How does Elodea living in the dark affect the pH of the solution? What does this imply about the process plants carry out in the dark and why?
The correct answer and explanation is:
Correct Answer:
When Elodea lives in the dark, the pH of the solution decreases (becomes more acidic). This implies that in the dark, the plant is not photosynthesizing but continues to respire, producing carbon dioxide which forms carbonic acid in the solution.
Explanation:
Elodea is an aquatic plant that performs both photosynthesis and cellular respiration. Photosynthesis occurs only in the presence of light, while cellular respiration happens both day and night. During photosynthesis, Elodea uses carbon dioxide and releases oxygen. This process removes CO₂ from the water, which raises the pH by reducing the formation of carbonic acid.
However, when Elodea is kept in the dark, photosynthesis stops because light is required for this process. Despite this, cellular respiration continues. In respiration, Elodea breaks down sugars and releases carbon dioxide as a byproduct. The carbon dioxide released dissolves in the surrounding water and reacts with water molecules to form carbonic acid (H₂CO₃). Carbonic acid dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻), increasing the concentration of H⁺ in the solution. As a result, the pH of the water decreases, becoming more acidic.
This change in pH confirms that Elodea is no longer removing CO₂ through photosynthesis in the dark but is actively releasing it through respiration. The presence of carbon dioxide is critical evidence that cellular respiration is still occurring, while the absence of light prevents the plant from using that CO₂ in photosynthesis. This demonstrates the importance of light in regulating the balance of gases in aquatic environments and highlights that plants do not photosynthesize in the dark, but they do continue to carry out respiration, which has a direct impact on the chemistry of their surroundings.