Skip to content

Photosynthesis produces starch

AIM 

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

HOW DOES IT WORK?                                                   

Preparation Time: 24 hours
Time allocation: 35 Min

The leaf sample you have has grown due to photosynthesis that has taken place, the alcohol used in the experiment 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 is then exposed to this bleached area on the leaf and 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.

Note that ethanol is heated using a hot water bath. Ethanol boils at 78°C, it is advisable to set up a hot water bath instead of using a Bunsen Burner as the Ethanol will gradually heat up  instead of rapidly and uncontrollably heating up when exposed to direct flame of a Bunsen Burner.

Variegated leaves have green parts (where the cells contain chlorophyll) and white parts (where there is no chlorophyll). Only the parts that are green will become blue-black with iodine solution, thus showing the importance of chlorophyll in photosynthesis.

A plant can be ‘de-starched’ by leaving it in the dark for a few hours. We can observe this by having parts of the leave covered with dark paper and other parts exposed, then plant is placed in the light/sunlight for a few hours. Only the uncovered parts become blue-black with iodine solution, showing the importance of light in photosynthesis as it demonstrates the presence of Starch.

MATERIALS

The box includes:

  • 1 x 250ml Beaker
  • 1 x White spotting tile
  • 1 x test tube
  • 100ml Ethyl Alcohol
  • 10ml Iodine solution
  • 1 x Long Pair of forceps
  • 1 x strip of foil
  • 1 x Paperclip
  • Gas stove and fuel cartridge

To be supplied by Educator

  • 1 Seedling or plant exposed to sun
  • Paperclips

Refillable items:

  • Ethyl Alcohol
  • 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. Make a water bath by filling the beaker with enough water approximately 150ml and bringing it to a boil (noticeable effervescence)
  3. Fill 1/3 of water into a test tube and bring to a boiling on the gas stove inside a water bath
  4. Remove the leaf from the plant and dip the leaf in boiling water for 60 seconds, using the forceps.
  5. Remove the leave from the boiling water.
  6. 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.
  7. Place leaf sample on watch glass.
  8. 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 starch is actively photosynthesizing in the plant. This is because starch is a compact storage system for sugars which act as energy reserves for respiration. This ensures that the plant, when removed from sunlight or during night time it will have enough energy stress to continue metabolizing.

PRECAUTIONS:

  • Students and educator may not leave any open flame unattended
  • Chemicals must be stored correctly
  • Ingestion or inhalation of any chemicals is 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:
    • 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:
  • First aid measures
    • Eye: Immediately flush with fresh water for 15 minutes.
    • External: Wash continuously with fresh water for 15 minutes.
    • Internal: Rinse out mouth, give 1 to 2 cups of water or milk, and induce vomiting. Call a physician or poison control at once.
  • Spillage: Practice reasonable care and caution. Remove all contaminated clothing and shoes. Wash before reuse. Wash thoroughly after handling.