Respiration and Photosynthesis



Examining the interactions between Elodea and snails by detecting the presence of CO2.

Objective: To design and conduct and experiment to compare the products of photosynthesis and respiration.

Background: In this lab you will use an indicator called bromothymol blue (BTB) to indicate whether CO2, carbon dioxide is present. Carbon dioxide reacts with water to form carbonic acid (H2CO3). Then, carbonic acid immediately forms into hydrogen ion and a bicarbonate ion. See the reaction below:

CO2

H2O H2CO3 ( H+ + HCO3 –

Neutral Acidic

color change

BLUE GREEN/ YELLOW

When carbonic acid dissociates into free H+, it lowers the pH of the solution, turning the color of the BTB from blue to yellow. (Recall that the more hydrogen ions there are, the lower the pH – the more acidic the solution is). The color change is proportional to the amount of acid (H+) in the solution, which is proportional to the amount of carbon dioxide produced. In other words, the more CO2 dissolves in the water, the more acid the solution becomes and the pH of the solution will decrease. Therefore, a pH indicator such as BTB can indicate the relative amount of CO2 dissolved in water based on the color of the solution. BTB’s color ranges from blue to yellow in an acidic solution.

Materials:

Test tubes and test tube racks

Rubber stoppers

Aluminum foil

Elodea (water plant) and snails

Bromothymol blue solution: ~30 drops of BTB in 100 mls of water

(may include 1 drop of NaOH).

Before you begin, read the above information and answer the questions below. Please write neatly!

1. When carbon dioxide mixes with water, what ion is produced to make the solution more acidic?

2. What indicator is being used in this lab and what does it indicate?

3. If you were to exhale (high content of CO2) into a solution of bromothymol blue, what color would it become?

4. Write the chemical equations for photosynthesis and respiration below:

Ps:

Rs:

You will be testing for the products of photosynthesis and/or respiration by setting up test tubes with the materials provided. Each small group will be given a topic to focus upon. Then, your group will decide upon your experiment. Fill out the parts in the spaces below:

Hypothesis:

Procedure:

(please number each step and write neatly)

Results: (on separate paper, attached, labeled ‘Results’)

1. Make a data table before you do the experiment – everyone in your group should have the same data table (each person should have his/her own table).

2. Record data in the table during the experiment.

3. Create a graph or other pictorial diagram based on your data.

4. Write a one paragraph figure legend for your graph that clearly tells what the graph shows – you will NOT explain, interpret or offer inferences.

Discussion Section:

1. Ms. Juo will provide the graphs/ diagrams and figure legends from the other Honor credit student experiments. Write a brief summary (1 paragraph) to compare and contrast your results with the other groups’ results.

2. Explain the interaction between the snail and the Elodea in terms of biochemical processes? (hint: What gases are being produced and used during respiration and photosynthesis?) Answer this by showing which gases come from and go into the elodea plant and the snail. Label your picture so that it is clear that you understand the interaction.

[pic] [pic]

Please type the following and attach:

Sources of Error:

Describe in great detail TWO sources of error: state the error, explain its effects on the results, and describe how you would change the procedure to prevent the error next time. Be specific and thorough – 1 paragraph each.

Conclusion: Briefly summarize what you did during the lab. What did you learn from this lab about photosynthesis and respiration? 1 long paragraph or 2 short paragraphs.

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