Photosynthesis Investigations and Starch Test

Investigations in Photosynthesis

  • Investigations can determine if a factor is essential for photosynthesis or measure the rate of photosynthesis.
  • Experimental Setup:
    • Experiment: Plant given all requirements except the factor being tested.
    • Control: Plant given all requirements under the same conditions.
    • A starch test is often performed at the end to confirm photosynthesis occurred.

Destarching a Plant

  • Purpose: To remove existing starch before starting an investigation.
  • Method:
    • Place the plant in a dark cupboard for 48 hours.
    • The plant uses its stored starch during this period.
    • Any starch present at the end of the investigation is due to photosynthesis during the experiment.

The Starch Test

  • Principle: Detects the presence of starch, indicating photosynthesis.
  • Photosynthesis produces glucose, which is converted to starch.
  • Positive Result: Starch present → Photosynthesis occurred.
  • Negative Result: Starch absent → Photosynthesis did not occur.

Method:

  1. Place a leaf in a beaker of boiling water to soften it and kill the cells, stopping metabolic processes.
  2. Transfer the leaf to a test tube with boiling ethanol (alcohol).
  3. Place the test tube in a beaker of boiling water (water bath) for ~10 minutes.
    • Ethanol is flammable and has a lower boiling point than water, hence the water bath.
    • Chlorophyll is soluble in alcohol and will be extracted, turning the leaf white and brittle.
  4. Rinse the leaf in water to soften it.
  5. Spread the leaf on a tile and add iodine solution.

Result:

  • Blue-black color indicates the presence of starch, proving photosynthesis.

Investigation 2: Light Requirement

  • Aim: To prove light is required for photosynthesis.
  • Method:
    1. Destarch a potted plant (48 hours in darkness).
    2. Cover a portion of a leaf with aluminum foil.
    3. Place the plant in a sunny area for 48 hours.
    4. Remove the foil and test the leaf for starch.
  • Experimental Setup:
    • Experiment: The part of the leaf covered by foil (no light).
    • Control: The uncovered part of the leaf (receives light).

Results:

  • Experiment (foil-covered): Iodine remains light brown (no starch).
  • Control (uncovered): Iodine turns blue-black (starch present).

Conclusion:

  • Light is essential for photosynthesis.

Investigation 3: Carbon Dioxide Requirement

  • Aim: To prove carbon dioxide is required for photosynthesis.
  • Materials used to remove CO_2: Sodium hydroxide, potassium hydroxide, or soda lime.
  • Materials used to add CO_2: Sodium bicarbonate or potassium bicarbonate.
  • Method:
    1. Destarch two potted plants (48 hours in darkness).
    2. Set up two bell jars:
      • Bell jar 1: Contains a plant and sodium hydroxide to absorb CO_2.
      • Bell jar 2: Contains a plant and sodium bicarbonate to release CO_2.
    3. Seal the bell jars and place them in a sunny area for 48 hours.
    4. Test leaves from each plant for starch.

Results:

  • Bell jar 1 (no CO_2): Iodine remains light brown (no starch).
  • Bell jar 2 (with CO_2): Iodine turns blue-black (starch present).

Conclusion:

  • Photosynthesis requires carbon dioxide.

Investigation 4: Chlorophyll Requirement

  • Aim: To prove chlorophyll is required for photosynthesis.
  • Use a variegated leaf (green parts with chlorophyll, white parts without).
  • No destarching is needed as the experiment and control are on the same leaf.
  • Method:
    1. Place a potted plant with variegated leaves in a sunny place for a few hours.
    2. Remove a leaf and test for starch.

Results:

  • Experiment (white part): Iodine remains light brown (no starch).
  • Control (green part): Iodine turns blue-black (starch present).

Conclusion:

  • Photosynthesis requires chlorophyll.

Investigation 5: Oxygen Production

  • Aim: To prove oxygen is produced during photosynthesis.
  • Method:
    1. Set up an aquatic environment with pond weed in water enriched with sodium bicarbonate (to provide CO_2).
    2. Invert a test tube over a funnel placed over the pond weed to collect gas bubbles.
    3. Place the apparatus in a sunny area.
    4. Allow gas bubbles to collect in the test tube, displacing the water.
    5. Seal the test tube with a rubber stopper while submerged.
    6. Remove the test tube from the water and insert a glowing wooden splint.

Results:

  • The glowing splint re-ignites or burns more brightly.

Conclusion:

  • Oxygen is produced during photosynthesis.

Activity 2: Investigating Photosynthesis

  • Experiment involving a variegated plant.
    • Plant left in the dark for 3-4 days.
    • Initial starch test conducted.
    • Plant left in the light for four hours.
    • Drawing made to show the distribution of green and white areas.
    • Leaf tested for starch again.
    • Drawing made to show the distribution of blue-black and brown areas after iodine solution was added.