Comments on asking questions and analyzing results

Photosynthesis Exercise

Background:

Students are given a lab protocol in which they measure the use of CO₂ by Elodea during photosynthesis. They must read the exercise, perform the experiment, and draw conclusions.

• You are given:

In the introduction to this exercise, the lab manual tells you that the objectives are:

 

1. To gain experience in using the scientidfic method to investigate nature by phrasing a question, writing an hypothesis, recording observations, and analyzing collected data.
2. To follow a simple procedure designed to determine if plants use carbon dioxide during photosynthesis.
3. To understand that photosynthesis in green plants supplies a source of food and oxygen for other living organisms.

The lab manual also gives you the basic chemical reaction of photosynthesis, which tells you that carbon dioxide is used up in a reaction that forms sugar.

The last paragraph of the introduction asks: " How do scientists know that plants use carbon dioxide during photosynthesis? The following experiment will help you to answer this question."

The Procedure section tells you that bromthymol blue is an indicator that is sensitive to the amount of acid, and that carbon dioxide in water makes a weak acid.

• You must infer:

1. That bromthymol blue will be sensitive to the amount of carbon dioxide dissolved in the water,
2. That if carbon dioxide is used in photosynthesis, then the solution will be less acidic, and
3. That bromthymol blue will change color as the amount of carbon dioxide in the water is reduced due to photosynthesis.

Stating a question:

After you have read the lab exercise, you need to ask a question that can be answered by the procedure you will use.

NOTE: The information you have been given contains many clues about the question you should ask.

Examples of student questions:

The lab exercise asks you to: Phrase a question that expresses the purpose of this scientific investigation:

Students answered:

1. How do we know that plants use carbon dioxide during photosynthesis?

2. Does Elodea use carbon dioxide during photosynthesis?

3. Why do plants need starch and oil?

4. Does carbon dioxide interfere with photosynthesis?

5. Which will change color, and which will stay the same?

6. How will the photosynthesis of the plant effect how much carbon dioxide is released into the air?

7. Does photosynthesis affect carbon dioxide?

8. How does the indicator bromthymol blue affect the variables being tested?

How would you evaluate these questions?

Click here to see how the instructor evaluated them.

Analyzing the results

Later in the exercise, students are asked to draw conclusions by comparing their experimental and control results, and relating them to the initial hypothesis and purpose:

Based on (your analysis of the results in) your answers to Questions 5 and 6, how would you explain the observed color changes in your experimental results.

Examples of student answers:

1. Adding Elodea to tube E will cause it to change color because the Elodea will use up the carbon dioxide in the solution. The carbon dioxide in tube C will eventually diffuse over time, causing color change to occur from the top.

2. The Elodea used the CO₂ in the solution for photosynthesis, thus using up the acidic part of the solution, causing the pH to rise to form a more basic solution making it blue.

3. Tube E had the plant and it photosynthesized, the other one had nothing in it so it didn't change color.

4. Because of photosynthesis the plant was able to not only harvest the light energy but also stored energy in the chemicals. The oxygen released by photosynthesis may also affect the color.

5. The photosynthesis from the light and the carbon dioxide in the water was going into the air because there was less CO₂ in the air than in the tubes of Elodea.

6. The color changes in tube E were caused because the Elodea used CO₂ during photosynthesis, which increased the pH level (made the solution less acid). The color didn't change in tube C because there was no Elodea to use the CO₂, and the acid levels remained high.

How would you analyze these answers?

Stating a Question

The instructor analyzed the questions this way:

Questions 1 and 2 are reasonable questions for a course at this level. Which one is a better question?

Question 3 comes from left field. It has no relationship to the lab, and can't be answered by the exercise.

Questions 4 and 8 can't be answered by the exercise, because we're not varying the amount of CO₂ or bromthymol blue in the experiment.

Question 5 is incomplete and misses the point entirely. The question doen't define what is going to change color, and worse, focuses on the assay rather than the big question about photosynthesis.

Question 6 makes a link between carbon dioxide and photosynthesis, but garbles the science. (What does happen to the CO₂?) It can't be answered by the exercise, because we have no way to test how much CO₂ is released to the air.

Question 7 also links photosynthesis and CO₂, but is incomplete. As stated, photosynthesis must have a direct effect on CO₂ in some way. The student may have MEANT "How does photosynthesis affect carbon dioxide concentrations in solution", but this is NOT the question they asked.

Go back to the questions.

Analysis

The instructor analyzed the answers this way:

Answer 1 is good, but not great. It doesn't specifically mention photosynthesis, nor the effect of dissolved CO₂ on pH, but it does convey the basic idea, and compares the difference between use of CO₂ and diffusion into the atmosphere.

Answer 2 is also good, in that it mentions use of CO₂ in photosynthesis, and the effect on pH. But....it doesn't say anything at all about the control.

Answer 3 makes a vague connection to the experiment. This is the type of logic that educators look for in the elementary science curriculum. Not acceptable in a college-level science class.

Answer 4 misses the point entirely.

Answer 5 nicely addresses the concept of diffusion. Unfortunately, we weren't studying diffusion in this exercise. The answer is irrelevant.

Answer 6 does a good job of addressing the topic of the lab: photosynthesis. It also relates photosynthesis to decreasing CO₂ concentrations and drop in pH, which in turn cause the bromthymol blue indicator to turn color. It also compares the experimental and control tubes. This is a good answer for a course at this level.

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