Question

what is happening to the level of carbon dioxide in the experimental test tube with the elodea leaf as indicated by the red color in the tube?

a. the level of carbon dioxide is decreasing.
b. the level of carbon dioxide is unchanged.
c. the level of carbon dioxide is increasing.

what is happening to the level of carbon dioxide in the negative control test tube without the elodea leaf as indicated by the yellow color in the tube?

a. the level of carbon dioxide is decreasing.
b. the level of carbon dioxide is increasing.
c. the level of carbon dioxide is unchanged.

Explanation:

Analyze the experimental test tube with Elodea

Using the Photosynthesis Reactants and Cellular Respiration in Plants knowledge points

• The experimental tube contains an active Elodea leaf.
• Under light, the rate of photosynthesis exceeds cellular respiration.
• Photosynthesis consumes carbon dioxide (\(\text{CO}_2\)) as a reactant:

$$ 6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 $$

• This consumption causes the concentration of dissolved \(\text{CO}_2\) to decrease.
• A decrease in \(\text{CO}_2\) reduces carbonic acid formation, raising the pH and shifting the indicator (typically phenol red or bromothymol blue) toward its basic color (red/pink or blue).
• Therefore, the level of carbon dioxide in this tube is decreasing.

Analyze the negative control test tube

Using the Experimental Controls knowledge point

• The negative control tube contains the indicator solution but lacks the Elodea leaf.
• Without a living organism, there is no metabolic activity (neither photosynthesis nor cellular respiration) to consume or produce carbon dioxide.
• The initial yellow color indicates an acidic environment with dissolved \(\text{CO}_2\).
• Because the system is closed and lacks a biological driver, the amount of dissolved gas remains constant.
• Therefore, the level of carbon dioxide in this control tube is unchanged.

Answer:

Question 1

What is happening to the level of carbon dioxide in the experimental test tube with the Elodea leaf as indicated by the red color in the tube?

• A. The level of carbon dioxide is decreasing. (Correct answer)
• B. The level of carbon dioxide is unchanged.
• C. The level of carbon dioxide is increasing.

Question 2

What is happening to the level of carbon dioxide in the negative control test tube without the Elodea leaf as indicated by the yellow color in the tube?

• A. The level of carbon dioxide is decreasing.
• B. The level of carbon dioxide is increasing.
• C. The level of carbon dioxide is unchanged. (Correct answer)