.
  6. Place leaf sample on watch glass.
  7. Drop a few drops of Iodine solution on to the leaf sample. The part of the leaf that was exposed to light will turn blue-black, indicating the presence of starch. The part of the leaf that was covered will turn brown, indicating the absence of starch.

CONCLUSION

The experiment proved that photosynthesis is a light dependent energy system as only the part of the leaf that was exposed to light tested positive for the presence of starch. It also showed that a leaf will not die when parts of it does not photosynthesize , and thus photosynthesis that is inhibited may be a reversible reaction as it can be reactivated when the plant is re-exposed to light. The dependency of photosynthesis and thus the plant on light was displayed in this experiment.

PRECAUTIONS:

  • Students and educator may not leave any open flame unattended.
  • Chemicals must be stored correctly.
  • Ingestion or inhalation of any chemicals are dangerous.
  • When removing a hot glass apparatus off the heat, place on a surface which conducts heat well (e.g. chopping board or wood surface) to prevent it from shattering.
  • Ethyl Alcohol (Ethanol):
    • Do not ingest. If it is, do not induce vomiting or ingest milk, seek professional medical attention immediately.
    • Do not smell while boiling.
    • If contact is made with skin wash with water immediately.
    • If splashed in eye , rinse eye with water and seek professional medical attention immediately
  • Iodine Solution:
    • In presence of light Iodine solution undergoes a photo-decomposition reaction so it must be stored in a dark bottle, generally in amber bottle to minimize the amount of light that the solution is exposed to. If stored in a clear bottle it will gradually break down as the iodide ions which are gradually converted to iodine and the solution turns a yellowish color.