Need for Chlorophyll, Light & Carbon Dioxide | Biology for GCSE/IGCSE - Year 11 PDF Download

Investigating the Need for Chlorophyll

• Plants produce glucose through photosynthesis, but it's challenging to detect glucose in leaves due to its rapid utilization, conversion into other compounds, or storage as starch.
• Starch is stored in chloroplasts, the sites of photosynthesis, making testing for starch a reliable method to identify regions of active photosynthesis in a leaf.
• Procedure for testing leaves for starch:
  • A leaf is immersed in boiling water to terminate cell activity and facilitate cell membrane breakdown.
  • The leaf is then placed in hot ethanol for 5-10 minutes in a boiling tube to eliminate chlorophyll, enhancing visibility of color changes from iodine.
  • Subsequently, the leaf is softened by dipping it in boiling water before being spread on a white tile and covered with iodine solution.
  • In a green leaf, the entire leaf turns blue-black, indicating active photosynthesis across all leaf areas.
  • Using a variegated leaf (partially green and partially white) allows testing to determine the necessity of chlorophyll for photosynthesis.
  • The absence of chlorophyll in white leaf areas results in orange-brown coloration, demonstrating the lack of photosynthesis and starch storage in those regions.

• Care must be taken during the experiment with ethanol due to its high flammability. Ensure the Bunsen burner is turned off at the required stage to prevent accidents.
• It is safest to heat ethanol using an electric water bath rather than a beaker over an open flame from a Bunsen burner.

Investigating the Importance of Light in Photosynthesis

• When exploring the necessity of light for photosynthesis, a standard procedure involves initially destarching a plant by storing it in darkness for 24 hours.
• This destarching step ensures that any existing starch in the leaves is depleted, preventing it from influencing the experiment's outcomes.
• Following destarching, a leaf from the plant can be partially shielded with aluminum foil and exposed to sunlight for a day.
• After exposure, the leaf is removed and tested for starch using iodine solution.
• Areas of the leaf covered by foil retain an orange-brown color, indicating the absence of sunlight and thus no photosynthesis, while sun-exposed regions turn blue-black.
• This observation confirms that light is a critical factor for photosynthesis and the formation of starch.

Investigating the Importance of Carbon Dioxide in Photosynthesis

• Prepare two plants for the experiment by keeping them in the dark for an extended period to remove any stored starch.
• Place one plant in a bell jar along with a beaker of sodium hydroxide, which absorbs carbon dioxide from the air.
• Keep the other plant in a bell jar with a beaker of water as a control setup, where carbon dioxide absorption does not occur.
• Expose both plants to bright light for several hours to initiate photosynthesis.
• After the light exposure, test both plants for starch presence using iodine solution.
• The leaf from the plant near the sodium hydroxide will maintain an orange-brown color, indicating the inability to photosynthesize due to the lack of carbon dioxide.
• The leaf from the plant near water should turn blue-black, signifying that it had all the necessary components for successful photosynthesis.