Gas Exchange in Respiration:
Qualitative Observation of Carbon Dioxide Release
It is the time of year for teaching photosynthesis and respiration to my biology students. I think that all biology teachers will agree that these are difficult concepts to teach to our students. Other than genetics, I think that the topics of photosynthesis and respiration may be my favorite topics to teach. I enjoy the chemistry aspects, and I especially enjoy sitting back and marveling at how beautiful these two processes are. I really get excited in my classroom and can often be heard saying, "Isn't it cool how this works?!"
Which do you teach first? Personally, I like to teach respiration first. After 28 years of teaching, I find that this approach works best for me. I have tried it both ways, and now I always start with respiration.
It is so important to make sure that the students first have a firm grasp on the "big picture". Emphasize the reactants and the products. Make sure the students understand what the end result will be. Then start adding in the details of the chemical reactions. Once the student has a basic understanding of the relationship between these two processes, you can begin to add the details just like hanging ornaments on a Christmas tree. As I move into the more complicated aspects of these chemical reactions, my students become more and more excited. They quickly realize that they can really "see" the chemistry that is taking place.
Unfortunately, I have found it very difficult to find good labs to reinforce the concepts of respiration. I find photosynthesis labs to be more more effective. At any rate, today I did this lab with my students: Gas Exchange in Respiration. The basic idea is to show students that living organisms give off carbon dioxide. In the test tubes shown in the picture above, a small amount of phenol red has been added to each tube. Glass beads are added to provide a barrier between the phenol red and the specimens that will be added to each tube. As you look at the picture, the tube to the far left contains a piece of paper toweling that was dipped in a boiled yeast solution. Tube 2 has paper toweling that was dipped into a fresh yeast solution. Tube three contains 5-10 germinated seeds. Tube four contains 5-10 dry seeds. And tube 5 contains only the phenol red and no additional materials. If the organism gives off carbon dioxide it will cause the phenol red to change from red to orange or yellow. The tubes containing boiled yeasts and dry seeds will not show any change in the phenol red. Fresh yeast and germinated seeds are actively respiring and will quickly cause a color change in the phenol red.
If you have great lab ideas for respiration, I would love to hear them! Happy Teaching!