Investigating Gas Exchange: How Light and Dark Affect Aquatic Plants

Plants take in carbon dioxide $(CO_2)$ and release oxygen $(O_2)$ during photosynthesis. In the dark, they switch to respiration, using oxygen and releasing carbon dioxide. In this investigation, we’ll use hydrogencarbonate indicator to observe these gas exchange processes in aquatic plants.

 

Key Concepts

Photosynthesis and Gas Exchange:

• In the light, plants absorb $(CO_2)$ or photosynthesis and release $(O_2)$
• In the dark, plants respire, absorbing $(O_2)$ and releasing $(CO_2)$

Hydrogencarbonate Indicator:

• A chemical that changes color based on
  • Low $CO_2$ (Less acidic): Purple
  • Normal $CO_2$ (neutral): Red
  • High $CO_2$ (more acidic): Yellow

 

 

Equipment Needed

• Aquatic plant (e.g., Elodea)
• Hydrogencarbonate indicator solution
• Test tubes or beakers
• Light source (e.g., lamp)
• Dark covering (e.g., black paper or foil)

 

Method: Investigating Gas Exchange

Aim:

To study how light and dark conditions affect gas exchange in an aquatic plant.

Steps:

1. Set Up the Experiment:

   • Place a piece of aquatic plant in a test tube filled with hydrogencarbonate indicator solution.

2. Control Conditions:

   • Tube 1 (Control): No plant, just the indicator solution.
   • Tube 2 (Light): Plant exposed to a light source.
   • Tube 3 (Dark): Plant covered with black paper or foil.

3. Observe Changes:

   • Leave the setup for a few hours.
   • Note the color changes in the indicator solution for each test tube.

 

 

Expected Results

1. Tube 1 (Control):

   • No change in color (indicator stays red) because there is no plant to affect $CO_2$ levels.

2. Tube 2 (Light):

   • Indicator turns purple (low $CO_2$) because the plant is photosynthesizing, absorbing $CO_2$ and releasing $O_2$

3. Tube 3 (Dark):

   • Indicator turns yellow (high $CO_2$) because the plant is respiring, releasing $CO_2$ and using $O_2$

 

 

Why This Happens

• In Light:
  Photosynthesis removes $CO_2$ from the water, lowering $CO_2$ levels and making the solution less acidic.

• In Dark:
  Without light, photosynthesis stops. The plant only respires, releasing $CO_2$ into the water, increasing $CO_2$ levels and acidity.