Measuring the Rate of Cellular Respiration

What is the rate of respiration in these germinating seeds?

Many biology teachers tell me that they dread teaching photosynthesis and respiration to their students.  Since I love teaching these concepts, I always ask why they feel this way.  Responses include:  "My students think it is boring.  It is too abstract for the students to understand.  There is too much chemistry involved.  There aren't any good labs to do."

I would have to disagree with all of these statements.  Photosynthesis and respiration may be the two topics I love teaching the most!  What is more fundamental to the study of biology than photosynthesis and respiration?  I have several labs (that I love!) that I do with my students while teaching about respiration.  I have already written articles on two of these labs:  Gas Exchange in Respiration, and Energy in Foods.

This blog post is about a simple and effective method of measuring the rate of respiration in a living organism.  As you can see in the photo above, I used Sugar Snap Peas as my choice for a respiring organism.  The objective?  To determine how much oxygen is consumed during respiration by these peas over a given amount of time.

The experiment includes 2 experimental set-ups and 1 control set up.  The rate of respiration will be measured in germinating peas and in dry peas that are dormant.  The peas will be placed inside a device known as a respirometer.  To insure equal volume in each set-up, the volume of germinating peas is first determined by water displacement.  An equal number of dry peas will be used and the volume will be made equal by the addition of small glass beads.  The third respirometer will contain an equal volume of glass beads only.

The respirometers are assembled as seen in the photo to the left.  The essential components of the experiment include:
1.   Respirometers are assembled and placed in a large pan of water.
2.   As oxygen is consumed by the seeds, the water will be drawn into the pipets.  This can be measured with the calibrate pipet that has been inserted into the rubber stopper.
3.  Since carbon dioxide is also released during respiration, there will be no movement of water into the pipet unless this is removed as a factor affecting the experiment.  A small amount of absorbent cotton is placed in the bottom of the vial.  This cotton is saturated with KOH.  As carbon dioxide is released by the respiring peas, it reacts with KOH to form solid potassium carbonate.  This removes the carbon dioxide and allows only the oxygen to be measured.
4.   Measurements will be taken every 5 minutes for some length of time.  Since we have a long lab period, we were able to set up the experiment and then allow it to run for 30 minutes.

This lab has all the best components of a lab:  

• Easy set-up!
• Easy clean-up!
• No fancy equipment required!
• Works every time!
• Kids love it!

Happy Teaching!!

Lab: Gas Exchange in Respiration

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!

Gas Exchange in Respiration:

Qualitative Observation of Carbon Dioxide Release