Photosynthesis Core Practicals and Experiments

Key Objectives

• Understand how to demonstrate that starch is produced in photosynthesis.
• Show that oxygen is given off by water plants during photosynthesis.
• Demonstrate the requirements of light, chlorophyll, and CO2 in photosynthesis.

Production of Starch in Photosynthesis

Testing for Starch

• Process Overview:
  • Plants make glucose during photosynthesis and store it as starch.
  • Test for starch in leaves using iodine.
• Iodine Test:
  • Iodine turns from yellow-brown to blue-black if starch is present.
• Experiment Steps:
  1. Boil a leaf in water for 30 seconds to kill it.
  2. Boil it in ethanol after turning off the Bunsen burner (safety: ethanol is flammable).
  3. Ethanol extracts chlorophyll, making the leaf colorless.
  4. Rinse the leaf in cold water.
  5. Spread on a white tile and add iodine.
  6. A blue-black color indicates the presence of starch.

De-starching

• Purpose: To demonstrate a plant’s reliance on photosynthesis for starch.
• Method: Keep a plant in the dark to deplete starch reserves, test with iodine, and observe no color change.

Chlorophyll Requirement

• Variegated Leaves Test:
  • Use leaves with white (no chlorophyll) and green (chlorophyll) areas.
  • Conduct starch test to show blue-black only in green areas.

Carbon Dioxide Requirement

• Soda Lime Experiment:
  • Use soda lime to absorb CO2 and create a low CO2 environment.
  • Perform the starch test showing no starch production.

Oxygen Production in Photosynthesis

Pond Weed Experiment

• Objective: Observe oxygen production visually via pond weed.
• Plant Types: Elodea or Cabomba.
• Method:
  1. Place plant underwater and observe oxygen bubbles from the cut stem.
  2. Count bubbles produced per minute to gauge photosynthesis rate.
  3. Repeat and measure for reliability.
  4. Experiment with changing light intensity or CO2 levels to study effects on photosynthesis rate.

Measurement Accuracy

• Accuracy Tips:
  • Count and record bubble frequency.
  • Use repeat experiments for reliability.
  • Change only one independent variable at a time (light, temperature, or CO2).
  • Measure dependent variable (bubbles per minute).
  • Collect volume of gas for more accuracy.

Key Takeaways

• Controlled variables are crucial for valid experiments.
• Always ensure safety when using flammable materials like ethanol.
• Use graphs to represent data and analyze effects of various conditions on photosynthesis.