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Wednesday, December 5, 2012

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!!

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