Investigate and describe the effect of light and dark conditions on gas exchange in an aquatic plant using hydrogencarbonate indicator solution

The effect of light and dark conditions on gas exchange in aquatic plants can be effectively investigated using hydrogencarbonate indicator solution. This indicator serves as a pH indicator that changes color based on the concentration of carbon dioxide (CO₂) in the solution, allowing for the observation of gas exchange during photosynthesis and respiration.

Experimental Setup

Materials Needed

• Hydrogencarbonate indicator solution
• Aquatic plant (e.g., Elodea)
• Test tubes or boiling tubes
• Aluminum foil (to block light)
• Gauze (to allow partial light)
• Rubber bungs

Method

1. Prepare four test tubes with equal volumes of hydrogencarbonate indicator solution.
2. Label the tubes as follows:
   • Tube A: Control (no plant)
   • Tube B: Contains a leaf from the aquatic plant, exposed to light
   • Tube C: Contains a leaf wrapped in aluminum foil (no light)
   • Tube D: Contains a leaf wrapped in gauze (partial light)
3. Seal each tube with a rubber bung to prevent gas exchange with the atmosphere.
4. Place all tubes in a well-lit environment for a set duration (e.g., 30 minutes).

Observations and Results

Tube A: Control

• Observation: The hydrogencarbonate indicator remains orange/red.
• Interpretation: No net gas exchange occurs; CO₂ levels are at atmospheric levels.

Tube B: Light Condition

• Observation: The hydrogencarbonate indicator changes to purple.
• Interpretation: The rate of photosynthesis exceeds the rate of respiration, resulting in a net uptake of CO₂. The decrease in CO₂ concentration leads to an increase in pH, causing the color change to purple.

Tube C: Dark Condition

• Observation: The hydrogencarbonate indicator turns yellow.
• Interpretation: In the absence of light, photosynthesis does not occur, but respiration continues. This results in an accumulation of CO₂, which lowers the pH and causes the indicator to change color to yellow.

Tube D: Partial Light Condition

• Observation: The hydrogencarbonate indicator remains orange/red.
• Interpretation: The rate of photosynthesis is approximately equal to the rate of respiration under partial light conditions, resulting in no net change in CO₂ levels.