Introduction
Cellular respiration and fermentation are 2 of the most challenging concepts for introductory biology students, who may become so consumed by memorizing steps of the Krebs cycle and glycolysis that they lose sight of the big picture. The following demonstrations place aerobic cell respiration and fermentation firmly in grasp. First, students observe respiration in germinating seeds by detecting the carbon dioxide produced. Next, they observe the carbon dioxide gas produced by yeast fermentation.
Activity Details
- Prep Time: 15 minutes
- Class Time: 45-60 minutes
Materials
- Hydrated Seeds
- Sealable Bag
- Bromthymol Blue, 0.04%
- Sodium Hydroxide, 1%
- Test Tubes
- Rubber Stoppers
- 250-mL Flaask
- Test Tube Rack
- Distilled Water or Springwater
- Paper Towel
Demonstrating Aerobic Cellular Respiration with Germinating Seeds
Have students observe the respiration of germinating seeds, using bromthymol blue as a pH indicator. You can keep this demonstration simple by observing pH changes in solution as seeds respire. Alternatively, you might add variables to the demonstration (e.g., compare the respiration rates of various plant seeds with the same total mass). Additionally, you can test whether photosynthesis is occurring by placing duplicate samples in the dark.
Safety Tip
Note:Wear gloves and safety goggles while handling the bromthymol blue solution and sodium hydroxide.
Procedure
- Two days before the demonstration, rehydrate the seeds (we recommend wheat, barley, peas, or mung beans). Place the seeds in a cup or beaker. Use dechlorinated water to cover the seeds to a depth at least 3 times their height in the container to compensate for the expansion of the seeds as they swell. Allow the seeds to soak overnight.
- Pour off the remaining water and fold the seeds into a wet paper towel. Place the towel in a sealable bag. Close the bag and store the seeds in a dark place over a second night.
- Prepare the bromthymol blue solution by adding 1.5 mL (about 30 drops) of 0.04% bromthymol blue to 80 mL of springwater or distilled water. The prepared bromthymol blue solution should be green, but the shade of green may vary depending on the pH of your water source. A color change will easily be observed if the solution given to students is slightly basic. To create a slightly basic solution, add the sodium hydroxide dropwise to the bromthymol blue until the color changes from green to a deep blue (10 to 20 drops).
- Explain the use of bromthymol blue as a pH indicator to the class. The chemical bromthymol blue is an indicator that appears blue in an alkaline (base) solution and yellow in an acidic solution. Carbon dioxide that is added to the solution surrounding the seeds combines with water to form carbonic acid (H2CO3), turning the bromthymol blue to a yellow-green color. Remove the carbon dioxide from solution and the bromthymol blue will turn a deeper blue.
- Fill a test tube ¾ full with rehydrated seeds. Pour the bromthymol blue solution over the seeds until the tube is completely full. Seal the tube with a rubber stopper.
- Prepare a control tube with only bromthymol blue solution. Fill the tube until it is completely full, and seal it with a rubber stopper.
- Store the samples until the end of the class period or overnight and have students make observations regarding the color of the solution or any other changes they observe. (You may set up duplicate samples in the dark to rule out any possible effects of photosynthesis on the experiment.)
Class Discussion
Have students consider the seeds’ respiration. Oxygen is present in the test tube, and the carbon dioxide gas is a product of the process of aerobic cellular respiration. (To a much lesser extent and for a limited time, germinating seeds may also perform anaerobic respiration, which also produces carbon dioxide.) Discuss the purpose of the control tube with only bromthymol blue solution.
Students may think that the seeds are photosynthesizing. If chlorophyll and light are present, the seeds may be photosynthesizing. Students should know that plants’ photosynthesis uses carbon dioxide and produces oxygen. While additional oxygen would not significantly change the pH, removal of carbon dioxide from solution would make the solution more basic and turn the bromthymol blue a deeper blue color.
