Background:

Experimentally tracking changes in dissolved carbon dioxide (C0₂) and oxygen (O₂) can be an excellent and interesting way to investigate photosythesis and cellular respiration.

If exposed to sunlight, aquatic plants will lower dissolved C0₂ and raise dissolved O₂ is a test environment as a result of photosynthetic activity. On the other hand, animals such as snails and fish will do the opposite - they will raise dissolved C0₂ and lower dissolved O₂ as a result of cellular respiration activity. Plants will do the same as animals if their rate of photosynthesis falls below that of their rate of cellular respiration, as is the case when an aquatic plant is kept in the dark.

However, gas production in plants and animals can be challenging to measure for several reasons:

1. The production of C0₂ or O₂ is often slow, and accumulates slowly. Therefore, any experiment attempting to detect differences in the concentrations of these gases while likely have to take place over several hours, perhaps days.
2. The indicators that change colour to detect differences in C0₂ or O₂ concentration often yield very subtle colour changes that are very difficult to see, especially over long time periods.
3. Bromothymol blue (BTB) is an acid-base indicator, it measures the concentration of dissolved C0₂ indirectly by detecting the change in pH as the result of changes in dissolved carbonic acid. But the BTB will also react to anything else that changes pH, which can be caused by the release of waste products other than C0₂.

How can experimental technique by altered to deal with these challenges?

In regard to the first two points above, one idea to deal with subtle changes and long experimental time periods is to use digital imagery to help spot colour change, and "accelerate" the entire experiment. Before and after pictures can be very helpful to spot subtle colour changes (this may be the only option for experiments carried out in the dark), but even better is the use of time-lapse video. Many camcorders have the ability to record intervals - simply set up your camcorder at the beginnning of the experiment, configure it to record a second or two of video every five or ten minutes, then view the tape at the end of the experiment.

The following QuickTime video (2.2 MB) is an example of such a time-lapse recording. Digital video software was used to further speed the results, effectively shrinking as 10 hour time frame to 25 seconds - click on the image below (start of the experiment) to play the video:

Note that the arrangment of the tubes is the NOT quite the same as the first 5 tubes in Activity C13 (difference in red):

• Tube 1 - Water and BTB
• Tube 2 - Water and BTB and C0₂
• Tube 3 - Water and BTB and plant (Elodea)
• Tube 4 - Water and BTB and C0₂ and plant (Elodea) and snails
• Tube 5 - Water and BTB and snails

Analyzing and Interpreting:

The time-lapse video clearly shows a colour change in the last tube, indicating an accumulation of C0₂ in the tube.

• Why did this occur?
• It may be expected that the 4th tube should have changed from yellowish to blue as the plant removes C0₂ as the result of photosynthesis. However, according to the video, this did not occur. Why not?

Extension:

To deal with challenge 3 above, other techniques can be used to measure dissolved gas concentrations. For example, dissolved oxygen sensors are readlily available that give highly accurate and real-time measurments of changes in dissolved 0₂ conentrations.

The section below offers more ideas and images to help you with your use of a dissolved oxygen sensor to investigate photosynthesis and cellular respiration.

The Question:

Does light have an effect on oxygen production or use by plants and animals?

Variables:

Identify the type of data you will collect to support your hypothesis and state the manipulated, responding and controlled variables in this investigation.

Materials:

Note - material list is similar to Activity C13, text page 306.

• 4 - 250 ml beakers or large test tubes
• aquarium water
• sprigs of aquatic plant
• 4-6 aquarium snails or small goldfish
• area of lab that can be darkened (closed cabinet)
• dissolved oxygen sensor
• USB Link to computer (or alternatively, a handheld recording unit such as an Xplorer)

Procedure:

Step 2:

Step 3:

Fish in Water:

Elodea in Water:

The following graph represents a recording interval of about an hour.

The following graph is a continuation of the first, but represents a much shorter time interval of about 4 minutes. With the scale adjusted, the increase in dissolved oxygen is still very noticeable.

Step 5:

Repeat step 4 with the remaining three beaker configurations.

Step 6:

Repeat step 4 again with all the beaker configurations, but this time keep each beaker in a dark place. Keep the beaker in the dark for a few minutes before beginning data collection.

Step 7:

Use the graph to determine and record the overall change in dissolved oxygen for each of the beakers. This can be facilitated by choosing the maximum and minimum values from the statistics menu button at the top of the graph.

Analyzing and Interpreting:

1.	Based on the changes in dissolved oxygen in the beakers, which of the beakers showed evidence of photosynthesis? Explain.
2.	Based on the changes in dissolved oxygen in the beakers, which of the beakers showed evidence of cellular respiration? Explain.
3.	Based on the data collected from the beakers in the light and dark, what is the effect of light on photosynthesis and cellular respiration? Explain.

Forming Conclusions:

4.	Based on the data you have collected, write a summary statement for the following question: Does light have an effect on oxygen production or use by plants and animals?